Evolving Assessments of Human and Natural
Contributions to Climate Change
Updated August 11, 2021
Congressional Research Service
https://crsreports.congress.gov
R45086

Evolving Assessments of Human and Natural Contributions to Climate Change

Summary
This CRS report provides the 2021 scientific statements regarding human and natural
contributions to global climate change from the Intergovernmental Panel on Climate Change
(IPCC). It also provides context for the ongoing policy deliberations in Congress by tracing the
evolution of scientific understanding and confidence in attribution of observed climate change.
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Evolving Assessments of Human and Natural Contributions to Climate Change

Introduction
As Congress continues to deliberate whether and how to address climate change, a question
sometimes raised has been the degree to which humans and natural factors have influenced
observed global climate change. The importance of the question relates partly to potential
efficacy of policies to abate climate change: To the degree the climate is being altered by human
influence, policies may be able to reduce or avoid undesired changes. To the degree changes are
natural, policies may be ineffectual in avoiding undesired climate change. Ultimately, scientific
confidence in understanding the specific drivers of climate change may also influence efforts to
establish accountability for past and projected climate change. It may also support improving
capacities to anticipate, prepare for, and adapt to future conditions. Policymakers have called on
researchers and major scientific assessment processes to analyze and report on the attribution of
observed climate change to various possible causes.1 Over time, scientific assessments of both
climate change and the extent to which humans have influenced it have expressed increasing
confidence, while moving the broader scientific community toward greater consensus.
The Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change (IPCC)
released its latest scientific conclusions in August 2021:2
It is unequivocal that human influence has warmed the atmosphere, ocean and land.
Widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere have
occurred…. Observed increases in well-mixed greenhouse gas (GHG) concentrations since
around 1750 are unequivocally caused by human activities. (p. SPM-5)
The likely range of total human-caused global surface temperature increase from 1850–
1900 to 2010–2019 is 0.8°C [Celsius] to 1.3°C, with a best estimate of 1.07°C. It is likely
that well-mixed [greenhouse gases] GHGs contributed a warming of 1.0°C to 2.0°C, other
human drivers (principally aerosols) contributed a cooling of 0.0°C to 0.8°C, natural
drivers changed global surface temperature by –0.1°C to 0.1°C, and internal variability
changed it by –2°C to 0.2°C. It is very likely that well-mixed GHGs3 were the main driver
of tropospheric warming since 1979, and extremely likely that human-caused stratospheric
ozone depletion was the main driver of cooling of the lower stratosphere between 1979 and
the mid-1990s.
The general “unequivocal” statement of attribution, above, expresses an increase in scientific
confidence compared with the preceding major U.S. assessment, the Climate Science Special
Report (CSSR), released in October 2017 by the U.S. Global Change Research Program
(USGCRP). It stated the following:
It is extremely likely [>95% likelihood] that human influence has been the dominant cause
of the observed warming since the mid-20th Century. For the warming over the last century,

1 See, among others, the Global Change Research Act of 1990 (P.L. 101-606); United Nations General Assembly,
“Protection of the global climate for present and future generations of mankind,” 43/53, 70th Plenary Meeting,
December 6, 1988.
2 Intergovernmental Panel on Climate Change. “AR6 Climate Change 2021: The Physical Science Basis - Summary for
Policy Makers,” August 9, 2021, https://www.ipcc.ch/report/ar6/wg1/. A change of 0.8°C [Celsius] to 1.3°C is
equivalent to a change of 1.4°F [Fahrenheit] to 2.3°F.
3 Well-mixed GHGs are those that remain in the atmosphere long enough—several years to thousands of years—such
that they diffuse and mix globally throughout the lower atmosphere (troposphere) and a global average concentration
by volume can be established. For a more technical explanation, see IPCC, op. cit., Annex II—Glossary. The well-
mixed GHGs include water vapor (H2O), as well as others associated with human activities: carbon dioxide (CO2),
nitrous oxide (N2O), methane (CH4) and ozone (O3). GHGs associated with human activities also include sulfur
hexafluoride (SF6), hydrofluorocarbons (HFCs), chlorofluorocarbons (CFCs), and perfluorocarbons (PFCs).
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there is no convincing alternative explanation supported by the extent of the observational
evidence.4
This CRS report provides context for scientists’ statements of attribution by tracing the evolution
of scientific understanding and confidence regarding the drivers of recent global climate change.5
Climate change science can be traced back to the early 1800s. Through the 20th century, academic
institutions, federal and state agencies, foreign governments, and other entities invested
significant time and tens of billions of dollars in climate research. This investment has led to
substantial advances in empirical observations, atmospheric and ocean physics and chemistry,
climate and economic simulation models, statistical methods, and other achievements. As a result,
scientists have increased their confidence in their detection and understanding of climate change
and attribution of observed changes to their causes. There is now high scientific confidence that
the global climate is warming, primarily as a result of increased human-related greenhouse gas
(GHG) emissions and other activities.6 This confidence has evolved from nearly two centuries of
research and assessments.
This report describes a chronology (in the Appendix) of 200 years of major scientific statements,
selected to represent views at each time, regarding the human and natural contributions to global
climate change. The chronology demonstrates how scientific views and confidence in those views
evolved over time.
That GHGs, including carbon dioxide (CO2),7 water vapor, and other gases, warm the Earth’s
climate is not a recent concept. The greenhouse effect,8 as it is sometimes called, was deduced as

4 Donald J. Weubbles et al., Climate Science Special Report: Fourth National Climate Assessment, Volume I,
USGCRP, 2017, https://science2017.globalchange.gov/downloads/CSSR2017_FullReport.pdf. A brief explanation of
development of the CSSR is available at https://www.globalchange.gov/content/cssr. The CSSR is the first volume of
the fourth national assessment required periodically by the Global Change Research Act of 1990 (P.L. 101-606).
5 Scientists and commentators often discuss climate change referring to different periods of time. Such time differences
can explain some or all of the differences among some perspectives. The factors having greatest influence on global
climate typically vary depending on whether the period of interest is one decade, several decades to a century, or tens
of thousands to millions of years. While scientific assessments of the drivers of climate change cover all time scales,
most assessments consider the human contribution since the Industrial Revolution beginning in the 19th century.
6 For example, by 2017, Weubbles et al., Climate Science Special Report. The CSSR (p. 127) provided the following
summary paragraph regarding scientific confidence in detection of climate change and attribution to human and natural
causes:
Detection and attribution studies, climate models, observations, paleoclimate data, and physical understanding
lead to high confidence (extremely likely) that more than half of the observed global mean warming since 1951
was caused by humans, and high confidence that internal climate variability played only a minor role (and possibly
even a negative contribution) in the observed warming since 1951. The key message and supporting text
summarizes extensive evidence documented in the peer-reviewed detection and attribution literature, including in
the [Fifth Assessment Report of the Intergovernmental Panel on Climate Change].
7 CO2 was referred to as carbonic acid in the 1800s, as will appear in some entries in Table A-1.
8 GHGs in the atmosphere allow shortwave solar radiation to pass through to the Earth’s surface but block longwave
energy (i.e., heat) from re-radiating into space. GHGs are similar to glass that allows sunlight to enter a greenhouse but
keeps heat from escaping. An early description is present in T. Sterry Hunt, “On the Earth’s Climate in Palæozoic
Times.” The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, vol. 26, no. 175
(October 1, 1863):
The late researches of Tyndall on the relation of gases and vapours to radiant heat are important in
their bearing upon the temperature of the earth’s surface in former geological periods. He has
shown that heat, from whatever source, passes through hydrogen, oxygen, and nitrogen gases, or
through dry air, with nearly the same facility as through a vacuum.... Gases and some other solid
substances, which are readily permeable to light and to solar heat, offer, as is well known, great
obstacles to the passage of radiant heat from non-luminous bodies; and Tyndall has recently shown
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early as 18279 with relatively little dispute since the 19th century among scientists about the role
of GHGs: Some level of GHGs in the atmosphere is necessary for maintaining a temperate
climate on Earth. Instead, the debate that unfolded involved whether the climate had been
warming overall10 and, if so, to what the changes may be attributable (such as industrial releases
of GHGs, volcanoes, solar activity, orbital ellipses of the Earth around the Sun, or other natural
variations). (See text box, Human and Natural Influences on Climate.)
Human and Natural Influences on Climate11
There are several ways that climate can be affected by both human and natural causes.
Humans: Climate is affected by changing concentrations in the atmosphere of GHGs such as CO2, methane,
water vapor, chlorofluorocarbons, hydrofluorocarbons, and other gases and aerosols. GHGs trap heat in the
atmosphere and warm the atmosphere. Most human-related GHGs are emitted in the production and use of fossil
fuels. Though not typically considered GHGs, emissions of polluting sulfur and carbonaceous aerosols also alter
the atmosphere’s reflection of solar radiation and its absorption of heat. Humans change land cover, affecting both
the reflectivity of land and the removal of CO2 from the atmosphere by vegetation. Higher CO2 concentrations in
the atmosphere fertilize vegetation, increasing removals of CO2 from the atmosphere when other factors (e.g.,
nutrient and water availability, temperature, solar insolation, etc.) are not constraining.
Natural: Natural influences on the climate may include changes in solar energy, naturally occurring water vapor
and CO2 in the atmosphere, volcanic aerosol and GHG emissions, and cyclical oscillations in the oceans. These
factors impact climate by affecting the amount of solar radiation reaching earth’s surface, modulating how much
heat is retained within the atmosphere, or changing oceanic and atmospheric temperature circulation. Over scales
of centuries to tens of thousands of years, the Earth’s rotational wobble and orbit around the sun also affect the
pattern of incoming solar radiation and affect climate on geologic time scales (e.g., glacial and inter-glacial periods).
Chronology of Climate Change Science
As indicated by the information presented in Table A-1, scientists have noted, dating back to
early in the 19th century, both human and natural factors potentially influencing climate. As one
scholar observed, “by 1900, most of the chief theories of climate change had been proposed, if
not yet fully explored.”12 There were a number of contending theories—including changes in
solar energy, the Earth’s orbital geometry, volcanoes, the geography of continents, and changes in
GHGs—in the late 1800s as the quotations in Table A-1 indicate. Well into the 1900s, the state of
the science relating CO2 concentrations in the atmosphere to the Earth’s temperature was
primarily theoretical inference. Scientists debated whether increases in CO2 in the atmosphere
due to increasing emissions from fossil fuels would lead to further warming.
Since then, a number of factors—including better measurement technologies; development of
physics- and empirically based simulation models; more research, review, and revision; and

