IPCC Fourth Assessment Report
Publication Date: February 2007
Publication Format: Digital Publications
Executive Summary: The Independent Summary for Policymakers is a detailed and thorough overview of the state of climate change science as laid out in the UN International 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, including:
• Data collected by weather satellites since 1979 continue to exhibit little evidence of atmospheric warming, with estimated trends ranging from nearly zero to 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 thickness.
• 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 reconstructions.
• 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 forecasting.
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