New findings from the National Academy of Sciences (NAS) provide evidence that global climate model forecasts are unreliable indicators of future climate.
Earlier this year, the NAS’s National Research Council (NRC) released a report titled “Reconciling the Observations of Global Temperature Change.” The report panel found that global temperatures have not been changing as state-of-the-art climate models predicted they would–a fact that lends support to the premise that the models should not be used in forming policy that could potentially lead to great economic harm.
Although the average temperature at the Earth’s surface has risen somewhere between 0.13°C and 0.2°C per decade during the past 21 years, temperatures in the lower atmosphere as measured by NASA satellites virtually have not changed at all, the report states.
“There really is a difference between temperatures at the two levels that we don’t fully understand,” panel chairman and University of Washington Atmospheric Sciences professor John Wallace told the Washington Post. Wallace went on to say that the disparity between the temperature trends at the surface and those in the lower atmosphere points to a weakness in scientists’ ability to predict future warming.
Global climate models built to forecast the Earth’s future climate indicate that temperatures in the lower atmosphere should be rising faster than those at the surface. But the NRC report demonstrates that the observations do not support these claims.
A turning point
That observations of surface temperatures show a warming trend while those of the lower atmosphere do not is a disparity that calls into clear question scientists’ ability to model climate change.
But never before has such a diverse group of experts as those on the NRC panel embraced this fact–not just satellite gurus John Christy and Roy Spencer, for example, but also scientists who have typically supported GCM predictions, including NASA’s James Hansen, Lawrence Livermore National Laboratory’s Benjamin Santer, Kevin Trenberth of the National Center for Atmospheric Research, and orbital drift theorist Frank Wentz.
Instead, a lot of effort has been spent (by many of these same scientists) either trying to discredit the satellite observations or, as in the case of the series of reports from the Intergovernmental Panel on Climate Change (IPCC), sweeping the issue under the computer in an attempt to ignore it altogether. Not a single mention of satellite temperature observations and the difficulties they portend appears in the Summary for Policymakers (the most-read portion) of the latest IPCC report. In this light, this NRC panel’s findings and their subsequent publication mark a true turning point, by admitting this scientific problem and the implications associated with it. But no one has provided an adequate explanation for the IPCC’s refusal to acknowledge it.
Verifying models against reality
The panel examined the output from several climate models in order to assess how well they mimicked the observed surface and lower atmospheric temperature trends. They found that, “although climate models indicate that changes in greenhouse gases and aerosols play a significant role in defining the vertical structure of the observed atmosphere, model-observation discrepancies indicate that the definitive model experiments have not been done.”
The report did not even conclude that the observed surface warming was due to anthropogenic changes in the Earth’s greenhouse effect. Although Wallace stated that “there is a high level of confidence among the panel members that the surface temperature is indeed rising,” he noted the rise is not necessarily due to manmade changes in the atmosphere.
“The rapid increase in the Earth’s surface temperature over the past 20 years is not necessarily representative of how the atmosphere is responding to long-term, human-induced climate changes, such as increasing amounts of carbon dioxide and other ‘greenhouse’ gases,” Wallace explained. “The nations of the world should develop an improved climate monitoring system to resolve uncertainties in the data and provide policy makers with the best available information.” Note that Wallace did not say the nations of the world should dramatically reduce their greenhouse emissions.
The NRC panel’s overall conclusions are well summarized in their final paragraph:
It is clear from the foregoing that reconciling the discrepancy between the global-mean trends in temperature is not simply a matter of deciding which one of them is correct or determining the ideal “compromise” between them. In the long term, it will require major advances in the ability to interpret and model the subtle variations in the vertical temperature profile of the lower atmosphere that occur in association with the internal variability of the climate system in response to volcanic eruptions [and] solar forcing and in connection with changes in atmospheric composition due to human activities. It will also require more precise and extensive satellite- and ground-based observations for monitoring climate change, and changes in the way these observations are implemented and processed.
The best course of action would appear to be to work toward a better understanding of the observed differences, and then to build that knowledge into climate models. Only when climate models can accurately explain known phenomena, and not before, can we begin to use them to explore the possible future effect of changes in the climate system. Based upon the NRC panel’s findings, this ability is apparently still a long way off.
Robert E. Davis is an associate professor of environmental science at the University of Virginia.
Reconciling Observations of Global Temperature Change, 2000. Panel on Reconciling Temperature Observations, National Research Council, National Academy Press, Washington, DC, 85pp.
Houghton, J.T., et al. (editors), 1996. Climate Change 1995: The Science of Climate Change, Intergovernmental Panel on Climate Change, Cambridge University Press, New York, NY, 572pp.