that many colourless vapours and gases have a similar effect, intercepting the heat from such
sources, by which they become warmed and in their turn radiate heat.... The diffusion of olefiant
gas ... carbonic acid gas ... ozone ... watery vapour present in the air.... Like a covering of glass, it
allows the sun’s rays to reach the earth but prevents to a great extent the loss by radiation of the
heat thus communicated. (p. 323, emphasis added)
9 Jean-Baptiste Joseph Fournier, “Mémoire Sur Les Températures Du Globe Terrestre et Des Espaces Planétaires,”
Mémoires de l’Académie Royale Des Sciences de l’Institute de France, 1827, https://geosci.uchicago.edu/~rtp1/papers/
Fourier1827Trans.pdf.
10 The focus of this report is on attribution of global temperature changes, but much research and many scientific
assessments address other metrics of climate, such as precipitation and seasonality.
11 Weubbles et al., Climate Science Special Report.
12 James Rodger Fleming, Historical Perspectives on Climate Change, Oxford University Press (2005).
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longer series of observations—have improved the foundations of climate science. As a result,
scientists have improved quantification of the relationships between observed conditions:
1. Natural and human-related GHG emissions (the latter mostly from fossil-fuel-
based energy) to the atmosphere;
2. Increasing GHG concentrations in the atmosphere and changes in other
influences on climate (e.g., changes in solar and volcanic activity);
3. Rising global average surface temperature; and
4. Other observed changes in the spatial and temporal patterns of climate.
The magnitudes of factors, and therefore their influences on climate, vary over time. With
acceleration of human population growth and industrialization since the 19th century, the factors
related to human activities have increased relative to those of natural processes. Increased
scientific capacity has made climate change increasingly detectable and attributable to the varying
influences over the past two centuries.13
In the late 1930s, Guy Callendar compiled existing data on atmospheric CO2 concentrations and
regional temperatures.14 Through imprecise calculations, he showed a correlation between
observed increases in both over time. Some scientists considered the correlation merely
coincidence. Callendar’s calculations provided early quantitative indications of a climate
warming as a result of human activity. At the time, however, the relative contribution of human
activity compared with natural factors could not be determined.15 David Keeling later established
more consistent and repeatable measurements of atmospheric CO2 in the 1950s.16 Keeling’s
precise measurements provided strong evidence of a connection between increasing human-
related CO2 emissions and the increasing CO2 concentrations in the atmosphere. The
measurements established a quantitative benchmark for later studies examining the linkage
between increasing CO2 concentrations and rising global temperatures. Keeling’s concentration
data facilitated additional research on the global carbon cycle, the oceans, and the effects of
human activities.
In the middle of the 20th century, scientists (and many in the public) recognized that a general
warming of the climate had occurred (see quotations in Table A-1).17 This was followed by a 30-
year period of relatively flat or decreasing global average temperatures from around 1946 to
197718 (Figure 1). Arguably, the apparent change in trajectory heightened scientific uncertainty

13 Before robust observations of the climate and the factors that influence it became available, scientists theorized how
future climate would evolve with, for example, continued increases in fossil fuel emissions, as can be seen in the
quotations in Table A-1. Since the 1990s, projections of future climate changes rely on formally produced and peer-
reviewed scenarios of how different climate forcing factors may evolve, including how economic activities and public
policies may alter those factors.
14 Guy S. Callendar, “The Artificial Production of Carbon Dioxide and Its Influence on Temperature,” Quarterly
Journal of the Royal Meteorological Society, vol. 64, no. 275 (April 1938), pp. 223-40, https://www.eas.ualberta.ca/
jdwilson/EAS372_15/exams/Callendar_QJRMS1938.pdf.
15 Substantial scientific research suggests that solar radiation and other natural factors are important in the global
temperature increase in the first half of the 20th century. See, among other research, P.A. Stott et al, “Attribution of
Twentieth Century Temperature Change to Natural and Anthropogenic Causes,” Climate Dynamics, vol.17, no. 1
(January 1, 2001), pp. 1-21, https://doi.org/10.1007/PL00007924.
16 Scripps Institute of Oceanography, “The Early Keeling Curve,” http://scrippsco2.ucsd.edu/history_legacy/
early_keeling_curve.
17 See, for example, Albert Abarbanel and Thorp McClusky, “Is the World Getting Warmer?,” The Saturday Evening
Post, July 1, 1950.
18 National Academy of Sciences, Energy and Climate (Washington, DC: National Academies Press, 1977),
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about the direction of future climate changes and any human influence on them. It coincided with
concerns about “global dimming,”19 at least in part attributed to sulfur and particulate pollution,20
which increased rapidly during that period before leveling off around 1980.21 Current assessments
indicate that the mid-20th century warming hiatus may have been due to a combination of human
(GHG, pollution) and natural influences (solar variability, volcanoes).22 As temperatures began to
rise again in the late 1970s, authoritative scientific assessments performed by various
governmental and nongovernmental institutions, supported by an expanding body of peer-
reviewed published research, pointed to an emerging consensus regarding a probable human
contribution to climate change, primarily due to increasing GHG emissions.
Figure 1. Anomalies of Global Average Surface Temperature, 1880-2020
(in degrees Celsius and Fahrenheit)

Source: National Oceanic and Atmospheric Administration, National Centers for Environmental Information,
“Climate at a Glance: Global Time Series,” annual data for 1880-2020, retrieved on August 12, 2021,
https://www.ncdc.noaa.gov/cag/global/time-series/globe/land_ocean/ann/12/1880-2020.
Notes: A temperature “anomaly” is the difference between the measured temperature and the average over the
20th century. Statisticians consider computation of the anomalies to be more reliable than using absolute
temperature measurements. For documentation, see https://www.ncdc.noaa.gov/ghcnm/v3.php. Four other

https://www.nap.edu/download/12024.
19 For a relatively recent review of research on the variability of solar radiation reaching the Earth’s surface—
sometimes called “global dimming” and “brightening” by scientists—and how it may have influenced non-linear
changes in 20th century climate, see Martin Wild, “Global Dimming and Brightening: A Review,” Journal of
Geophysical Research, vol. 114, June 27, 2009, https://doi.org/200910.1029/2008JD011470.
20 According to this theory, global dimming and cooling during this time was probably at least partially influenced by
human activities in the form of aerosol pollution, as explained in the article cited above.
21 See, among others, Martin Wild, “Enlightening Global Dimming and Brightening,” Bulletin of the American
Meteorological Society, vol. 93, no. 1 (July 15, 2011), http://journals.ametsoc.org/doi/pdf/10.1175/BAMS-D-11-
00074.1.
22 N. L. Bindoff et al, “Detection and Attribution of Climate Change: from Global to Regional,” chapter 10 in T. F.
Stocker et al. (eds.), Climate Change 2013: The Physical Science Basis: Contribution of Working Group I to the Fifth
Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge: U.K.: Cambridge University Press,
2013), p. 883, http://ipcc.ch/report/ar5/wg1/. This chapter also describes the methods and data for “detection and
attribution” of climate change over different time scales.
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research organizations analyze and publish global temperature anomalies. They use different methods and
produce similar estimates—particularly regarding trends—with NOAA’s typically being slightly below those of
the National Aeronautics and Space Administration, for example.
The relative role of human versus natural influences became more clear in the early 2000s.
Longer series of improved observations (e.g., solar radiation, clouds, land cover change),
statistical methods, and computational models enabled more robust analyses and comparisons of
research methods and results. Major, collaborative, authoritative assessments—U.S. and
international—were established to compile, debate, and consider the strengths or weaknesses of
scientific analysis regarding climate change in order to inform policymakers.
What Are Scientific Assessments of Climate?
Policymakers, when seeking thoroughly vetted and balanced scientific information, sometimes commission
scientific assessments. On climate change, periodic assessments have been mandated under domestic law (e.g., the
Global Change Research Act of 1990, P.L. 101-606) and by international agreements (United Nations General
Assembly Resolution 43/53 of 6 December 1988). According to the Office of Management and Budget, a scientific
assessment is
an evaluation of a body of scientific or technical knowledge, which typically synthesizes multiple
factual inputs, data, models, assumptions, and/or applies best professional judgment to bridge
uncertainties in the available information. These assessments include, but are not limited to,
state-of-science reports; technology assessments; weight-of-evidence analyses; meta-
analyses....23
The National Academy of Sciences, Engineering and Medicine (NASEM, formerly NAS), the Intergovernmental
Panel on Climate Change (IPCC), and the United States Global Change Research Program (USGCRP) have
organized scientific climate assessments involving hundreds to thousands of individual scientists and subject to
expert peer reviews and, for the IPCC and USGCRP, public peer reviews.
Table A-2 contains the relevant conclusions regarding human and natural influences on climate
change from the major assessments conducted, with those of
 NASEM beginning in 1977,24
 IPCC beginning in 1990, and
 USGCRP beginning in 2001.
These inclusive assessments underpinned growing scientific confidence that human activities
were likely the major cause of the observed global warming since the mid-20th century.
Conclusions
Many factors have contributed to increased scientific confidence in quantifying the human and
natural contributions to climate change. Longer records of observational data have provided more
evidence of the concordance between higher GHG emissions, higher GHG concentrations in the
atmosphere, and temperature increases as well as other metrics of climate change. Satellites have
provided important observations of temperatures; atmospheric pollution; and land, snow, and ice
cover beginning in the late 1970s. Additionally, improved scientific understanding of atmospheric
physics, together with vastly more powerful computers, has led to climate models that better
simulate atmospheric and oceanic conditions, including at regional scales. Uncertainties remain in

