Levels of CO2 Still Good Predictor of Global Warming

One way to anticipate the progress of global warming is to examine the evidence for changes in global climate over the last 550 million years and compare it with levels of carbon dioxide, a professor at Yale said in a recent journal article.

“Carbon dioxide (CO2) causes warming. The more CO2, the more warming,” said Robert Berner, the Alan M. Bateman Professor of Geology and Geophysics at Yale University and co-author of an article published in a recent issue of the journal Science. “In general there is a good correlation of long periods of intense warming in the geological past with high levels of CO2 and periods of long-lived continental glaciation with periods of low CO2.”

Berner and a colleague, Thomas Crowley, a professor of oceanography at Texas A&M University, said variations in CO2 are very important for explaining past ice ages. “The bottom line remains the same - CO2 is still very important to the whole process of climate change. We just don’t have the whole story yet,” Crowley said.

Geologists have long known that on time scales of tens of millions of years, intervals of continental glaciation were interspersed with times of little or no ice. “The magnitude of warmth during these warm intervals is impressive,” the researchers said. “At times during the Cretaceous, about 65 to 145 million years ago, duck-billed dinosaurs roamed the northern slope of Alaska. Deep and bottom waters of the ocean, now near freezing, could reach a balmy 15 C degrees. This was a time of high estimated levels of CO2.”

In the 1980s, paleoclimate data and geochemical and climate models suggested that such long-term variations in climate were strongly influenced by natural variations in the CO2 content of the atmosphere. But there has been more recent conflicting data. For example, during certain periods of high levels of CO2, calculated temperatures in the tropics were quite cold.

In an attempt to resolve the issue, Berner and Crowley compared estimates of the heating due to changes in CO2 and solar radiation over the past 550 million years with both the continental glaciation record and low latitude temperature estimates by other researchers.

They said there were discrepancies between the presence or absence of major continental glaciation and estimated tropical temperatures, and to explain this disagreement more investigation is needed. Possible explanations include errors in estimating tropical temperatures, and/or complexities of paleoclimate modeling such as unusual continental configurations, changes in ocean circulation that could have altered ocean heat transport, and the response of the atmosphere-ocean circulation during times of low continental ice volume.

“The good agreement between the CO2 record and continental glaciation continues to support the conclusion that CO2 has played an important role in long term climate change,” Berner and Crowley said. “For factors responsible for the presence or absence of continental ice, the CO2 model works very well.”

“Predictions of future greenhouse warming due to increases in atmospheric CO2 should not be clouded by some unresolved disagreements involving the distant geologic past,” the authors said.

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