Some of the world's most powerful supercomputers are devoted to ingeniously devised model simulations intended to help us understand what our future climate will be like. So you'd think they would be pretty good at it.
But people most familiar with how the climate system behaved in the past -- before humans came along to alter the composition of its atmosphere -- see patterns of natural change that are fundamentally different from what the biggest, most complex models project for the future.
Remarkably, the character of this "disconnect" between geological data of the past and computer simulations of the future is not at all what people who complain about climate science and climate scientists would have us believe.
"The models seem to be too stable," Paul Valdes, a climate modeler and "paleoclimate" specialist at the University of Bristol in the UK, argues in the new issue of the journal Nature Geoscience.
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Judging from the evidence, Earth's climate is more sensitive than the computer models designed to simulate its behavior.
Thousands of years of climates past are punctuated by episodes of change that are rapid and radical -- big, sudden shifts in temperature and precipitation. This inclination toward "abrupt climate change" -- found in ice cores, ocean sediments and other geological data -- surprised earth scientists as it was uncovered in the 1990s, and still it remains outside of much conventional thinking on the subject.
Valdes argues that the models used by the UN's Intergovernmental Panel on Climate Change in its most recent report (2007) "have not proved their ability to simulate abrupt change when a critical threshold is crossed."
Scientists calculate the likely accuracy of models to foretell the future by testing their ability to accurately recreate the past.
Valdes cites four examples of abrupt changes in the distant past which "could have direct bearing on climate predictions for the twenty-first century" that the current generation of big, complex climate models are unable to accurately recreate:
- Rapid warming 55.8 million years ago began at a time when temperatures at the equator were not very different from temperatures at the poles, but climate models cannot match those starting conditions, making it "unrealistic to simulate the further abrupt warming" that followed during that epoch.
- Temperature records of the last ice age, from about 120,000 to 12,000 years ago, taken from ice cores and ocean sediments, show two types of abrupt change: numerous cold snaps and rapid warm episodes -- gyrations of 20 degrees F or more -- that theorists have a hard time explaining and modern climate models can't realistically replicate.
- More recently, between 9,000 and 5,500 years ago, in the span of just a matter of decades, the climate system transformed the region of North Africa now known as the Sahara from a relatively wet, vegetated steppe into the vast, barren desert we see today. "Complex climate models fail to simulate the vegetated state," notes Valdes, "and can not therefore capture this event of rapid change."
Valdes calls for more computing power, better scientific understanding of the mechanism of change in the climate system, better data for the computer models, and concludes:
"In the meantime, we need to be cautious. If anything, the models are underestimating change, compared with the geological record. According to the evidence from the past, Earth's climate is sensitive to small changes, whereas the climate models seem to require a much bigger disturbance to produce abrupt change. Simulations on the coming century with the current generation of complex models may be giving us a false sense of security."
IMAGE: Right to left, the temperature profile of air over Greenland during the past 100,000 years, taken from Greenland Ice Core Project II. CREDIT: National Research Council
Tags: Climate Change, Global Warming, Meteorology
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