23 U.S. Office of Management and Budget, “Final Information Quality Bulletin for Peer Review,” 70 Federal Register
2667, January 14, 2005, https://www.gpo.gov/fdsys/pkg/FR-2005-01-14/pdf/05-769.pdf.
24 NAS is a private, nonprofit society of distinguished scholars. It was established by Congress in statute in 1863 and is
charged with providing independent, objective advice to the nation on matters related to science and technology.
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the models on how they simulate the effects of clouds, for example, and model simulations at
high-resolution scales of space (i.e., local) and time. Despite uncertainties, current climate
scientific assessments state high confidence (very likely to extremely likely) that human influence
is the dominant cause of the observed warming over the past half-century. While near consensus
has developed relatively recently, it has evolved with repeated research, reviews, and
reproducibility, based on scientific concepts established as early as 200 years ago. Future climate
outcomes depend on many additional factors, such as the future rates and character of socio-
economic development and efforts to curtail the growth of GHG emissions.
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Appendix. Chronology of Scientific Statements on
Attribution of Global Climate Change
This appendix contains bibliographic references and quotations regarding scientific understanding
of global climate change and the influence of CO2, other GHGs, and natural factors on observed
and prospective global climate. Because the capacities and methods of science have changed
markedly over the past 200 years, the references appear in two tables representing selected
scientific literature and national or international scientific assessments.
Table A-1 presents representative statements excerpted from key scientific literature from 1827 to
1987 regarding human-related and other contributions to climate change. Sources include
selected, widely cited academic papers, government reports, and NAS reports. The table’s
selections largely precede the establishment of broadly inclusive scientific assessments to
compile and assess the weight of scientific evidence. For the period up to 1987, CRS selected key
academic scientific papers and reports that were influential to scientific contemporaries during
and after their respective times.
Scientific assessments began in the mid-20th century to more systematically and inclusively
evaluate the full body of scientific literature on specific topics.
Table A-2 compiles the conclusions pertinent to this report from major U.S. and international
scientific assessments, beginning in 1977, that address the human contribution to global climate
change. The assessments have been produced by the USGCRP, NAS, and IPCC.25
Table A-1. Select Scientific Statements Regarding the Influences on Global Climate
of Human Activities and Natural Factors, 1827-1987
Year
Title
Key Quotation(s)
Reference
1827
Mémoire sur les
“The establishment and progress of
Jean-Baptiste Joseph Fournier,
Températures du
human societies, and the action of
“Mémoire Sur Les Températures Du
Globe Terrestre
natural forces, can notably change the
Globe Terrestre et Des Espaces
et des Espaces
state of the ground surface over vast
Planétaires,” Mémoires de l’Académie
Planétaires [On
regions, as well as the distribution of
Royale Des Sciences de l’Institute de
the Temperatures waters and the great movements of the
France, 1827,
of the Terrestrial
air. Such effects have the ability to make https://geosci.uchicago.edu/~rtp1/
Sphere and
the mean degree of heat vary over the
papers/Fourier1827Trans.pdf.
Interplanetary
course of several centuries."
Space]
1856
Circumstances
“An atmosphere of [carbonic acid]
Eunice Foote, “Circumstances Affecting
Affecting the
would give to our earth a high
the Heat of the Sun’s Rays,” American
Heat of the Sun’s
temperature; and if as some suppose, at
Journal of Science and Arts, vol. 22 (1856),
Rays
one period of its history the air had
pp. 382-83, https://books.google.com/
mixed with it a larger proportion than
books?id=6xhFAQAAMAAJ&pg=
at present, an increased temperature
PA382#v=onepage&q&f=false.
from its own action as well as from
increased weight must have necessarily
resulted."
1896
On the Influence
“If the quantity of carbonic acid
Svante Arrhenius, “On the Influence of
of Carbonic Acid
increases in geometric progression, the
Carbonic Acid in the Air upon the

25 Individual scientists, and some organizations, may not agree with the conclusions of major scientific assessments.
CRS is not aware of any national or international climate-related scientific body that dissented from the recent
assessments quoted in this report.
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Year
Title
Key Quotation(s)
Reference
in the Air upon
augmentation of the temperature will
Temperature of the Ground,”
the Temperature
increase nearly in arithmetic
Philosophical Magazine and Journal of
of the Ground
progression.”
Science, vol. 41, series 5 (April 1896),
“This quantity of carbonic acid, which is
pp. 237-76, http://www.trunity.net/files/
supplied to the atmosphere chiefly by
108501_108600/108531/
modern industry, may be regarded as
arrhenius1896_greenhouse-effect.pdf.
completely compensating the quantity
of carbonic acid that is consumed in the
formation of limestone (or other
mineral carbonates) by the weathering
or decomposition of silicates.”
”We must regard volcanic exhalations
as the chief source of carbonic acid for
the atmosphere.”
1897
A Group of
“A comparison of early with later life,
Thomas C. Chamberlin, “A Group of
Hypotheses
stripped of theoretical presumptions,
Hypotheses Bearing on Climatic
Bearing on
does not seem to me clearly to imply
Changes,” Journal of Geology, vol. 5, no. 7
Climatic Changes
any great difference in the content of
(1897), pp. 653-683,
carbon dioxide.”
https://www.jstor.org/stable/pdf/
“For it is almost axiomatic to say that
30054630.pdf?refreqid=
climatic changes would attend changes
excelsior%3Aa77b10a39bbbe564c432a9
in the constitution of the atmosphere. I
204d719cec.
assume that atmospheric poverty,
especially in the critical item of carbon
dioxide, is correlated with low
temperature.”
1898
Monthly Weather “We have therefore an alternation of
U.S. Department of Agriculture,
Review and
cold and warm periods due to the
Weather Bureau, Monthly Weather
Annual Summary
interaction of elevation [of the Earth’s
Review and Annual Summary, July 1898,
surface] and carbon dioxide [in the
http://congressional.proquest.com/
atmosphere].... [W]e hope eventually to congressional/result/
be able to deduce the resulting climatic
pqpresultpage.gispdfhitspanel.pdflink/
peculiarities, and to show that very
$2fapp-s3$2fjlh-content$2fPT4-A2906-
slight changes in oceans and continents
8$2fPT4-A2906-8_From_1_to_752.pdf/
have produced all the variations of
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geological climate, and that little or
gis%7Cexecutive-branch%7Ca2906-8.
nothing need be hypothecated as to the
variations of solar heat, of atmospheric
gases, of terrestrial latitudes, or the
many other climatological elements.”
1911
USGS: The Data
“In a thousand years, then, if the rate
Frank Wigglesworth Clark, U.S.
of Geochemistry
were constant and no disturbing factors
Geological Survey, “The Data of
interfered, the amount of CO2 in the
Geochemistry”, Bulletin 491, 2nd ed., U.S.
atmosphere would be doubled. If we
House of Representatives, 1911, pp. 47,
take into account the combustion of
135,
fuels other than coal and the large
https://congressional.proquest.com/
additions to the atmosphere from the
congressional/result/
sources previously mentioned the result pqpresultpage.gispdfhitspanel.pdflink/
becomes still more startling. Were
$2fapp-bin$2fgis-
there no counterbalancing of this
serialset$2f0$2fd$2f4$2fa$2f5997_hdoc
increase in atmospheric carbon, animal
1499_from_1_to_779.pdf/
life would soon become impossible
entitlementkeys=1234%7Capp-
upon our planet.”
gis%7Cserialset%7C5997_h.doc.1499.
“Whether the theory of Arrhenius is in
harmony with the facts of historical
geology—that is, whether periods of
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Year
Title
Key Quotation(s)
Reference
volcanic activity [releasing carbon
dioxide] have coincided with warmer
climates, and a slackening of activity
with lowering of temperatures is also in
dispute. Apparently, the controversy is
not yet ended.”
“Water vapor, then, is the chief agent in
the atmospheric regulation of climate
and to this conclusion; The climatic
conditions may vary as Chamberlin
claims, but the relative dryness or
wetness of the atmosphere may be the
true cause of fluctuating temperatures,
rather than the carbon dioxide.”
1939
The Composition
“It is a commonplace that man is able to Guy S. Callendar, “The Composition of
of the
speed up the processes of Nature, and
the Atmosphere Through the Ages,”
Atmosphere
he has now plunged heavily into her
Meterological Magazine, vol. 74, no. 878
Through the Ages slow-moving carbon cycle by throwing
(March 1939), pp. 33-39,
some 9,000 tons of carbon dioxide into
https://www.google.com/url?sa=t&rct=
the air each minute. This great stream
j&q=&esrc=s&source=web&cd=1&ved=
of gas results from the combustion of
0ahUKEwjBjtHfoP3WAhXB4SYKHYGF
fossil carbon (coal, oil, peat, etc.), and it
A9UQFggmMAA&url=
appears to be much greater than the
https%3A%2F%2Fdigital.nmla.metoffice.g
natural rate of fixation.”
ov.uk%2Fdownload%2Ffile%2Fsdb%253
“From the best laboratory observations
AdigitalFile%257Cedc8aafc-a27a-4afe-
it appears that the principal result of
bd29-619b052bd199%2F&usg=
increasing atmospheric carbon dioxide,
AOvVaw0R8QW-XaTql-iGP-
apart from a slight speeding up of rock
cJjF11https://www.google.com/url?sa=t&
weathering and plant growth, would be
rct=j&q=&esrc=s&source=web&cd=1&
a gradual increase in the mean
ved=
temperature of the colder regions of
0ahUKEwjBjtHfoP3WAhXB4SYKHYGF
the earth.”
A9UQFggmMAA&url=
https%3A%2F%2Fdigital.nmla.metoffice.g
ov.uk%2Fdownload%2Ffile%2Fsdb%253
AdigitalFile%257Cedc8aafc-a27a-4afe-
bd29-619b052bd199%2F&usg=
AOvVaw0R8QW-XaTql-iGP-cJjF11
1948
Circular Causal
“Callendar (1940), considering the best
George Evelyn Hutchinson, “Circular
Systems in
data available since 1866, concludes that Causal Systems in Ecology,” Annals of
Ecology
during the present century there has
the New York Academy of Sciences, vol.
been an increase of the order of 10 per
50, (1948), pp. 221-246,
cent in the CO2 content of the
http://people.wku.edu/charles.smith/
atmosphere. This he attributes to the
biogeog/HUTC1948.htm.
modern industrial combustion of fuel....
It seems far more likely that the
observed increment in the carbon
dioxide of air at low levels in both
Europe and eastern North America is
due to changes in the biological
mechanisms of the cycle rather than to
an increase in industrial output.... The
self-regulating mechanisms of the
carbon cycle can cope with the present
influx of carbon of fossil origin."
1966
U.S. House of
"Carbon dioxide accumulations from
Rep. George P. Miller, “Environmental
Representatives:
the burning of gas, petroleum, and coal
Pollution: A Challenge to Science and
Congressional Research Service

