Long-standing puzzle in climate science resolved

Washington, Oct 11 : A team of scientists has resolved a long-standing puzzle in climate science, by reconciling the differences between simulated and observed temperature trends in the tropics.

Using state-of-the-art observational datasets and results from computer model simulations archived at Lawrence Livermore National Laboratory (LLNL), researchers and colleagues from 11 other scientific institutions have refuted a recent claim that simulated temperature trends in the tropics are fundamentally inconsistent with observations.

This claim was based on the application of a flawed statistical test and the use of older observational datasets.

Computer model simulated changes in surface temperature and sea-ice extent.

The models used to simulate these changes were first run with best estimates of historical changes in human and natural factors over the 20th century, and then driven by estimated future changes in greenhouse gases.

Temperature and sea-ice changes are shown at four different times. Results are averages from the output of nearly two dozen individual climate models.

Climate model experiments invariably predict that human-caused greenhouse gas increases should lead to more warming in the tropical troposphere (the lowest layer of the atmosphere) than at the tropical land and ocean surface.

This predicted "amplification" behavior is in accord with basic theoretical expectations.

Until several years ago, however, most satellite and weather balloon records suggested that the tropical troposphere had warmed substantially less than the surface.

For nearly a decade, this apparent discrepancy between simulations and reality was a major conundrum for climate scientists.

The discrepancy was at odds with the overwhelming body of other scientific evidence pointing toward a "discernible human influence" on global climate.

A paper published online last year in the International Journal of Climatology claimed to show definitively that "models and observations disagree to a statistically significant extent" in terms of their tropical temperature trends.

This claim formed the starting point for an investigation by a large team of climate modelers and observational data specialists, which was led by LLNL's Benjamin Santer.

In marked contrast to the earlier claim, Santer's international team found that there is no fundamental discrepancy between modeled and observed trends in tropical temperatures.

"We've gone a long way toward reconciling modeled and observed temperature trends in the problem area of the tropics," said Santer.

"We should apply what we learned in this study toward improving existing climate monitoring systems, so that future model evaluation studies are less sensitive to observational ambiguity," he added.

ANI