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March 6, 2020
Weather and Climate Change: What’s the Difference?
States, in the oceans, from international sources, and from
Weather is day-to-day temperature, precipitation, and
other federal agencies. NOAA analyzes weather data
other conditions.
historically and provides multiday forecasts or seasonal
outlooks.
Climate is weather observed over multidecadal
periods. For example, the National Oceanic and
Climate is defined by the long-term statistics of weather
Atmospheric Administration (NOAA) measures a
compiled for a specified place (Figure 1), including the
climate normal over 30 years.
globe. For example, climate would include the maximum
hourly precipitation in April in a particular county averaged
Climate variability is how weather may vary for seasons
over 30 years or the global mean surface temperature from
or years from the long-term average.
1981 to 2010. The availability of long-term data varies by
Climate change occurs when the long-term climate
location, making climate analysis for some locations
shifts, without apparent return to previous normals.
challenging.
With much discussion of climate change in Congress, in the
NOAA’s National Centers for Environmental Information
news, and among constituents, some commonly used words
defines U.S. climate normals as “three-decade averages of
may be misunderstood or misused. Below are explanations
climatological variables including temperature and
of several key terms: weather, climate, climate variability,
and climate change, as well as some associated concepts.
precipitation” for defined locations. The most recent
climate normal for a defined location uses weather
observations for 1981-2010. The next will cover 1991-
Weather is the state of the atmosphere at a specific place
2020. People use climate normals for a wide array of
and time, described by such conditions as temperature,
purposes, such as placing daily weather into a historical
precipitation, humidity, cloudiness, and wind speed.
context or determining technical specifications for the
Weather can include conditions of the Earth’s water and
resilience of buildings and infrastructure against extreme
land surfaces as well, with such variables as wave height
weather.
and snow cover. Weather conditions can vary rapidly or last
up to a few days in the mid-latitudes. NOAA compiles
observations from weather stations around the United
Figure 1. Illustration of How Weather Makes Up the Long-term Climate, Using Temperature
Daily Maximum Temperature in Topeka, Kansas, 1981-2010, the Most Recent Climate Normal
Source: CRS using data from NOAA, National Climate Data Center, Climate Data Online. Extracted February 26, 2020.
Notes: The highest temperature on any given day and place is its daily maximum temperature, which is one aspect of weather. In this figure,
each dot represents the daily maximum temperature in Topeka on one day of the most recent 30-year climate normal from 1981 to 2010.
Overal , the statistics of this figure show aspects of Topeka’s recent climate. The darkest line, in the center, is the normal daily maximum
temperature in NOAA’s climate normal. The smooth lines above and below it represent one (statistical) standard deviation from the average
(about 18 percentage points above and below). The very light two lines, above and below, represent the fifth and 95th percentiles, computed by
CRS to show extreme daily maximum temperatures. (CRS used rol ing three-day averages of these values to smooth the lines and better show
instances of extreme weather—those dots above the 95th percentile or below the fifth. About 90% of the time, the daily maximum temperature
could be expected to fal between the fifth and 95th percentile lines. CRS selected Topeka because it is roughly in the center of the continental
United States.
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Weather and Climate Change: What’s the Difference?
Climate variability can include several different concepts.
such as solar variability or human-related greenhouse gas
Generally, it means the differences from the long-term
emissions, are external forcings. Each factor has its own
averages (or other statistics) of climate-related variables,
rhythm and pattern, temporally ranging from less than a
such as precipitation, over longer time or wider areas than
year to tens of thousands of years. Geographically, some
individual weather events. Climate variability may describe
forcings have stronger regional or hemispheric effect, while
how a season is above or below the climate normal average
others are global. The temporal and geographical
for, say, temperature or snowfall. Climate variability may
characteristics of each is like a fingerprint on the climate
also describe longer, multiyear patterns that are not average,
system. Using such “fingerprints,” with sufficient
such as when certain large scale weather patterns occur. An
observations and powerful computing systems, the 2017
example is the so-called El Niño/La Niña (ENSO) cycle,
U.S. National Climate Assessment (NCA4) concluded:
with cycles of warm and cool periods in the Pacific Ocean
It is extremely likely [>95%] that human influence
and the atmosphere. ENSO phases typically last from nine
to 12 months and on average occur every two to seven
has been the dominant cause of the observed
years. With only climate variability, scientists would expect
warming since the mid-20th century. For the
the climate to reflect historically average conditions.