10

Evolving Assessments of Human and Natural Contributions to Climate Change

Year
Title
Key Quotation(s)
Reference
Environmental
change the nature of the atmosphere.
Technology,” Report of the Subcommittee
Pollution: A
Weather patterns can be altered
on Science, Research, and Development.
Challenge to
purposefully or accidentally by human
Committee on Science and Astronautics,
Science and
activity. These powerful forces have
U.S. House of Representatives, 1966, pp.
Technology
only come about recently and are not
3, 43, 263,
well understood. As a consequence, in
https://congressional.proquest.com/
many risk-benefit questions, the
congressional/result/
magnitude of the risk is relatively
pqpresultpage.gispdfhitspanel.pdflink/
unknown."
$2fapp-bin$2fgis-
“By the year 2000 at present and
congresearch$2f6$2f7$2f2$2f9$2fcmp-
projected rate of fossil fuel
1966-sah-0009_from_1_to_67.pdf/
consumption, the carbon dioxide level
entitlementkeys=1234%7Capp-
in the air will cause a significant rise in
gis%7Ccongresearch%7Ccmp-1966-sah-
the air temperature.”
0009.
1970
Man-Made
“It appears that on the local scale man-
Helmut E. Landsberg, “Man-Made
Climatic Changes
made influences on climate are
Climatic Changes,” Science, vol. 170, no.
substantial but that on the global scale
3964 (December 18, 1970),
natural forces still prevail.... The
http://www.jstor.org/stable/pdf/
potential for anthropogenic changes of
1730486.pdf?refreqid=
climate on a larger and even a global
excelsior%3Ae79d4c4c72387997406c41
scale is real.... In my opinion, man-made
3dafdebbaf.
aerosols, because of their optical
properties and possible influences on
cloud and precipitation processes
constitute a more acute problem than
CO2.... Over longer intervals, energy
added to the atmosphere by heat
rejection and CO2 absorption remain
matters of concern.”
1972
NSF: Patterns and "Judging from the record of the past
National Science Board, National
Perspectives in
interglacial ages, the present time of
Science Foundation, “Patterns and
Environmental
high temperatures should be drawing to
Perspectives in Environmental Science,”
Science
an end, to be followed by a long period
1972, https://archive.org/details/
of considerably colder temperatures
patternsperspect00nati.
leading into the next glacial age some
20,000 years from now. However, it is
possible, or even likely, that human
interference has already altered the
environment so much that the climatic
pattern of the near future will follow a
different path. For instance, widespread
deforestation in recent centuries,
especially in Europe and North
America, together with increased
atmospheric opacity due to man-made
dust storms and industrial wastes,
should have increased the earth’s
reflectivity. At the same time, increasing
concentration of industrial carbon
dioxide in the atmosphere should lead
to a temperature increase by
absorption of infrared radiation from
the earth's surface. When these human
factors are added to such other natural
factors as volcanic eruptions, changes in
solar activity, and resonances within the
hydro-atmosphere, their effect can only
Congressional Research Service

11

Evolving Assessments of Human and Natural Contributions to Climate Change

Year
Title
Key Quotation(s)
Reference
be estimated in terms of direction, not
of amount.”
1974
NSF: Science and
"Human activity may be involved on an
National Science Board, National
the Challenges
even broader scale in changing the
Science Foundation, “Science and the
Ahead
global climate.... The cause of the
Challenges Ahead,” 1974,
cooling trend is not known with
https://archive.org/details/
certainty. But there is increasing
sciencechallenge00nati.
concern that man himself may be
implicated, not only in the recent
cooling trend but also in the warming
temperatures over the last century....
By the middle of this century, the
cooling effect of the dust particles more
than compensated for the warming
effect of the carbon dioxide, and world
temperature began to fall.”
1975
NAS:
“While the natural variations of climate
National Academy of Sciences.
Understanding
have been larger than those that may
Understanding Climate Change: A Program
Climate Change:
have been induced by human activities
for Action (Washington, DC: National
A Program for
during the past century, the rapidity
Academies Press, 1975),
Action
with which human impacts threaten to
https://ia801806.us.archive.org/7/items/
grow in the future, and increasingly to
understandingcli00unit/
disturb the natural course of events, is a understandingcli00unit.pdf.
matter of concern. These impacts
include man’s changes of the
atmospheric composition and his direct
interference with factors controlling the
all important heat balance.... Of the two
forms of pollution [CO2 and aerosols],
the carbon dioxide increase is probably
the more influential at the present time
in changing temperatures near the
earth's surface. If both the CO2 and
particulate inputs to the atmosphere
grow at equal rates in the future, the
widely differing atmospheric residence
times of the two pollutants means that
the particulate effect will grow in
importance relative to that of CO2.
There are other possible impacts of
human activities that should be
considered in projecting future
climates.”
1983
World Climate
“The existing observational evidence
International Council of Scientific
Research
does not allow concluding with
Unions, World Meteorological
Programme:
confidence that the observed variations
Organization, Report of the Fifth Session
Report of the
are caused specifically by the increasing
of the Joint Scientific Committee
Fifth Session of
concentration of atmospheric carbon
(Hangzhou, China, 12-17 March 1984),
the Joint Scientific dioxide. On the other hand, the surface
September 1984, https://library.wmo.int/
Committee
warming indicated by the record of the
pmb_ged/wmo-td_1.pdf.
global mean temperature is not
inconsistent with the computed effect
of the observed increase of atmospheric
CO2 since the beginning of the
industrial revolution.”
Congressional Research Service

12

Evolving Assessments of Human and Natural Contributions to Climate Change

Year
Title
Key Quotation(s)
Reference
1985
Villach
“Based on analyses of observational
World Meteorological Organisation,
Conference
data, the estimated increase in global
Report of the International Conference on
Statement
mean temperature during the last one
the assessment of the role of carbon
hundred years of between 0.3 and
dioxide and of other greenhouse gases in
0.7°C is consistent with the projected
climate variations and associated impacts,
temperature increase attributable to the Villach, Austria, October 9-15, 1985,
observed increase in CO2 and other
WMO No. 661, 1986,
greenhouse gases, although it cannot be
https://web.archive.org/web/
ascribed in a scientifically rigorous
20131121040937/http://
manner to these factors alone.”
www.scopenvironment.org/
downloadpubs/scope29/statement.html.
1987
Mankind's Impact
“There is now a strong consensus that
William W. Kellogg, “Mankind’s Impact
on Climate: The
the observed increase in the
on Climate: The Evolution of an
Evolution of an
atmospheric concentrations of carbon
Awareness,” Climatic Change, vol. 10
Awareness
dioxide and other infrared-absorbing
(1987), pp. 113-36,
trace gases is indeed warming the earth, https://link.springer.com/content/pdf/
and that this change is caused by
10.1007%2FBF00140251.pdf.
mankind.”
Source: Compiled by CRS.

