Understanding Climate Change
Highlights from the IPCC Working Group I Summary for Policymakers of "Climate Change 2007: The Physical Science Basis"
Below are some of the key statements in the Summary for Policymakers from Working Group I of the 2007 IPCC Fourth Assessment Report. The summary was released on Friday, February 2, 2007, and is available on the IPCC Web site, along with the full working group and synthesis reports (see box at right).
Some indices have a range of uncertainty, given in parentheses. Spelling
and punctuation have been adapted to Standard American style.
What’s happened to our climate, and why?
“Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global mean sea level.”
“Average Northern Hemisphere temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years and likely the highest in at least the past 1300 years.”
“Temperatures of the most extreme hot nights, cold nights and cold days are likely to have increased due to anthropogenic forcing. It is more likely than not that anthropogenic forcing has increased the risk of heat waves.”
Cause of the changes
“Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increased in anthropogenic greenhouse gas concentrations.”
“It is likely that there has been significant anthropogenic warming over the past 50 years averaged over each continent except Antarctica.”
“Since IPCC’s first report in 1990, assessed projections have
suggested global averaged temperature increases between about 0.15 and 0.3°C
[0.27 and 0.54°F] per decade for 1990 to 2005. This can now be compared
with observed values of about 0.2°C [0.36°F] per decade, strengthening
confidence in near-term projections.”
What are some of the other observed trends?
More moisture in the air: “The average atmospheric water vapor content has increased since at least the 1980s over land and ocean as well as in the upper troposphere. The increase is broadly consistent with the extra water vapor that warmer air can hold.”
Less ice and snow on land: “Mountain glaciers and snow cover have declined on average in both hemispheres. Widespread decreases in glaciers and ice caps have contributed to sea level rise (ice caps do not include contributions from the Greenland and Antarctic ice sheets).”
Less Arctic sea ice: “. . . annual average Arctic sea ice extent has shrunk by 2.7 (2.1 to 3.3)% per decade, with larger decreases in summer of 7.4 (5.0 to 9.8)% per decade.”
Rising sea level: “Global average sea level rose at
an average rate of 1.8 (1.3 to 2.3) millimeters per year over 1961 to 2003.
The rate was faster over 1993 to 2003, about 3.1 (2.4 to 3.8) millimeters
per year.” Whether the faster rate for 1993 to 2003 reflects decadal
variability or an increase in the longer-term trend is unclear.”
What are some of the areas where research has progressed since the Third Assessment Report (TAR)?
Temperatures aloft (in the lower and middle troposphere): “. . . show warming rates that are similar to those of the surface temperature record and are consistent within their respective uncertainties, largely reconciling a discrepancy noted in the TAR.”
Solar contribution to climate variability: “Changes in solar irradiance since 1750 are estimated to cause a radiative forcing of +0.12 (+0.06 to +0.30) watts per square meter, which is less than half the estimate given in the TAR.”
Human versus solar influence: “The
understanding of anthropogenic warming and cooling influences on climate
has improved since the TAR, leading to very high confidence that
the globally averaged net effect of human activities since 1750 has been
one of warming, with a radiative forcing of +1.6 (+0.6 to 2.4) watts per
What are some climatic features that haven’t changed since the last IPCC report?
Diurnal temperature range: “A decrease in diurnal temperature range (DTR) was reported in the TAR, but the data available then extended only from 1950 to 1993. Updated observations reveal that DTR has not changed from 1979 to 2004 as both day- and night-time temperature have risen at about the same rate.”
The extent of Antarctic sea ice: “Antarctic sea ice
extent continues to show inter-annual variability and localized changes but
no statistically significant average trends, consistent with the lack of
warming reflected in atmospheric temperatures averaged across the region.”
What can we expect to happen?
“There is now higher confidence in projected patterns of warming and other regional-scale features, including changes in wind patterns, precipitation, and some aspects of extremes and of ice.”
“Snow cover is projected to contract. Widespread increases in thaw depth are projected over most permafrost regions.”
“Sea ice is projected to shrink in both the Arctic and Antarctic.”
“It is very likely that hot extremes, heat waves, and heavy precipitation events will continue to become more frequent.”
“Based on a range of models, it is likely that future tropical cyclones (typhoons and hurricanes) will become more intense, with larger peak wind speeds and more heavy precipitation associated with ongoing increases of tropical SSTs [sea-surface temperatures]. There is less confidence in projections of a global decrease in numbers of tropical cyclones.”
“Both past and future anthropogenic carbon dioxide emissions will continue to contribute to warming and sea level rise for more than a millennium, due to the timescales required for removal of this gas from the atmosphere.”
The University Corporation for Atmospheric Research manages the National Center for Atmospheric Research under sponsorship by the National Science Foundation. Any opinions, findings and conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.