warming over the last century, there is no
convincing alternative explanation supported by the
When the climate continues to change over longer times,
extent of the observational evidence.
climatologists discern climate change. In effect, this means
For more information about scientific assessments of
that what was “normal” has shifted. See Figure 2.
climate change, see CRS Report R45086, Evolving
Assessments of Human and Natural Contributions to
Climate change may be discerned when the statistics of the
Climate Change, by Jane A. Leggett.
climate (e.g., the climate normal) continue to shift over
multiple decades. Distinguishing climate change from
Extreme Weather and Climate Change
climate variability can be challenging. Detecting climate
Some extreme weather can lead to emergencies or disasters.
change requires multiple decades of consistently collected
measurements. The greater the climate change relative to
Not all extreme weather events can be attributed to climate
the variability, the more rapidly scientific detection can
change. Nonetheless, climate models project that daily
occur. Observers have pointed to this as one reason that
maximum temperatures that would be extremely high in the
scientists did not reach consensus until the 1990s that the
current climate normal—for example, higher than 92oF in
Earth’s climate had changed over the 20th century. Global
July in Topeka (Figure 1)—would occur more and more
warming is one aspect of climate change, being a sustained
frequently as the Earth’s climate warms with continued
increase of global average temperature, usually measured at
greenhouse gas emissions. Extremely cold days are
the surface of the Earth, though it may also be measured at
expected to become less frequent with global warming.
specific altitudes in the atmosphere.
According to the 2017 NCA4, since the mid-20th century
Figure 2.Illustration of Climate Variability and Change
“[s]ome extremes have already become more frequent,
in Annual Global Mean Surface Temperatures, 1980-
intense, or of longer duration, and many extremes are
2019
expected to continue to increase or worsen.” As examples,
Anomalies (differences) from average 1901-2000 in oCelsius
“what in the past have been considered to be extreme
precipitation events are becoming more frequent” on most
continents, and “[t]he annual minimum and maximum sea
ice extent have decreased over the last 35 years.” However,
“[w]hether global trends in high-intensity tropical cyclones
are already observable is a topic of active debate.”
So while not all extreme weather experienced today can be
attributed to climate change, climate change may make
some of today’s extreme events more likely (especially
those related to high temperatures, hydrological drought, or
heavy precipitation). Scientists use complex attribution
studies, such as “fingerprinting,” to draw such conclusions.
Source: CRS, using data from NOAA, Climate at a Glance.
Global Change Is Broader Than Climate Change
Notes: The blue dots represent each year’s annual average
Another concept, broader than climate change, is that of
temperature and, as a set, il ustrate interannual variability of
global change. The Global Change Research Act of 1990
temperature. The yel ow line shows climate variability as a rol ing
(P.L. 101-606) defined it as “changes in the global
seven-year average of annual temperatures. The red line depicts the
environment (including alterations in climate, land
climate change of increasing annual temperature, averaged over
productivity, oceans or other water resources, atmospheric
rol ing 30-year periods.
chemistry, and ecological systems) that may alter the
capacity of the Earth to sustain life.”
Attributing Climate Variability and Climate
Change to Different “Forcings”
Jane A. Leggett, Specialist in Energy and Environmental
A number of factors drive climate variability and change.
Policy
Some factors are internal to the climate system, and some,
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Weather and Climate Change: What’s the Difference?
IF11446
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