Congressional Research Service

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Evolving Assessments of Human and Natural Contributions to Climate Change

Table A-2. Excerpted Statements on the Human Contribution to Global Climate in
Scientific Assessments of the NAS, the IPCC, and the USGCRP, 1977-2021
Year
Title
Key Quotation(s)
Reference
1977
NAS: Energy and
"In the first half of the twentieth
National Academy of Sciences, Energy
Climate: Studies
century ... a general warming of the
and Climate (Washington, DC: National
in Geophysics
earth occurred.... There is considerable
Academies Press, 1977),
evidence that, between the 1940s and
https://www.nap.edu/download/12024.
about 1970, the climatic changes of the
earlier part of this century had tended
to undergo a reversal. Temperatures
had mostly fallen.”
“Today, we find ourselves uneasy
because through our use of energy we
may be significantly disturbing the
natural climate system. This uneasiness
is justified.”
1979
NAS: Carbon
“None of the model calculations
National Academy of Sciences, “Carbon
Dioxide and
predicts negligible warming. The
Dioxide and Climate: A Scientific
Climate: A
primary effect of an increase of CO2 is
Assessment,” Report of an Ad Hoc
Scientific
to cause more absorption of thermal
Study Group on Carbon Dioxide and
Assessment
radiation from the earth’s surface and
Climate, Woods Hole, Massachusetts,
thus to increase the air temperature in
July 23-27, 1979, to the Climate
the troposphere.”
Research Board, Assembly of
“We conclude that the predictions of
Mathematical and Physical Sciences,
CO
National Research Council,
2-induced climate changes made
with the various models examined are
https://www.nap.edu/download/19856.
basically consistent and mutually
supporting.”
1990
IPCC: Climate
“The size of the warming over the last
J. T. Houghton et al., Climate Change:
Change: The
century is broadly consistent with the
The IPCC Scientific Assessment. Report
IPCC Scientific
predictions of climate models but is also prepared for Intergovernmental Panel
Assessment - 1st
of the same magnitude as natural
on Climate Change by Working Group I
Report
climate variability. If the sole cause of
(Cambridge, U.K.: Cambridge University
the observed warming were the human
Press, 1990), https://www.ipcc.ch/
made greenhouse effect, then the
ipccreports/far/wg_I/
implied climate sensitivity would be
ipcc_far_wg_I_full_report.pdf.
near the lower end of the range
inferred from the models. The observed
increase could be largely due to natural
variability, alternatively this variability
and other man-made factors could have
offset a still larger man-made
greenhouse warming. The unequivocal
detection of the enhanced greenhouse
effect from observations is not likely for
a decade or more, when the
commitment to future climate change
will then be considerably larger than it
is today.”
1994
NAS: Solar
“Taken collectively, the above evidence,
National Academy of Sciences, Solar
Influences on
although circumstantial, does suggest
Influences on Global Change (Washington,
Global Change
that solar variability could influence
DC: National Academies Press, 1994),
future global change.... Lack of
https://www.nap.edu/download/4778.
knowledge of solar influences will limit
Congressional Research Service

14

Evolving Assessments of Human and Natural Contributions to Climate Change

Year
Title
Key Quotation(s)
Reference
the certainty with which anthropogenic
climate change can be detected. But it is
unlikely that solar influences on global
change will be comparable to the
expected anthropogenic influences.”
1995
IPCC Second
“Carbon dioxide remains the most
B. Bolin et al., Climate Change 1995: A
Assessment:
important contributor to anthropogenic report of the Intergovernmental Panel on
Climate Change
forcing of climate change; projections of
Climate Change (Cambridge, U.K.:
1995
future global mean temperature change
Cambridge University Press, 1995),
and sea level rise confirm the potential
http://www.ipcc.ch/ipccreports/sar/
for human activities to alter the Earth's
wg_I/ipcc_sar_wg_I_full_report.pdf.
climate to an extent unprecedented in
human history.... [O]bservations suggest
a ‘discernible human influence on global
climate.’”
“Most of these studies have detected a
significant change and show that the
observed warming trend is unlikely to
be entirely natural in origin.”
2001
USGCRP: 1st
“The observed magnitude, pattern, and
National Assessment Synthesis Team,
National Climate
timing of the global warming indicate
Climate Change Impacts on the United
Assessment
that the rising concentrations of CO2
States: The Potential Consequences of
and other greenhouse gases caused by
Climate Variability and Change, Report for
human activities are contributing
the USGCRP (Cambridge, U.K.:
significantly to the recent warming.”
Cambridge University Press, 2001),
https://data.globalchange.gov/assets/e9/
97/436129058f2107f4925aeec13ed8/
nca-2000-foundation-report.pdf.
2001
NAS: Climate
“The changes observed over the last
National Academy of Sciences, Climate
Change Science:
several decades are likely mostly due to
Change Science: An Analysis of Some Key
An Analysis of
human activities, but we cannot rule out Questions (Washington, DC: National
Some Key
that some significant part of these
Academy Press, 2001),
Questions
changes is also a reflection of natural
https://www.nap.edu/download/10139.
variability."
2001
IPCC: Climate
“There is new and stronger evidence
R. T. Watson et al. (eds.), Climate
Change 2001:
that most of the warming observed
Change 2001: Synthesis Report,
The IPCC Third
over the last 50 years is attributable to
Contribution of Working Groups I, II,
Assessment
human activities.”
and III to the Third Assessment Report
Report
of the Intergovernmental Panel on
Climate Change (Cambridge, U.K.:
Cambridge University Press, 2001),
https://www.ess.uci.edu/researchgrp/
prather/files/2001IPCC_SyR-
Watson.pdf.
2007
IPCC: The Fourth “Most of the observed increase in global R. K. Pachuri et al. (eds.), Climate
Assessment
average temperatures since the mid-20th Change 2007: Synthesis Report,
Report: Climate
century is very likely [>90%] due to the
Contribution of Working Groups I, II,
Change
observed increase in anthropogenic
and III to the Fourth Assessment
GHG concentrations.”
Report of the Intergovernmental Panel
on Climate Change (Geneva,
Switzerland: IPCC Secretariat, 2007),
http://www.slvwd.com/agendas/Full/
2007/06-07-07/Item%2010b.pdf.
Congressional Research Service

15

Evolving Assessments of Human and Natural Contributions to Climate Change

Year
Title
Key Quotation(s)
Reference
2009
USGCRP: 2nd
“The global warming of the past 50
Thomas R. Karl et al. (eds.), Global
National Climate
years is due primarily to human-induced
Climate Change Impacts in the United
Assessment
increases in heat trapping gasses....
States (Cambridge, UK: Cambridge
Recent scientific assessments find that
University Press, 2009),
most of the warming of the Earth’s
https://downloads.globalchange.gov/
surface over the past 50 years has been
usimpacts/pdfs/climate-impacts-
caused by human activities.”
report.pdf.
2010
NAS: Advancing
“Climate change is occurring, is caused
National Academy of Sciences,
the Science of
largely by human activities, and poses
Advancing the Science of Climate Change,
Climate Change
significant risks for—and in many cases
America's Climate Choices: Panel on
is already affecting—a broad range of
Advancing the Science of Climate
human and natural systems.”
Change (Washington, DC: National
Academy Press, 2010),
https://www.nap.edu/download/12782.
2012
NAS: Climate
“The overwhelming majority of climate
National Academy of Sciences, Climate
Change: Evidence, scientists agree that human activities,
Change: Evidence, Impacts, and Choices,
Impacts, and
especially the burning of fossil fuels
2012, http://nas-sites.org/
Choices
(coal, oil, and gas), are responsible for
americasclimatechoices/files/2012/06/
most of the climate change currently
19014_cvtx_R1.pdf
being observed.”
2014
NAS: Climate
“Rigorous analysis of all data and lines
National Academy of Sciences, Climate
Change: Evidence
of evidence shows that most of the
Change: Evidence and Causes, 2014,
and Causes
observed global warming over the past
http://dels.nas.edu/resources/static-
50 years or so cannot be explained by
assets/exec-office-other/climate-change-
natural causes and instead requires a
full.pdf
significant role for the influence of
human activities.”
2014
IPCC: The Fifth
“[Anthropogenic GHG emissions]
R. K. Pachuri et al. (eds.), Climate
Assessment
effects, together with those of other
Change 2014: Synthesis Report,
Report: Climate
anthropogenic drivers, have been
Contribution of Working Groups I, II,
Change 2014
detected throughout the climate system and III to the Fifth Assessment Report
and are extremely likely [>95%] to have
of the Intergovernmental Panel on
been the dominant cause of the
Climate Change (Geneva, Switzerland:
observed warming since the mid-20th
IPCC Secretariat, 2014),
century.”
http://www.ipcc.ch/pdf/assessment-
report/ar5/syr/
SYR_AR5_FINAL_full_wcover.pdf.
2014
USGCRP: 3rd
“The majority of the warming at the
Jerry M. Melillo et al., (eds.), Highlights of
National Climate
global scale over the past 50 years can
Climate Change Impacts in the United
Assessment
only be explained by the effects of
States: The Third National Climate
human influences, especially the
Assessment, USGCRP (Washington, DC:
emissions from burning fossil fuels (coal, U.S. Government Printing Office, 2014),
oil, and natural gas) and from
http://www.globalchange.gov/sites/
deforestation.”
globalchange/files/
NCA3_Highlights_LowRes-small-
FINAL_posting.pdf.
2017
USGCRP:
“It is extremely likely [>95%] that
Donald J. Weubbles et al., Climate
Climate Science
human influence has been the dominant
Science Special Report: Fourth National
Special Report:
cause of the observed warming since
Climate Assessment, Volume I, USGCRP,
Fourth National
the mid-20th century. For the warming
2017,
Climate
over the last century, there is no
https://science2017.globalchange.gov/
Assessment
convincing alternative explanation
downloads/CSSR2017_FullReport.pdf.
Congressional Research Service

