The Independent Summary for Policymakers is a
detailed and thorough overview of the state of climate change
science as laid out in the UN Intergovernmental Panel on Climate
Change (IPCC) draft report. This independent summary has been
reviewed by more than 50 scientists around the world and their
views on its balance and reliability are tabulated for readers.
It carefully connects summary paragraphs to the chapters and
sections of the IPCC report from which they are drawn, allowing
readers to refer directly to what is in the IPCC Report,
• Data collected by weather satellites since 1979 continue to
exhibit some evidence of lower atmospheric warming, with
estimated trends ranging near the low end of past IPCC forecasts.
There is no significant warming in the tropical troposphere (the
lowest portion of the Earth's atmosphere), which accounts for
half the world's atmosphere, despite model predictions that
warming should be amplified there.
• Temperature data collected at the surface exhibits an upward
trend from 1900 to 1940, and again from 1979 to the present.
Trends in the Southern Hemisphere are small compared to those in
the Northern Hemisphere.
• There is no compelling evidence that dangerous or unprecedented
changes are underway. Perceptions of increased extreme weather
events are potentially due to increased reporting. There is too
little data to reliably confirm these perceptions.
• There is no globally-consistent pattern in long-term
precipitation trends, snow-covered area, or snow depth. Arctic
sea ice thickness showed an abrupt loss prior to the 1990s, and
the loss stopped shortly thereafter. There is insufficient data
to conclude that there are any trends in Antarctic sea ice
• Current data suggest a global mean sea level rise of between
two and three millimeters per year. Models project an increase of
roughly 20 centimeters over the next 100 years, if accompanied by
a warming of 2.0 to 4.5 degrees Celsius.
• Natural climatic variability is now believed to be
substantially larger than previously estimated, as is the
uncertainty associated with historical temperature
• Attributing an observed climate change to a specific cause like
greenhouse gas emissions is not formally possible, and therefore
relies on computer model simulations. These attribution studies
do not take into account the basic uncertainty about climate
models, or all potentially important influences like aerosols,
solar activity, and land use changes.
• Computer models project a range of future forecasts, which are
inherently uncertain for the coming century, especially at the
regional level. It is not possible to say which, if any, of
today's climate models are reliable for climate prediction and
Text revised March 2, 2007
• Entries for 2 reviewers were corrected for spelling and/or area
• A problem in the wording of 2.1b was noted by a reader, who
asked to remain anonymous but whose input is gratefully
acknowledged. While the AR4 refers to "a global warming of the
troposphere from T2 of 0.04 to 0.20°C decade-1 for the period
1979-2004", the trend coefficients of specific interest reflect
processing to remove an estimated stratospheric component and
fall in the narrower interval 0.12-0.19 C/decade. The wording of
Sct 2.1b has been expanded and revised to remove any
misunderstanding on this point.
• Further revisions may be made to ensure consistency with the
final IPCC text in May.