DAVID BIELLO, SCIENTIFIC AMERICAN - The Intergovernmental Panel on
Climate Change in its first report in 1990 predicted that temperatures
would warm by 0.5 degree Fahrenheit (0.3 degree Celsius) per decade if
no efforts were made to restrain greenhouse gas emissions. But the panel
of scientists and other experts was wrong: By 2001, the group estimated
that average temperatures would increase by 2.7 to 8.1 degrees F (1.5 to
4.5 degrees C) in the 21st century, and they raised the lower end to 3.6
degrees F (2 degrees C) this year in their most recent report. In
essence, neither this international team of experts nor any other can
say with any certainty just how bad global warming may get.
There is a simple explanation for this, says atmospheric physicist
Gerard Roe of the University of Washington in Seattle: Earth's climate
is extremely sensitive. In other words, small changes in various
physical processes that control climate lead to big results. "If nothing
else changed by [warming], a doubling of carbon dioxide would ultimately
lead to a temperature change of about 1.2 [degrees] C," [(2.1 degrees
F)] Roe says. "In fact, because of internal processes within the climate
system, such as changing snow cover, clouds and water vapor in the
atmosphere, our best estimate is that the actual warming would be two to
four times larger than that."
Some of these feedback processes are poorly understood-- like how
climate change affects clouds--and many are difficult to model,
therefore the climate's propensity to amplify any small change makes
predicting how much and how fast the climate will change inherently
difficult. "Uncertainty and sensitivity are inextricably linked," Roe
says. "Some warming is a virtual certainty, but the amount of that
warming is much less certain.". . .
Waiting for more scientific certainty before acting is a mistake, Roe
says. "People are comfortable with the idea that stock markets, housing
prices and the weather are uncertain, and they are used to making
decisions on that basis," he notes. . .