Climate Sensitivity and Feedbacks

climate sensitivity IS generally defined as the global mean surface temperature change that is followed by the doubling of carbon dioxide (CO2) in the atmosphere. Put simply, climate sensitivity is the amount that temperatures rise or fall in response to alterations in climate.

Since 1990, official estimates of climate sensitivity have ranged between 2.7-8.1 degrees F (1.5-4.5 degrees C). In the summer of2004, at the Workshop on Climate Sensitivity, researchers generally agreed that temperatures have warmed by 0.6 degrees C over the last 100 years. Feedbacks are mechanisms that amplify or diminish climate change. Examples include water vapor, land ice-cover, vegetation cover, and ocean heat transport. Feedbacks may be affected by physical processes such as wind, snow, and seasonal variations.

Climatologists have identified three major ways in which climate sensitivity and feedbacks may be studied using computer models. The first method uses a three-dimensional global climate model according to a specified formula based on CO2 doubling. The second uses CLIMAP climate boundary conditions to produce an analysis based on the cooling of the last ice age, which occurred approximately 18,000 years ago. Finally, climatologists analyze climate sensitivity by estimating the changes in global temperature, in conjunction with the presence of greenhouse gases, to estimate empirical climate sensitivity. In computer models designed to estimate climate change, feedbacks are used to determine the magnitude of predicted climate changes, as well as the time span in which the changes are likely to occur. Results vary among climate sensitivity models, because of the differences in the ways in which climate feedbacks are handled in the particular model used.

As early as 1984, a study of climate sensitivity and feedbacks conducted by James Hensen, et al., determined that increased levels of greenhouse gases in the atmosphere were indicative of eventual global mean warming of approximately 1 degree C, a reading that was comparable to the Altithermal, a dry postglacial interval that is believed to be the warmest period on earth over the last 100,000 years. Beginning in the late 20th century, the Intergovernmental Panel on Climate Change (IPCC), established by the United Nations Environment Programme (UNEP) and the United Nations World Meteorological Organization (WMO), began issuing assessments of the scientific and socioeconomic data that were available on climate change. These assessment reports have greatly contributed to the understanding of global warming and climate change, although they have continued to generate considerable controversy.

steady temperature rise

The Third Assessment Report (TAR) was released in 2001, predicting that average surface temperatures of the Earth would increase steadily between 1990 and 2100, with estimates ranging from 2.5-10 degrees F (1.4-5.8 degrees C). At the same time, IPCC researchers predicted that sea levels would continue to rise steadily. Despite these pessimistic predictions, IPCC scientists generally believe that human behaviors, which are considered contributory factors in global warming and climate change, may alter predictions if humans begin to engage in more environmentally responsible behavior. Examples of possible responsible actions include making drastic reductions in CO2

emissions and eliminating use of aerosols and some pesticides.

TAR findings have consistently been used to support the notion that a consensus exists among scientists concerning global warming. The Third Report acknowledges that considerable confidence exists in the ability of climate sensitivity and feedback models to predict future climate changes, but notes that many such models fail to account for all elements of climate change. A fourth IPCC report, released on April 27, 2007, employed tens of thousands of data series on climate sensitivity and feedbacks to more fully assess current conditions and make more accurate predictions on the future of global warming and climate change.

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