Weather and Climate Change: What’s the Difference?

Weather and Climate Change: What’s the Difference?

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Updated May 11, 2021
Weather and Climate Change: What’s the Difference?
and snow cover. Weather conditions can vary rapidly or last
Weather is day-to-day temperature, precipitation, and
up to a few days in the mid-latitudes. NOAA compiles
other conditions.
observations from weather stations around the United
States, in the oceans, from international sources, and from
Climate is weather observed over multidecadal periods
other federal agencies. NOAA analyzes historical weather
for a defined location. For example, the National
data and provides multiday forecasts or seasonal outlooks.
Oceanic and Atmospheric Administration (NOAA)
measures a climate normal over specific 30-year
Climate is defined by the long-term statistics of weather
periods for locations in the United States.
compiled for a specified place (Figure 1), including the
globe. For example, climate would include the maximum
Climate variability is how weather may vary for multiple
hourly precipitation in July in a particular county over 30
weeks or years from the long-term average.
years or the global mean surface temperature from 1900 to
Climate change occurs when the long-term climate
2000. The availability of long-term data varies by location,
shifts, without an apparent return to previous
making climate analysis for some locations challenging.
normals.
NOAA’s National Centers for Environmental Information
With much discussion of climate change in Congress, in the
defines U.S. climate normals as “three-decade averages of
news, and among constituents, some commonly used words
climatological variables including temperature and
may be misunderstood or misused. Below are explanations
precipitation” for defined locations. The most recent
of several key terms: weather, climate, climate variability,
and climate change, as well as some associated concepts.
climate normal for a defined location uses weather
observations for 1991-2020. People use climate normals for
a wide array of purposes, such as placing daily weather into
Weather is the state of the atmosphere at a specific place
a historical context or determining technical specifications
and time, described by such conditions as temperature,
for the resilience of buildings and infrastructure against
precipitation, humidity, cloudiness, and wind speed.
extreme weather.
Weather can include conditions of the Earth’s water and
land surfaces as well, with such variables as wave height
Figure 1. Illustration of How Weather Makes Up the Long-Term Climate, Using Temperature
Daily Maximum Temperature in Topeka, Kansas, the Past Climate Normal for 1981-2010

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 yellow dot represents the daily maximum temperature in Topeka, Kansas, per day in 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 pink lines above and below it represent one (statistical) standard deviation from the
average (about 18 percentage points above and below). The two yel ows lines, above and below, represent the 5th 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 5th.) About 90% of the time, the daily maximum
temperature could be expected to fal between the 5th 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.
gas emissions, are external forcings. Each factor has its own
Generally, it means the differences from the long-term
rhythm and pattern, temporally ranging from less than a
averages (or other statistics) of climate-related variables,
year to tens of thousands of years. Geographically, some
such as precipitation, over longer time or wider areas than
forcings have stronger regional or hemispheric effect, while
individual weather events. Climate variability may describe
others are global. The temporal and geographical
how a season is above or below the climate normal average
characteristics of each is like a fingerprint on the climate
for, say, temperature or snowfall. Climate variability may
system. Using such “fingerprints,” with sufficient
also describe longer, multiyear patterns that are not average,
observations and powerful computing systems, the 2017
such as when certain large-scale weather patterns occur. An
U.S. National Climate Assessment (NCA4) concluded:
example is the so-called El Niño/La Niña (ENSO) cycle,
It is extremely likely [>95%] that human influence
with cycles of warm and cool periods in the Pacific Ocean
and the atmosphere. ENSO phases typically last from nine
has been the dominant cause of the observed
to 12 months and on average occur every two to seven
warming since the mid-20th century. For the
years. With only climate variability, scientists would expect
warming over the last century, there is no
the climate to reflect historically average conditions.
convincing alternative explanation supported by the
extent of the observational evidence.
When the climate continues to change over longer times,
For more information about scientific assessments of
climatologists discern climate change. In effect, this means
climate change, see CRS Report R45086, Evolving
that what was “normal” has shifted. See Figure 2.
Assessments of Human and Natural Contributions to
Climate Change, by Jane A. Leggett.
Climate change may be discerned when the statistics of the
climate (e.g., the climate normal) continue to shift over
Extreme Weather and Climate Change
multiple decades. Distinguishing climate change from
Some extreme weather can lead to emergencies or disasters.
climate variability can be challenging. Detecting climate
Not all extreme weather events can be attributed to climate
change requires multiple decades of consistently collected
change. Nonetheless, climate models project that some
measurements. The greater the climate change relative to
events that would be extreme in the current climate—such
the variability, the more rapidly scientific detection can
as daily maximum temperatures higher than 92oF in July in
occur. Observers have pointed to this as one reason that
Topeka (Figure 1)—would occur more and more
scientists did not reach consensus until the 1990s that the
frequently as the Earth’s climate warms with continued
Earth’s climate had changed over the 20th century. Global
greenhouse gas emissions. Extremely cold days are
warming is one aspect of climate change; it is a sustained
expected to become less frequent with global warming.
increase of global average temperature, usually measured at
the surface of the Earth, though it may also be measured at
According to the 2017 NCA4, since the mid-20th century
specific altitudes in the atmosphere.
“[s]ome extremes have already become more frequent,
intense, or of longer duration, and many extremes are
Figure 2.Illustration of Climate Variability and Change
expected to continue to increase or worsen.” As examples,
in Global Mean Surface Temperatures, 1880-2019
“what in the past have been considered to be extreme
Anomalies (differences) from average 1901-2000 in oCelsius
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.
Global Change Is Broader Than Climate Change

Source: CRS, using data from NOAA, Climate at a Glance.
Another concept, broader than climate change, is that of
Notes: The blue dots represent each year’s annual average
global change. The Global Change Research Act of 1990
temperature and, as a set, il ustrate interannual variability of
(P.L. 101-606) defined it as “changes in the global
temperature. The yel ow line shows climate variability as a rol ing
environment (including alterations in climate, land
seven-year average of annual temperatures. The red line depicts
productivity, oceans or other water resources, atmospheric
climate change as increasing annual temperature, averaged over
chemistry, and ecological systems) that may alter the
rol ing 30-year periods.
capacity of the Earth to sustain life.”
Attributing Climate Variability and Climate
Jane A. Leggett, Specialist in Energy and Environmental
Change to Different “Forcings”
Policy
A number of factors drive climate variability and climate
IF11446
change. Some factors are internal to the climate system, and
some, such as solar variability or human-related greenhouse
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Weather and Climate Change: What’s the Difference?

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