16

Evolving Assessments of Human and Natural Contributions to Climate Change

Year
Title
Key Quotation(s)
Reference
supported by the extent of the
observational evidence.”
2021
IPCC: The Sixth
“It is unequivocal that human influence
V. Masson-Delmotte et al. (eds.),
Assessment
has warmed the atmosphere, ocean and
“Summary for Policymakers” in Climate
Report: The
land. Widespread and rapid changes in
Change 2021: The Physical Science Basis.
Physical Science
the atmosphere, ocean, cryosphere and
Contribution of Working Group I to
Basis
biosphere have occurred…. Observed
the Sixth Assessment Report of the
increases in well-mixed greenhouse gas
Intergovernmental Panel on Climate
(GHG) concentrations since around
Change, 2021,
1750 are unequivocally caused by
https://www.ipcc.ch/report/sixth-
human activities.” “It is very likely that
assessment-report-working-group-i/.
well-mixed GHGs were the main driver

of tropospheric warming since 1979,
and extremely likely that human-caused
stratospheric ozone depletion was the
main driver of cooling of the lower
stratosphere between 1979 and the
mid-1990s.” “Human-induced climate
change is already affecting many
weather and climate extremes in every
region across the globe. Evidence of
observed changes in extremes such as
heatwaves, heavy precipitation,
droughts, and tropical cyclones, and, in
particular, their attribution to human
influence, has strengthened since [the
Fifth Assessment Report].”
Source: Compiled by CRS.
Notes: The documents cited reflect the evolution of select aspects of climate change science. This list is not
comprehensive of, and is not intended to highlight, all aspects of the scientific efforts in climate research.

Congressional Research Service

17

Evolving Assessments of Human and Natural Contributions to Climate Change

Table A-3. Select Scientific Statements Regarding the Influences on Global Climate
of Human Activities and Natural Factors, 1827-1987
Year
Title
Key Quotation(s)
Reference

1827
Mémoire sur les
“The establishment and progress of human
Jean-Baptiste Joseph Fournier, “Mémoire Sur
Températures du
societies, and the action of natural forces,
Les Températures Du Globe Terrestre et
Globe Terrestre et
can notably change the state of the ground
Des Espaces Planétaires,” Mémoires de
des Espaces
surface over vast regions, as well as the
l’Académie Royale Des Sciences de l’Institute de
Planétaires [On the
distribution of waters and the great
France, 1827, https://geosci.uchicago.edu/
Temperatures of the
movements of the air. Such effects have the
~rtp1/papers/Fourier1827Trans.pdf.
Terrestrial Sphere and ability to make the mean degree of heat vary
Interplanetary Space]
over the course of several centuries."
1856
Circumstances
“An atmosphere of [carbonic acid] would
Eunice Foote, “Circumstances Affecting the

Affecting the Heat of
give to our earth a high temperature; and if
Heat of the Sun’s Rays,” American Journal of
the Sun’s Rays
as some suppose, at one period of its history Science and Arts, vol. 22 (1856), pp. 382-83,
the air had mixed with it a larger proportion
https://books.google.com/
than at present, an increased temperature
books?id=6xhFAQAAMAAJ&
from its own action as well as from increased pg=PA382#v=onepage&
weight must have necessarily resulted."
q&f=false.
1896
On the Influence of
“If the quantity of carbonic acid increases in
Svante Arrhenius, “On the Influence of

Carbonic Acid in the
geometric progression, the augmentation of
Carbonic Acid in the Air upon the
Air upon the
the temperature will increase nearly in
Temperature of the Ground,” Philosophical
Temperature of the
arithmetic progression.”
Magazine and Journal of Science, vol. 41, series
Ground
“This quantity of carbonic acid, which is
5 (April 1896), pp. 237-76,
supplied to the atmosphere chiefly by
http://www.trunity.net/files/108501_108600/
modern industry, may be regarded as
108531/arrhenius1896_greenhouse-
completely compensating the quantity of
effect.pdf.
carbonic acid that is consumed in the
formation of limestone (or other mineral
carbonates) by the weathering or
decomposition of silicates.”
”We must regard volcanic exhalations as the
chief source of carbonic acid for the
atmosphere.”
1897
A Group of
“A comparison of early with later life,
Thomas C. Chamberlin, “A Group of

Hypotheses Bearing
stripped of theoretical presumptions, does
Hypotheses Bearing on Climatic Changes,”
on Climatic Changes
not seem to me clearly to imply any great
Journal of Geology, vol. 5, no. 7 (1897), pp.
difference in the content of carbon dioxide.” 653-683, https://www.jstor.org/stable/pdf/
“For it is almost axiomatic to say that
30054630.pdf?refreqid=excelsior%3Aa77b10a
climatic changes would attend changes in the 39bbbe564c432a9204d719cec.
constitution of the atmosphere. I assume that
atmospheric poverty, especially in the critical
item of carbon dioxide, is correlated with
low temperature.”
1898
Monthly Weather
“We have therefore an alternation of cold
U.S. Department of Agriculture, Weather

Review and Annual
and warm periods due to the interaction of
Bureau, Monthly Weather Review and Annual
Summary
elevation [of the Earth’s surface] and carbon
Summary, July 1898,
dioxide [in the atmosphere].... [W]e hope
http://congressional.proquest.com/
eventually to be able to deduce the resulting
congressional/result/pqpresultpage. gispdfhits
climatic peculiarities, and to show that very
panel.pdflink/$2fapp-s3$2fjlh-content$2fPT4-
slight changes in oceans and continents have
A2906-8$2fPT4-A2906-8_From_1_
produced all the variations of geological
to_752.pdf/entitlementkeys=1234%7Capp-
climate, and that little or nothing need be
gis%7Cexecutive-branch%7Ca2906-8.
hypothecated as to the variations of solar
heat, of atmospheric gases, of terrestrial
Congressional Research Service

18

Evolving Assessments of Human and Natural Contributions to Climate Change

Year
Title
Key Quotation(s)
Reference

latitudes, or the many other climatological
elements.”
1911
USGS: The Data of
“In a thousand years, then, if the rate were
Frank Wigglesworth Clark, U.S. Geological

Geochemistry
constant and no disturbing factors interfered, Survey, “The Data of Geochemistry”, Bulletin
the amount of CO2 in the atmosphere would 491, 2nd ed., U.S. House of Representatives,
be doubled. If we take into account the
1911, pp. 47, 135, https://congressional.
combustion of fuels other than coal and the
proquest.com/congressional/result/pqpresult
large additions to the atmosphere from the
page.gispdfhitspanel.pdflink/$2fapp-bin$2fgis-
sources previously mentioned the result
serialset$2f0$2fd$2f4$2fa$2f5997_hdoc
becomes still more startling. Were there no
1499_from_1_to_779.pdf/entitlementkeys=
counterbalancing of this increase in
1234%7Capp-gis%7Cserialset%7
atmospheric carbon, animal life would soon
C5997_h.doc.1499.
become impossible upon our planet.”
“Whether the theory of Arrhenius is in
harmony with the facts of historical
geology—that is, whether periods of volcanic
activity [releasing carbon dioxide] have
coincided with warmer climates, and a
slackening of activity with lowering of
temperatures is also in dispute. Apparently,
the controversy is not yet ended.”
“Water vapor, then, is the chief agent in the
atmospheric regulation of climate and to this
conclusion; The climatic conditions may vary
as Chamberlin claims, but the relative
dryness or wetness of the atmosphere may
be the true cause of fluctuating temperatures,
rather than the carbon dioxide.”
1939
The Composition of
“It is a commonplace that man is able to
Guy S. Callendar, “The Composition of the

the Atmosphere
speed up the processes of Nature, and he has Atmosphere Through the Ages,”
Through the Ages
now plunged heavily into her slow-moving
Meterological Magazine, vol. 74, no. 878
carbon cycle by throwing some 9,000 tons of (March 1939), pp. 33-39,
carbon dioxide into the air each minute. This https://www.google.com/url?sa=t&rct=j&q=&
great stream of gas results from the
esrc=s&source=web&cd=1&ved=
combustion of fossil carbon (coal, oil, peat,
0ahUKEwjBjtHfoP3WAhXB4SYK
etc.), and it appears to be much greater than HYGFA9UQFggmMAA&url=https%3A
the natural rate of fixation.”
%2F%2Fdigital.nmla. metoffice.gov.uk
“From the best laboratory observations it
%2Fdownload%2Ffile%2Fsdb%253AdigitalFile
appears that the principal result of increasing %257Cedc8aafc-a27a-4afe-bd29-619b052
atmospheric carbon dioxide, apart from a
bd199%2F&usg=AOvVaw0R8QW-XaTql-
slight speeding up of rock weathering and
iGP-cJjF11https://www.google.com/url?sa=t&
plant growth, would be a gradual increase in
rct=j&q=&esrc=s&source=web&cd=1&ved=
the mean temperature of the colder regions
0ahUKEwjBjtHfoP3WAhXB4SYKHYGFA
of the earth.”
9UQFggmMAA&url=https%3A%2F%2
Fdigital.nmla. metoffice.gov.uk%2Fdownload
%2Ffile%2Fsdb%253AdigitalFile%257Cedc
8aafc-a27a-4afe-bd29-619b052bd199%2F&
usg=AOvVaw0R8QW-XaTql-iGP-cJjF11
1948
Circular Causal
“Callendar (1940), considering the best data
George Evelyn Hutchinson, “Circular Causal

