New Analysis of Global Temperature: Year-to-Year Correlations Reveal Human Fingerprint

BOULDER--A new statistical analysis of 115 years of global temperaturedata, compared with output from two leading computer models of climate,has strengthened the argument that human-caused emissions of greenhousegases, such as carbon dioxide, are warming the earth's atmosphere. Thenew results by Tom Wigley, a senior scientist at the National Center forAtmospheric Research (NCAR); Richard Smith (University of NorthCarolina); and Benjamin Santer (Lawrence Livermore National Laboratory)appear in the November 27 issue of Science. NCAR's primary sponsor isthe National Science Foundation.

The paper is the first one to examine each year's average temperaturesfor the Northern and Southern Hemispheres by correlating them with thereadings taken up to 20 years earlier or later. If the numbers rise andfall randomly over time, then the correlations are weaker than if thereis a consistent long-term trend. In their paper, "AnthropogenicInfluence on the Autocorrelation Structure of Hemispheric-MeanTemperatures," Wigley and colleagues found that the correlations werefar stronger for actual temperature data than for simulations taken fromtwo global climate models. For this project, the models purposelyomitted this century's increase in greenhouse gases, holding themconstant, which means the models replicated only the natural year-to-year variability of the climate system. The implication is that thiscentury's warming trend has overpowered the climate system's naturalvariability. Correlations were higher in the actual temperature data,where the thread of warming runs through the year-to-year record, thanin the constant-greenhouse-gas simulations, where no such thread exists.

In examining the strong year-to-year correlations, Wigley and hiscoauthors considered volcanoes and changes in solar output. According tothe scientists, volcanic eruptions are so infrequent and their effectsso short-lived that "they may be rejected" as an explanation for thedifferences between the data and the models. However, variations insolar output over the last century could have been large enough toaffect some long-term trends. Global temperature rose sharply fromroughly 1900 to 1940, leveled off until the 1970s, and then begananother period of warming that has accelerated in the 1990s. Could thesun be responsible for this century's entire warm-up of about 1 degreeFahrenheit (0.6 degree Celsius) across the globe?

The answer depends largely on how sensitive the earth's climate is. Eachglobal computer model has a unique sensitivity level determined by themodel's physics. This allows the model atmosphere to warm by a givenamount for a given increase in either greenhouse-gas levels or solarinput, or both. In separate experiments using a third, simpler model,Wigley and colleagues subtracted estimates of these possible influencesfrom the actual temperature record. The scientists discovered that, inorder to explain their results using solar effects alone, this model hadto be about six times more sensitive to changes in solar input than isbelieved realistic. Thus, they write, "solar forcing alone isinsufficient to explain the behavior of the observed temperature data."

In contrast, they found, when changes in greenhouse-gas levels areincluded with solar output, then a sensitivity in keeping with currentunderstanding of the climate system was enough to reconcile thecorrelations in actual and simulated temperature data. The results, theynote, "are noticeably degraded if a sensitivity outside this range isassumed."

The authors' work is based on their confidence--bolstered by otherevidence presented in their Science paper--that the two simulations ofclimate with constant greenhouse-gas levels realistically simulate theyear-to-year changes in hemispheric temperature that are distinct fromlong-term trends. These simulations were performed with climate modelsat the United Kingdom's Hadley Centre and at the Geophysical FluidDynamics Laboratory of the National Oceanic and AtmosphericAdministration.

"Our results imply that both anthropogenic and solar forcing havesignificantly affected global climate," conclude the authors. Accordingto Wigley, "These results provide another important piece in the jigsawpuzzle of climate change, strengthening yet further our confidence thatthere has been a discernible human influence on climate. Furthermore,they provide additional evidence that the models used to makeprojections of future climate change are realistic."

NCAR is managed by the University Corporation for Atmospheric Research,a consortium of more than 60 universities offering Ph.D.s in atmosphericand related sciences.

-The End-

Writer: Bob Henson

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