*This page contains information on the basics of climate change from the educational booklet, Ecological Impacts of Climate Change. For more detailed discussion of the physical basis of climate change, please see Understanding and Responding to Climate Change, another publication from the National Academies.
The Earth Is Getting Warmer
Image courtesy of the Joint Institute for the
Study of the
Atmosphere and Ocean, University
of Washington.A relatively rapid increase in temperature has been documented during the past century, both at
Earth's surface and in the oceans. The average surface temperature for Earth as a whole has risen
some 1.3°Fahrenheit since 1850, the starting point for a global network of thermometers. If emission
rates for greenhouse gases (which
trap heat inside Earth's atmosphere) continue
on their current track, models indicate
that the globe will be 4.3 to 11.5°F
warmer by 2100 than it was in 1990.
Human Activities Are Contributing
to Climate Change
The physical processes that cause climate change are scientifically well
documented: both human activities and natural variability are contributing
to global and regional warming. According to the Intergovernmental Panel
on Climate Change, whose documents are considered the most authoritative
source for information on the "state of the science" on climate change,
it is very likely that most of the observed warming over the past 50 years
is the result of increased greenhouse gases generated by human activities.
Numerous expert reports from the National Research Council have supported
this conclusion as well.
Carbon dioxide has increased dramatically
since the
Industrial Revolution.
Adapted from Climate Change
2007: The Physical Science
Basis. Working Group 1
Contribution to the 4th Assessment
Report of the
Intergovernmental Panel on Climate Change.
Figure
SPM.5. Cambridge University Press.The release of greenhouse gases has increased significantly since the Industrial
Revolution, mostly from the burning of fossil fuels for energy, agriculture, industrial
processes, and transportation. Carbon dioxide, a major greenhouse gas, is
increasing in the atmosphere faster than at any time measured in the past,
having grown by about 35 percent since 1850. Two other greenhouse gases,
methane and nitrous oxide, are present in the atmosphere at much lower
concentrations than carbon dioxide but have increased rapidly. Methane has
increased by 150 percent; in addition, it is 25 times more effective per molecule
at trapping heat than carbon dioxide. Nitrous oxide, nearly 300 times more
effective, has increased by more than 20 percent.
Much remains to be learned about the factors that control the sensitivity of climate to increases
in greenhouse gases, rates of change, and the regional outcomes of the global changes. Although
scientific knowledge of climate is far from complete, the uncertainties concern the details: the scientific
community is highly confident in the basic conclusions.
Sea Levels Are Rising
Warmer temperatures not only cause glaciers and land ice to melt (adding
more volume to oceans) but also cause seawater to expand in volume as
it warms. The global average sea level rose by just under .07 inches per
year during the 20th century, but that number has risen to .12 inches per
year since the early 1990s. Under a "business-as-usual" greenhouse gas
emissions scenario, models indicate that sea levels could rise 2 feet or
more by 2100 compared to 1990 levels.
Changes Are Rippling Through the
Water Cycle
Climate change has complex effects on water supply and demand. The seasonal
rhythms of streams and rivers have changed as winter precipitation falls increasingly as
rain instead of snow, and as earlier spring temperatures cause snow in the mountains to melt
earlier and faster. Climate change may mean that some places will experience more days with
very heavy rain; other places may see more frequent, intense, and long-lasting droughts. Warmer
temperatures also mean higher evaporation rates and thirstier plants and people, increasing
demands for water. A warmer world will experience more precipitation on a global scale, but the changes will not be the same everywhere. Projections indicate that on average
dry areas will tend to get drier, and wet areas will tend to get wetter.
The Ocean Is Acidifying
Much of the carbon dioxide emitted by human activity has already
been taken up by the ocean, thus moderating the increase of carbon
dioxide in the atmosphere. However, as carbon dioxide dissolves in
seawater, it forms carbonic acid, acidifying the ocean. Ocean acidification
will likely cause serious harm to such treasured marine organisms
as corals, lobsters, and sea urchins.
Climate Change Is Reflected in
Extreme Weather
It is considered very likely that increasing global temperatures will lead to higher maximum
temperatures, more heat waves, and fewer cold days over most land areas. More severe
drought in some areas, combined with other factors, has contributed to larger and more
frequent wildfires.
This web page is based on the National Academies' educational booklet
Ecological Impacts of Climate Change.