Systems in Ecology
available since 1866, concludes that during
Systems in Ecology,” Annals of the New York
the present century there has been an
Academy of Sciences, vol. 50, (1948), pp. 221-
increase of the order of 10 per cent in the
246, http://people. wku.edu/charles.smith/
CO2 content of the atmosphere. This he
biogeog/HUTC1948.htm.
attributes to the modern industrial
combustion of fuel.... It seems far more likely
that the observed increment in the carbon
Congressional Research Service

19

Evolving Assessments of Human and Natural Contributions to Climate Change

Year
Title
Key Quotation(s)
Reference

dioxide of air at low levels in both Europe
and eastern North America is due to changes
in the biological mechanisms of the cycle
rather than to an increase in industrial
output.... The self-regulating mechanisms of
the carbon cycle can cope with the present
influx of carbon of fossil origin."
1966
U.S. House of
"Carbon dioxide accumulations from the
Rep. George P. Miller, “Environmental

Representatives:
burning of gas, petroleum, and coal change
Pollution: A Challenge to Science and
Environmental
the nature of the atmosphere. Weather
Technology,” Report of the Subcommittee on
Pollution: A Challenge patterns can be altered purposefully or
Science, Research, and Development. Committee
to Science and
accidentally by human activity. These
on Science and Astronautics, U.S. House of
Technology
powerful forces have only come about
Representatives, 1966, pp. 3, 43, 263,
recently and are not well understood. As a
https://congressional.proquest.com/
consequence, in many risk-benefit questions,
congressional/result/pqpresultpage.
the magnitude of the risk is relatively
gispdfhitspanel. pdflink/$2fapp-bin$2fgis-
unknown."
congresearch$2f6$2f7$2f2$2f9$2fcmp-1966-
“By the year 2000 at present and projected
sah-0009_from_1_to_67.pdf/
rate of fossil fuel consumption, the carbon
entitlementkeys=1234%7Capp-gis%7
dioxide level in the air will cause a significant
Ccongresearch%7Ccmp-1966-sah-0009.
rise in the air temperature.”
1970
Man-Made Climatic
“It appears that on the local scale man-made
Helmut E. Landsberg, “Man-Made Climatic

Changes
influences on climate are substantial but that
Changes,” Science, vol. 170, no. 3964
on the global scale natural forces still
(December 18, 1970), http://www.jstor.org/
prevail.... The potential for anthropogenic
stable/pdf/1730486.pdf?refreqid=excelsior
changes of climate on a larger and even a
%3Ae79d4c4c72387997406c413dafdebbaf.
global scale is real.... In my opinion, man-
made aerosols, because of their optical
properties and possible influences on cloud
and precipitation processes constitute a
more acute problem than CO2.... Over
longer intervals, energy added to the
atmosphere by heat rejection and CO2
absorption remain matters of concern.”
1972
NSF: Patterns and
"Judging from the record of the past
National Science Board, National Science

Perspectives in
interglacial ages, the present time of high
Foundation, “Patterns and Perspectives in
Environmental Science temperatures should be drawing to an end,
Environmental Science,” 1972,
to be followed by a long period of
https://archive.org/details/patternsperspect
considerably colder temperatures leading
00nati.
into the next glacial age some 20,000 years
from now. However, it is possible, or even
likely, that human interference has already
altered the environment so much that the
climatic pattern of the near future will follow
a different path. For instance, widespread
deforestation in recent centuries, especially
in Europe and North America, together with
increased atmospheric opacity due to man-
made dust storms and industrial wastes,
should have increased the earth’s reflectivity.
At the same time, increasing concentration of
industrial carbon dioxide in the atmosphere
should lead to a temperature increase by
absorption of infrared radiation from the
earth's surface. When these human factors
are added to such other natural factors as
Congressional Research Service

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Evolving Assessments of Human and Natural Contributions to Climate Change

Year
Title
Key Quotation(s)
Reference

volcanic eruptions, changes in solar activity,
and resonances within the hydro-
atmosphere, their effect can only be
estimated in terms of direction, not of
amount.”
1974
NSF: Science and the
"Human activity may be involved on an even
National Science Board, National Science

Challenges Ahead
broader scale in changing the global climate.... Foundation, “Science and the Challenges
The cause of the cooling trend is not known
Ahead,” 1974, https://archive.org/details/
with certainty. But there is increasing
sciencechallenge00nati.
concern that man himself may be implicated,
not only in the recent cooling trend but also
in the warming temperatures over the last
century.... By the middle of this century, the
cooling effect of the dust particles more than
compensated for the warming effect of the
carbon dioxide, and world temperature
began to fall.”
1975
NAS: Understanding
“While the natural variations of climate have National Academy of Sciences. Understanding
Climate Change: A
been larger than those that may have been
Climate Change: A Program for Action
Program for Action
induced by human activities during the past
(Washington, DC: National Academies Press,
century, the rapidity with which human
1975), https://ia801806.us.archive.org/7/
impacts threaten to grow in the future, and
items/understandingcli00unit/understanding
increasingly to disturb the natural course of
cli00unit.pdf.
events, is a matter of concern. These impacts
include man’s changes of the atmospheric
composition and his direct interference with
factors controlling the all important heat
balance.... Of the two forms of pollution
[CO2 and aerosols], the carbon dioxide
increase is probably the more influential at
the present time in changing temperatures
near the earth's surface. If both the CO2 and
particulate inputs to the atmosphere grow at
equal rates in the future, the widely differing
atmospheric residence times of the two
pollutants means that the particulate effect
will grow in importance relative to that of
CO2. There are other possible impacts of
human activities that should be considered in
projecting future climates.”
1983
World Climate
“The existing observational evidence does
International Council of Scientific Unions,

Research Programme: not allow concluding with confidence that the World Meteorological Organization, Report
Report of the Fifth
observed variations are caused specifically by of the Fifth Session of the Joint Scientific
Session of the Joint
the increasing concentration of atmospheric
Committee (Hangzhou, China, 12-17 March
Scientific Committee
carbon dioxide. On the other hand, the
1984), September 1984, https://library.
surface warming indicated by the record of
wmo.int/pmb_ged/wmo-td_1.pdf.
the global mean temperature is not
inconsistent with the computed effect of the
observed increase of atmospheric CO2 since
the beginning of the industrial revolution.”
1985
Villach Conference
“Based on analyses of observational data, the World Meteorological Organisation, Report of
Statement
estimated increase in global mean
the International Conference on the assessment
temperature during the last one hundred
of the role of carbon dioxide and of other
years of between 0.3 and 0.7°C is consistent
greenhouse gases in climate variations and
with the projected temperature increase
associated impacts, Villach, Austria, October
attributable to the observed increase in CO2 9-15, 1985, WMO No. 661, 1986,
Congressional Research Service

21

Evolving Assessments of Human and Natural Contributions to Climate Change

Year
Title
Key Quotation(s)
Reference

and other greenhouse gases, although it
https://web.archive.org/web/
cannot be ascribed in a scientifically rigorous 20131121040937/http://www.scop
manner to these factors alone.”
environment.org/downloadpubs/scope29/
statement.html.
1987
Mankind's Impact on
“There is now a strong consensus that the
William W. Kellogg, “Mankind’s Impact on

Climate: The
observed increase in the atmospheric
Climate: The Evolution of an Awareness,”
Evolution of an
concentrations of carbon dioxide and other
Climatic Change, vol. 10 (1987), pp. 113-36,
Awareness
infrared-absorbing trace gases is indeed
https://link.springer.com/content/pdf/
warming the earth, and that this change is
10.1007%2FBF00140251.pdf.
caused by mankind.”
Source: Compiled by CRS.

