March 23, 2000 — Results from a recently completed study of the climate of the past century have suggested that interactions between the atmosphere, ocean, and sea ice system may have played a prominent role in the global warming of the early 20th century, according to NOAA scientists. Using computer models run on high performance supercomputers, scientists at NOAA^s Geophysical Fluid Dynamics Laboratory in Princeton, N.J., conducted a set of six experiments to explore possible causes for the warming in the first half of the century. Their findings will be published in the March 24 issue of the journal Science. "It has been known for some time that the planet has warmed over the last century, with most of the warming concentrated in two distinct periods—from 1920 to 1944, and from 1970 to the present," said Thomas Delworth, research meteorologist. "While previous studies have concluded that the more recent warming is the result of increasing greenhouse gases such as carbon dioxide, the causes of the earlier warming are less clear." Delworth and Thomas Knutson used a comprehensive computer model of the earth^s climate system. They incorporated the effects of increasing concentrations of greenhouse gas and sulfate aerosols that have been observed over the last century. "We did not include the effects of volcanic or solar activity in the experiments, but according to our model simulations the observed early century warming could have occurred in the absence of such changes," said Knutson, also a research meteorologist. "However, we are not ruling out the possibility of such activity being major contributing factors." In one of the experiments they were able to replicate the characteristics of the observed global warming, including the warmings in the two distinct periods (1920-1944 and 1970 to present), and the strong high-latitude warming during the earlier period. Since they did not include the effects of changing volcanic and solar activity, the model results support the hypothesis that an early century warming could have occurred in the absence of such changes. "The fact that only one of the experiments reproduced the early century warming suggests that internal variability of the coupled ocean-atmosphere system, such as fluctuations of the heat transported northward in the Atlantic Ocean over many decades, may have played a very prominent role in early century warming," Delworth said. While an early 20th century warming as strong as the observed one was present in only one of the experiments, all the experiments, and the observations, show a rapid warming during the late 20th century with the latter warming encompassing much of the globe. "The fact that all experiments capture the warming from 1970 on is indicative of a robust response of the climate model to increasing concentrations of greenhouse gases," said Knutson. The researchers also noted that the study further demonstrates the ability of computer models used for studies of global change to reproduce the essential characteristics of past climate variations, thereby increasing the confidence that can be placed in using such models for studies of future climate change.

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