Order Code RS22970
October 17, 2008
Are Carbon Dioxide Emissions Rising More
Rapidly Than Expected?
Jane A. Leggett
Specialist in Environmental and Energy Policy
Resources, Science and Industry Division
Specialist in Energy Policy
Resources, Science and Industry Division
At least one recent report and numerous news articles suggest that carbon dioxide
(CO2) emissions are rising more rapidly than expected. This contention is often made
by comparing recent emissions estimates with the greenhouse gas (GHG) scenarios
published by the Intergovernmental Panel on Climate Change (IPCC) in 2000. While
CO2 emissions associated with human activities continue to rise — and may be worthy
of alarm because of their influence on climate change — any short-term comparisons
between actual emissions and IPCC scenarios miss the mark. First, the IPCC scenarios
explicitly are not predictions. Second, the IPCC scenarios are meant to represent
different possible GHG trajectories over many decades, and represent smooth emissions
paths averaged over at least 10 years. Just as the actual weather over a few years is not
necessarily representative of long-term climate, variability of emissions over one or
several years is not necessarily representative of long-term trends. Nonetheless,
monitoring of CO2 emissions and concentrations, and analysis of the factors driving
changes, is important to designing and evaluating policies to address climate change.
Rising Carbon Dioxide Emissions
At least one recent report and numerous news articles suggest that emissions of
carbon dioxide (CO2) — the leading greenhouse gas (GHG)1 — are rising more rapidly
For explanation of greenhouse gases and their influence on the Earth’s climate, see CRS Report
RL33849, Climate Change: Science and Policy Implications, by Jane A. Leggett.
than expected.2 Global CO2 emissions associated with fossil fuel combustion in 2007
were approximately 8.1 billion metric tons of carbon, compared to estimates of 6.5 billion
tons in 2000.3 Three sources of global emissions estimates are provided in Figure 1. The
estimates are all similar, but have some degree of uncertainty due to inconsistencies in the
quality of energy data provided and estimations made by the compilers.
Figure 1. Global Carbon Emissions from Fossil Fuel Combustion
Billions of Tons of Carbon
So urce: CDIA C, Carbo n Dio xide Info rmamatio n A nalysis Center [http://cdiac.o rnl.gov/]; EIA , Energy
Informatio n A dministratio n [http://www.eia.do e.go v/emeu/internatio nal/carbo ndio xide.html]; IEA ,
Internatio nal Energy A gency, CO2 Emissio ns fro m Fo ssil Fuel Combustio n 2007 .
There has been a notable increase in the rate of growth of CO2 emissions this decade.
Data from the Energy Information Administration (EIA) indicate that global CO2
emissions rose at an annual rate of 1.5% during the 1980s, 0.9% during the 1990s,4 and
3.2% during the first five years of this decade. Emissions increases in 2003 and 2004
J. Eilperin, “Carbon Is Building Up in Atmosphere Faster Than Predicted.” The Washington
Post, September 26, 2008. [http://www.washingtonpost.com/wp-dyn/content/article/2008/09/25/
AR2008092503989.html] This article was based on a report entitled “Carbon Budget 2007,”
issued by the Global Carbon Project on September 26, 2008. [http://www.globalcarbon
Carbon dioxide emissions presented in figures are measured in metric tons of carbon. Carbon
measurements can be converted to carbon dioxide units after multiplying by 3.67, the ratio of the
molecular weights of carbon dioxide to carbon.
The GHG emissions of the 1990s were strongly influenced by the collapse of the former Soviet
Union and Eastern European economies, as well as by restructuring of the energy systems in the
were particularly pronounced. As noted later, increased coal consumption, especially in
developing countries, is one of the main drivers behind the recent surge in emissions.5
When CO2 emissions accumulate in the atmosphere faster than the rate at which they
are removed from the atmosphere by “sinks,” atmospheric concentrations rise. Scientists
have concluded that the increased concentration of greenhouse gases (GHG) over the past
century, due to human activities, is likely responsible for most of the global warming
observed since the 1970s, and could lead to climate changes over the next century that
would be unprecedented in the course of human civilization. The potential impacts of
such climate change, including the possibility of non-linear or abrupt effects, cause alarm
among many scientists and citizens. Reports that CO2 emissions may be rising more
rapidly than expected often conclude, therefore, that the climate system may be changing
sooner than expected.
