Public Release: 

Yale Analysis Of Seasonal Temperature Trends For Past Century Contradict Computer Models Of Greenhouse Warming

Yale University

Yale Analysis of Seasonal Temperature Trends for Past Century
CONTACT: Cynthia L. Atwood
cynthia.atwood@yale.edu
For Immediate Release: July 17, 1996

Yale Analysis of Seasonal Temperature Trends for Past Century Contradict Computer Models of Greenhouse Warming

Trend Toward Earlier Onset of Seasons Surprises Scientists

New Haven, CT - In one of the most intensive analyses ever made of seasonal temperature trends in the northern hemisphere drawing upon more than a century of data, Yale University scientists found striking differences between the average onset of seasons and those predicted by two of the most sophisticated computer models on which many scientists have based their forecasts of global warming.

"According to the computer models, if global warming were occurring because of rising carbon dioxide levels from pollution and deforestation, the seasons would arrive gradually later each year," said co-author Jeffrey J. Park, associate professor of geology and geophysics at Yale. "Instead, we found the opposite trend, that the seasons on average have been arriving gradually earlier each year since shortly after the turn of the century. Furthermore, we found regional patterns that the models don't explain."

The study focused on regional seasonal shifts in temperature and not just on the average worldwide temperature, which is known to have risen by about 1 degree Fahrenheit in the last century, Professor Park explained.

The findings could mean that two leading computer simulations of global warming, that of the Geophysical Fluid Dynamics Lab (GFDL) in Princeton, N. J., and the Community Climate ModeL (CCM1) from the University Corporation for Atmospheric Research in Boulder, Colo. have significant shortcomings, said Professor Park, whose research with Yale graduate student Michael E. Mann was published in the May 15 issue of Geophysical Research Letters.

"If you find a trend opposite to that predicted by the models, you worry that the models are failing to capture the complicated physics of climate," said Professor Park, who explained that both computer simulations attempt to predict changes in atmospheric circulation and temperature patterns caused by rising CO2 levels. "Our comparison underscores the difficulty of attributing the observed global warming trend to either the greenhouse effect or normal climate variability, or to a combination of the two."

While no tell-tale signs of global warming were found in the Yale study, the unexpected shifts in the onset of seasons could be related to the burning of fossil fuels, he cautioned, because they began early in the 20th century and correlate to the rise of industrialization and the use of automobiles. But just exactly what impact those season shifts might have on climate or their relationship to global warming are unknown.

Another possible explanation for the discrepancies between the computer models' predictions and actual seasonal temperature trends could be that the models respond largely to changes at higher latitudes where ice sheets are melting, a region where historic temperature records are sparse. "If we had more data from the Arctic at the turn of the century, we might see a better match with computer predictions," Professor Park noted.

The Yale study drew upon temperature records from thousands of monitoring sites dating from 1854 to 1990 and compiled by the University of East Anglia in England. Professor Park calculated the onset of summer and winter by noting subtle changes in average monthly temperatures using "complex demodulation," a specialized data analysis technique. The study focused on the northern hemisphere because long-term records are not widely available from the southern hemisphere.

Differences Found Between West and East Coasts

While the Yale study found that, on average, the seasons are changing about one day earlier than expected since the turn of the century, some areas are experiencing significant delays in season shifts, especially western coastal regions of continents in the northern hemisphere. Professor Park attributes these regional variations at least in part to prevailing winds moving from adjacent oceans to coastal land.

Central and eastern regions, however, are shifting from one season to the next gradually earlier each year since early in the 20th century, pushing the average for the entire northern hemisphere to an earlier arrival of seasons, Professor Park said. He speculates the earlier shift could be caused by changes in regional weather patterns, winter snow cover, water vapor in the atmosphere and changes in the reflectivity of clouds and vegetation in the continental interiors.

The Yale study is the first to note that the trends in seasonal shifts are moving in opposite directions on the east and west coasts of continents, which could produce unexpected climate dynamics, Professor Park said. The Yale scientists also found smaller than expected extremes between winter and summer temperatures in scattered regions throughout the northern hemisphere, regardless of whether the seasons were delayed or earlier than expected in those locations, an effect predicted by global warming models.

"It could be that a model that doesn't capture large regional climate patterns or adequately take into account land-surface conditions in the interior of continents can't predict the future," said Mr. Mann, who has collaborated with Professor Park to study short-term weather fluctuations that occur every decade or so. "When you superimpose these climate fluctuations on long-term cycles, such as century-scale oscillations in ocean circulation patterns, it becomes more difficult to untangle greenhouse effects from natural climate changes, given our current knowledge."

A Smoking Gun for Human-Caused Climate Change

In their analysis, the Yale scientists also confirmed an unexpected acceleration in the delay of season shifts in some locations first reported last year by David J. Thomson of AT&T Bell Labs, which appears to be linked to the burning of fossil fuels beginning early in the 20th century.

For example, the seasons start two days later on average in southern Germany than a century ago, five days later in western Greenland and about one day later in northern California, although Dr. Thomson found localized sites with even greater delays, an eight-day delay in Paris and nearly three weeks in San Francisco. While the delays do not prove global warming is occurring, they indicate rapid unexpected changes in climate that could be related to human activity, Professor Park said.

Going a step farther, the Yale scientists confirmed that the earlier onset of seasons in central and eastern regions of continents, for example, a two day advance in southern New England, began about 1920, further supporting the notion of a link to the burning of fossil fuels but contradicting predictions of delays.

"The bottom line here is we were looking for a 'fingerprint' of global warming as predicted by two respected computer models, but we didn't find it," Mr. Mann said. "What we did find could provide clues about how scientists can revise their models to make them more accurate in predicting climate change in the next century."

Note to Editors: Jeffrey J. Park, (203) 432-3172, is a specialist in earthquakes and data analysis related to global change. He is former board chairman of the Incorporated Research Institutions for Seismology (IRIS), which is building a global seismic monitoring network. He. co-authored a recent report to Congress recommending the usecíG~xmì® pUZ

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