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Evolving Assessments of Human and Natural Contributions to Climate Change

Table A-4. Excerpted Statements on the Human Contribution to Global Climate in
Scientific Assessments of the NAS, the IPCC, and the USGCRP, 1977-2017
Year
Title
Key Quotation(s)
Reference
1977
NAS: Energy and
"In the first half of the twentieth century National Academy of Sciences, Energy
Climate: Studies in
... a general warming of the earth
and Climate (Washington, DC: National
Geophysics
occurred.... There is considerable
Academies Press, 1977),
evidence that, between the 1940s and
https://www.nap.edu/download/12024.
about 1970, the climatic changes of the
earlier part of this century had tended to
undergo a reversal. Temperatures had
mostly fallen.”
“Today, we find ourselves uneasy
because through our use of energy we
may be significantly disturbing the
natural climate system. This uneasiness is
justified.”
1979
NAS: Carbon
“None of the model calculations
National Academy of Sciences, “Carbon
Dioxide and
predicts negligible warming. The primary Dioxide and Climate: A Scientific
Climate: A Scientific effect of an increase of CO2 is to cause
Assessment,” Report of an Ad Hoc
Assessment
more absorption of thermal radiation
Study Group on Carbon Dioxide and
from the earth’s surface and thus to
Climate, Woods Hole, Massachusetts,
increase the air temperature in the
July 23-27, 1979, to the Climate
troposphere.”
Research Board, Assembly of
“We conclude that the predictions of
Mathematical and Physical Sciences,
CO
National Research Council,
2-induced climate changes made with
the various models examined are
https://www.nap.edu/download/19856.
basically consistent and mutually
supporting.”
1990
IPCC: Climate
“The size of the warming over the last
J. T. Houghton et al., Climate Change: The
Change: The IPCC
century is broadly consistent with the
IPCC Scientific Assessment. Report
Scientific
predictions of climate models but is also prepared for Intergovernmental Panel on
Assessment - 1st
of the same magnitude as natural climate Climate Change by Working Group I
Report
variability. If the sole cause of the
(Cambridge, U.K.: Cambridge University
observed warming were the human
Press, 1990), https://www.ipcc.ch/
made greenhouse effect, then the
ipccreports/far/wg_I/
implied climate sensitivity would be near ipcc_far_wg_I_full_report.pdf.
the lower end of the range inferred from
the models. The observed increase
could be largely due to natural
variability, alternatively this variability
and other man-made factors could have
offset a still larger man-made
greenhouse warming. The unequivocal
detection of the enhanced greenhouse
effect from observations is not likely for
a decade or more, when the
commitment to future climate change
will then be considerably larger than it is
today.”
1994
NAS: Solar
“Taken collectively, the above evidence,
National Academy of Sciences, Solar
Influences on
although circumstantial, does suggest
Influences on Global Change (Washington,
Global Change
that solar variability could influence
DC: National Academies Press, 1994),
future global change.... Lack of
https://www.nap.edu/download/4778.
knowledge of solar influences will limit
the certainty with which anthropogenic
climate change can be detected. But it is
Congressional Research Service

23

Evolving Assessments of Human and Natural Contributions to Climate Change

Year
Title
Key Quotation(s)
Reference
unlikely that solar influences on global
change will be comparable to the
expected anthropogenic influences.”
1995
IPCC Second
“Carbon dioxide remains the most
B. Bolin et al., Climate Change 1995: A
Assessment:
important contributor to anthropogenic report of the Intergovernmental Panel on
Climate Change
forcing of climate change; projections of Climate Change (Cambridge, U.K.:
1995
future global mean temperature change
Cambridge University Press, 1995),
and sea level rise confirm the potential
http://www.ipcc.ch/ipccreports/sar/wg_I/
for human activities to alter the Earth's
ipcc_sar_wg_I_full_report.pdf.
climate to an extent unprecedented in
human history.... [O]bservations suggest
a ‘discernible human influence on global
climate.’”
“Most of these studies have detected a
significant change and show that the
observed warming trend is unlikely to be
entirely natural in origin.”
2001
USGCRP: 1st
“The observed magnitude, pattern, and
National Assessment Synthesis Team,
National Climate
timing of the global warming indicate
Climate Change Impacts on the United
Assessment
that the rising concentrations of CO2
States: The Potential Consequences of
and other greenhouse gases caused by
Climate Variability and Change, Report for
human activities are contributing
the USGCRP (Cambridge, U.K.:
significantly to the recent warming.”
Cambridge University Press, 2001),
https://data.globalchange.gov/assets/e9/
97/436129058f2107f4925aeec13ed8/nca-
2000-foundation-report.pdf.
2001
NAS: Climate
“The changes observed over the last
National Academy of Sciences, Climate
Change Science: An several decades are likely mostly due to
Change Science: An Analysis of Some Key
Analysis of Some
human activities, but we cannot rule out Questions (Washington, DC: National
Key Questions
that some significant part of these
Academy Press, 2001),
changes is also a reflection of natural
https://www.nap.edu/download/10139.
variability."
2001
IPCC: Climate
“There is new and stronger evidence
R. T. Watson et al. (eds.), Climate Change
Change 2001: The
that most of the warming observed over 2001: Synthesis Report, Contribution of
IPCC Third
the last 50 years is attributable to human Working Groups I, II, and III to the
Assessment Report activities.”
Third Assessment Report of the
Intergovernmental Panel on Climate
Change (Cambridge, U.K.: Cambridge
University Press, 2001),
https://www.ess.uci.edu/researchgrp/
prather/files/2001IPCC_SyR-Watson.pdf.
2007
IPCC: The Fourth
“Most of the observed increase in global R. K. Pachuri et al. (eds.), Climate Change
Assessment Report: average temperatures since the mid-20th 2007: Synthesis Report, Contribution of
Climate Change
century is very likely [>90%] due to the
Working Groups I, II, and III to the
observed increase in anthropogenic
Fourth Assessment Report of the
GHG concentrations.”
Intergovernmental Panel on Climate
Change (Geneva, Switzerland: IPCC
Secretariat, 2007),
http://www.slvwd.com/agendas/Full/
2007/06-07-07/Item%2010b.pdf.
2009
USGCRP: 2nd
“The global warming of the past 50 years Thomas R. Karl et al. (eds.), Global
National Climate
is due primarily to human-induced
Climate Change Impacts in the United
Assessment
increases in heat trapping gasses....
States (Cambridge, UK: Cambridge
Recent scientific assessments find that
University Press, 2009),
most of the warming of the Earth’s
https://downloads.globalchange.gov/
Congressional Research Service

24

Evolving Assessments of Human and Natural Contributions to Climate Change

Year
Title
Key Quotation(s)
Reference
surface over the past 50 years has been
usimpacts/pdfs/climate-impacts-
caused by human activities.”
report.pdf.
2010
NAS: Advancing the “Climate change is occurring, is caused
National Academy of Sciences, Advancing
Science of Climate
largely by human activities, and poses
the Science of Climate Change, America's
Change
significant risks for—and in many cases is Climate Choices: Panel on Advancing the
already affecting—a broad range of
Science of Climate Change (Washington,
human and natural systems.”
DC: National Academy Press, 2010),
https://www.nap.edu/download/12782.
2012
NAS: Climate
“The overwhelming majority of climate
National Academy of Sciences, Climate
Change: Evidence,
scientists agree that human activities,
Change: Evidence, Impacts, and Choices,
Impacts, and
especially the burning of fossil fuels (coal, 2012, http://nas-sites.org/americasclimate
Choices
oil, and gas), are responsible for most of choices/files/2012/06/19014_cvtx_R1.pdf
the climate change currently being
observed.”
2014
NAS: Climate
“Rigorous analysis of all data and lines of National Academy of Sciences, Climate
Change: Evidence
evidence shows that most of the
Change: Evidence and Causes, 2014,
and Causes
observed global warming over the past
http://dels.nas.edu/resources/static-
50 years or so cannot be explained by
assets/exec-office-other/climate-change-
natural causes and instead requires a
full.pdf
significant role for the influence of
human activities.”
2014
IPCC: The Fifth
“[Anthropogenic GHG emissions]
R. K. Pachuri et al. (eds.), Climate Change
Assessment Report: effects, together with those of other
2014: Synthesis Report, Contribution of
Climate Change
anthropogenic drivers, have been
Working Groups I, II, and III to the Fifth
2014
detected throughout the climate system Assessment Report of the
and are extremely likely [>95%] to have Intergovernmental Panel on Climate
been the dominant cause of the
Change (Geneva, Switzerland: IPCC
observed warming since the mid-20th
Secretariat, 2014), http://www.ipcc.ch/
century.”
pdf/assessment-report/ar5/syr/
SYR_AR5_FINAL_full_wcover.pdf.
2014
USGCRP: 3rd
“The majority of the warming at the
Jerry M. Melillo et al., (eds.), Highlights of
National Climate
global scale over the past 50 years can
Climate Change Impacts in the United
Assessment
only be explained by the effects of
States: The Third National Climate
human influences, especially the
Assessment, USGCRP (Washington, DC:
emissions from burning fossil fuels (coal, U.S. Government Printing Office, 2014),
oil, and natural gas) and from
http://www.globalchange.gov/sites/
deforestation.”
globalchange/files/NCA3_Highlights_
LowRes-small-FINAL_posting.pdf.
2017
USGCRP: Climate
“It is extremely likely [>95%] that human Donald J. Weubbles et al., Climate
Science Special
influence has been the dominant cause
Science Special Report: Fourth National
Report: Fourth
of the observed warming since the mid-
Climate Assessment, Volume I, USGCRP,
National Climate
20th century. For the warming over the
2017,
Assessment
last century, there is no convincing
https://science2017.globalchange.gov/
alternative explanation supported by the downloads/CSSR2017_FullReport.pdf.
extent of the observational evidence.”
Source: Compiled by CRS.
Notes: The documents cited reflect the evolution of select aspects of climate change science. This list is not
comprehensive of, and is not intended to highlight, all aspects of the scientific efforts in climate research.

Congressional Research Service

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Evolving Assessments of Human and Natural Contributions to Climate Change

Author Information

Jane A. Leggett

Specialist in Energy and Environmental Policy

Acknowledgments
CRS acknowledges and appreciates Dr. Keith Jacobs for his original authorship of this report. Dr. Jacobs
was a Presidential Management Fellow in the Resources, Science, and Industry Division of CRS from June
to November 2018.

Disclaimer
This document was prepared by the Congressional Research Service (CRS). CRS serves as nonpartisan
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