Comparing CO2 Emissions with Established Scenarios
Carbon dioxide emissions have been rising, almost continuously, since the beginning
of the Industrial Revolution at least 120 years ago. One press report contended that “[t]he
rise in global carbon dioxide emissions last year outpaced international researchers’ most
dire projections”6 — a statement drawn by comparing annual emissions for the year 2007
(or even a multi-year average from 2000 to 2007) with the GHG scenarios published by
the Intergovernmental Panel on Climate Change (IPCC) in 2000.7 While CO2 emissions
associated with human activities continue to rise — and may be worthy of alarm because
of their influence on climate change — a comparison of one or a few years’ emissions
with the IPCC emissions scenarios misses the mark.
First, the IPCC scenarios are not predictions. The IPCC’s Special Report on
Emission Scenarios8 (SRES), which produced the CO2 emissions estimates cited above,
stated “[s]cenarios are alternative images of how the future might unfold and are an
appropriate tool with which to analyze how driving forces may influence future emission
outcomes and to assess the associated uncertainties.... The possibility that any single
emissions path will occur as described in the scenarios is highly uncertain.” While
observed increases in CO2 emissions merit attention and concern, comparing annual or
short-term multi-year emissions with the IPCC GHG scenarios reflects misunderstanding
or mischaracterization of the IPCC scenarios (and the models that produce them).
Second, the IPCC exercises are intended to produce scenarios (not predictions) over
multiple decades and to provide only long-term averages. The “time steps” of model
outputs from the six models that produced the “illustrative scenarios” typically is 5 to 10
International Energy Agency, “CO2 Emissions from Fuel Combustion,” 2007. p. xix.
J. Eilperin, op. cit.
The International Panel on Climate Change is a panel established by national governments that
assesses scientific research and publishes consensus findings for use by national policy-makers
and the public. The IPCC published Special Report on Emissions Scenarios in 2000. For more
information, see [http://www.ipcc.ch/].
Nakicenovic, Nebojsa, et al. 2000. Special Report on Emissions Scenarios. Published for the
Intergovernmental Panel on Climate Change by Cambridge University Press.
years, represented by the reporting of outcomes for 2000, 2010, 2020, etc. Even with
these long periods between model outputs, the scenarios are meant to provide conceptual
results over multi-decadal periods, not predictions for a year or a particular decade. As
the SRES report states, “[t]he modeling tools that have been used to develop these
scenarios that focus on the century time scale are less suitable for analysis of near term
(a decade or less) developments.”9 In addition, the assumptions and parameters used in
modeling will change over time, creating value in updating the scenarios periodically.
Normal inter-annual and inter-decadal variability of actual energy use and resulting
CO2 emissions would be expected to produce yearly emission values that fall both above
and below multi-year averages. For example, although recent economic growth in China
and India may have been higher than projected in the first part of this decade, those rates
may drop below the projected level (perhaps substantially) due to the current and expected
economic adjustments. Figure 2 provides estimates of recent annual changes in energyrelated global CO2 emissions. The average annual rate of increase between 1981 and
2005 was about 1.8%, but clear cyclical patterns are evident. In 1981, global emissions
declined by 1.4%. In 2004, they rose by 5.5%.
Percent Change on Previous Year (%)
Figure 2. Estimated Annual Change in Energy-Related Global CO2
So urce: EIA data fro m "Internatio nal Emissio ns Data," [http://www.eia.do e.go v/enviro nment.html]. B P data fro m B P
Statistical Review o f Wo rld Energy 2008, with co nversio ns to carbo n dio xide using the fo llo wing illustrative emissio ns facto rs:
co al (24.5 kg-C/GJ), o il (20 kg-C/GJ), natural gas (13.8 kg-C/GJ).
Economies routinely experience cycles of faster and slower growth; the GHG
emissions associated with these cycles normally fluctuates as well (if policies controlling
emissions remain constant). China, whose emissions have contributed substantially to
recent global CO2 increases, is also likely to experience periodic increases and slowdowns
in emissions. After a recent period of very strong growth, Chinese officials now expect
a period of slower emissions growth over the next few years.10
Ibid., p. 11.
Y. Wang, 2008. Transcript. Executive Vice President, China’s Academy of Macroeconomic
Research. Presented at Brookings Institution, September 18. [http://www.brookings.edu/events/
Comparisons with the IPCC Scenarios. Returning to the question of whether
recent carbon dioxide emission trends are higher than IPCC forecasts, Figure 3 shows
recent CO2 emissions (including from cement production) compared to IPCC scenarios.
