In the ancient past temperatures on Earth appeared to have been much warmer than today. It is possible that temperatures may rise as high as then based on current climate change projections.

The new study, by National Center for Atmospheric Research (NCAR) scientist Jeffrey Kiehl, will appear as a Perspectives piece in this week's issue of the journal Science.

Building on recent research, the study examines the relationship between global temperatures and high levels of carbon dioxide in the atmosphere tens of millions of years ago.

It warns that, if carbon dioxide emissions continue at their current rate through the end of this century, atmospheric concentrations of the greenhouse gas will reach levels that last existed about 30 million to 100 million years ago, when global temperatures averaged about 29 degrees F higher than now (in the high eighties F).

During the later portion of the Cretaceous, from 65 to 100 million years ago, average global temperatures reached their highest level during the last 200 million years. It is estimated that the average temperature was about 82 F. Even worse, locations bordering the Atlantic Ocean rose to about 95 F. The constant peak of high temperature was related to the fact that there were high carbon dioxide formulations.

So the Earth has been a lot warmer than it is at present. There are many factors that can impact global temperatures. Over the millennia the sun has gradually been getting warmer. There are also cycles driven by predictable changes in the Earth orbit known as Milankovitch cycles.

Kiehl, in his present study, said that global temperatures may gradually rise over the next several centuries or millennia in response to the carbon dioxide. Elevated levels of the greenhouse gas may remain in the atmosphere for tens of thousands of years, according to recent computer model studies of geochemical processes that the study cites.

The study also indicates that the planet's climate system, over long periods of times, may be at least twice as sensitive to carbon dioxide than currently projected by computer models, which have generally focused on shorter-term warming trends. This is largely because even sophisticated computer models have not yet been able to incorporate critical processes, such as the loss of ice sheets, that take place over centuries or millennia and amplify the initial warming effects of carbon dioxide.

The Perspectives article pulls together several recent studies that look at various aspects of the climate system, while adding a mathematical approach by Kiehl to estimate average global temperatures in the distant past. Its analysis of the climate system's response to elevated levels of carbon dioxide is supported by previous studies that Kiehl cites.

Kiehl focused on a fundamental question: when was the last time Earth's atmosphere contained as much carbon dioxide as it may have by the end of this century?

If society continues on its current pace of increasing the burning of fossil fuels, atmospheric levels of carbon dioxide might reach about 900 to 1,000 parts per million by the end of this century. That compares with current levels of about 390 parts per million, and pre-industrial levels of about 280 parts per million.

Kiehl drew on recently published research that, by analyzing molecular structures in fossilized organic materials, showed that carbon dioxide levels likely reached 900 to 1,000 parts per million about 35 million years ago.

At that time, temperatures worldwide were substantially warmer than at present, especially in polar regions—even though the Sun's energy output was slightly weaker. The high levels of carbon dioxide in the ancient atmosphere kept the tropics at about 9-18 degrees F above present day temperatures. The polar regions were some 27-36 degrees F above present-day temperatures.

Kiehl applied mathematical formulas to calculate that Earth's average annual temperature 30 to 40 million years ago was about 88 degrees F which is substantially higher than the pre-industrial average temperature of about 59 degrees F.

Computer models successfully capture the short-term effects of increasing carbon dioxide in the atmosphere. But the record from Earth's geologic past also encompasses longer-term effects, which accounts for the discrepancy in findings. The eventual melting of ice sheets, for example, leads to additional heating because exposed dark surfaces of land or water absorb more heat than ice sheets.

Because carbon dioxide is being pumped into the atmosphere at a rate that has never been experienced, Kiehl could not estimate how long it would take for the planet to fully heat up.