The United Nations Intergovernmental Panel on Climate Change claims to have found "new and stronger evidence that most of the observed warming over the last 50 years is attributable to human activities." This conclusion is based on improvements in computer models that can now "produce satisfactory simulations of the current climate." But two recent studies show that the models may not have adequately accounted for two important factors: soot and land use changes.
The first study, by University of Maryland climate researchers Eugenia Kalnay and Ming Cai, was published last May in Nature. It showed that, in addition to the well-known "urban heat island effect," in which temperatures increase in metropolitan areas, other land use changes, especially agricultural conversion and irrigation, cause surface warming as well. The authors wrote that "the trend in daily mean temperature due to land use changes is 0.35 degrees centigrade per century." This value is more than twice as high as previous estimates based on urbanization alone.
The second study, by James Hansen and Larissa Nazarenko of NASA's Goddard Institute for Space Studies, appeared in the December Proceedings of the National Academy of Sciences. Hansen and Nazarenko looked at the effect that the soot produced by burning fossil fuels and biomass has once it settles on ice and snow in the Northern Hemisphere. Dirty snow melts faster than clean snow because the black particles absorb more heat from the sun. Hansen and Nazarenko estimate that the soot effect may be responsible for about 0.2 degree Celsius of warming over 120 years. It also helps explain thinning Arctic sea ice, earlier spring snowmelt, and many melting glaciers often cited by environmental activists.
Both sets of researchers still firmly maintain that greenhouse gases are the most important man-made influence on climate. Hansen and Nazarenko insist, despite their findings, that "anthropogenic greenhouse gases have been the main cause of recent global warming."
Yet these studies suggest that land use changes account for 0.35 degrees Celsius of warming over 100 years and that soot is responsible for another 0.2 degrees of warming over a slightly longer period. Total warming during the century was 0.6 degrees Celsius, leaving something like 0.05 degrees attributable to increases in greenhouse gases. Cut both results in half, and accumulating greenhouse gases would still account for only about half of the warming seen in the last century.
While both studies could be wrong, they suggest the computer models may not adequately account for some important processes and may therefore overestimate future warming due to greenhouse gas increases. Despite these uncertainties, environmental activists insist the science is settled and demand that the world's governments reorganize the global economy to reduce greenhouse gas emissions.