The figure displays the six official IPCC “illustrative scenarios,” represented by solid
lines. It also displays six “averaged” IPCC scenarios represented by dashed lines that
were introduced by the Global Carbon Project (GCP), whose “Carbon Budget 2007”
report was cited in the media.11 The dashed lines are the numerical averages of all model
results for each of the IPCC scenario storylines.12 The IPCC did not calculate or publish
such averaged results; rather, the IPCC illustrative scenarios were chosen as best
representing each of the scenario storylines.13 The SRES illustrative scenarios
intentionally did not use all the models’ results because of certain models’ limitations and
Figure 3. Comparison of Actual Carbon Emissions
with SRES Illustrative and Average* Scenarios
Billions of Tons of Carbon
Note: * and dashed lines represent averaged scenarios used by the Global Carbon Project. Solid lines are IPCC illustrative scenarios.
Source: Observed emissions from Carbon Dioxide Information and Analysis Center, "Latest Published Global Estimates," and "2006-2007
Global and National Estimates by Extrapolation," [http://cdiac.ornl.gov/trends/emis/meth_reg.html]. SRES scenario data from
J. Eilperin, op. cit.
More information on the A1, A2, B1, and B2 scenarios is available in the IPCC’s Special
Report on Emissions Scenarios at [http://www.ipcc.ch/ipccreports/sres/emission/index.htm]. The
IPCC report notes 40 individual simulations in the A1, A2, B1, and B2 family of scenarios: 17
model runs used in the A1 scenario, six in the A2 scenario, nine in B1, and eight in B2.
However, some of the six models used to analyze the storylines were better able than others
to represent certain storylines; some models could not produce results consistent with certain
storyline assumptions. This is one reason that all the model results for a scenario were not
averaged, but the “most representative” was selected for publication.
The GCP averaged all models’ results for each scenario rather than using the IPCC’s
“most representative” model results when saying that “[t]his makes current trends in
emissions higher than the worst case IPCC-SRES scenario.”14 The GCP statement is true
only when comparing emissions with its averaged model results, not those published by
the IPCC. Recent emissions are below the A1B illustrative scenario and the top of the
range IPCC found in published research literature.
Moreover, comparing recent emissions with the IPCC scenarios is not, in itself,
adequate to understand future global GHG trajectories. Although the SRES A1B scenario
has the highest GHG emissions in the near term of all the IPCC illustrative scenarios, its
assumptions result in emissions beyond 2050 that are far below some other scenarios (like
A1F, which has lower near-term emissions). Understanding the factors driving emissions
in the short- and long-run is more revealing than comparing current emissions with
Monitoring Emissions Trends. Watchfulness over annual CO2 and other GHG
emissions is a vital element of addressing climate change and evaluating the effectiveness
of actions to mitigate it, and it is important to use appropriate data in monitoring progress.
Evaluating recent trends may help to discern which future GHG emission paths are most
likely and why.15 For example, recent population growth has been lower than many of the
SRES scenarios, while the use of some low-emission technologies (e.g. low emissions
vehicles, wind power, etc.) has been faster than in many of the scenarios. The combined
effects of such differences, however, require complex analysis.
Both the Environmental Protection Agency (EPA) and the Energy Information
Administration (EIA), in cooperation with additional federal agencies, track CO2 and
other GHG emissions and evaluate the factors that influence inter-annual variability and
longer-term trends. Several international entities provide similar services, including the
International Energy Agency, the World Bank, and BP (formerly British Petroleum). The
EPA and EIA also analyze how changes in the factors driving GHG emissions may affect
understanding of future climate trajectories. Some analysts believe, however, that the
EPA and EIA do not have adequate funding to analyze sufficiently how policies may
influence emissions and climate trajectories.
In sum, the past few years of exceptionally high growth may not provide a good basis
for setting expectations for emissions rates in the coming few years or in the long run.
Other changes, including the rate of growth in the global economy, emission control
policies and regulations, and the specific technologies drawing investment, may provide
greater insights into future GHG trajectories. Monitoring these factors, and how they may
be determining future emissions and climate paths, remains important as input to policymaking to address climate change.
For more discussion of factors that drive long-term GHG trends, see CRS Report RL33970,
Greenhouse Gas Emission Drivers: Population, Economic Development and Growth, and Energy
Use, by John Blodgett and Larry Parker.