About 100 million years ago, dinosaurs roamed a forest-covered Antarctica. Temperatures were about 50 degrees Fahrenheit, according to proxy records. Carbon dioxide was present in the atmosphere at higher levels than present day.

This period, known as "Cenomanian," poses a problem for today's computer-driven global climate models. These models are algorithmic representations of our climate used by scientists, and they are key for understanding and planning how to deal with a warmer world.

Climate models can also be run "backward" to re-create past climate changes. When Christopher Poulsen, a paleoscientist at the University of Michigan, tried to run climate models to re-create the Cenomanian climate, he ran into a problem that has been puzzling scientists for three decades.

Climate models show that Antarctica in the Cenomanian had subzero temperatures — too cool for dinosaurs. But temperature records, analyzed using cores and other proxies, suggested Antarctic temperatures were much higher.

The models, then, were getting the Cenomanian wrong. Some factor, not represented in climate models, had played an important role in the climate 100 million years ago and warmed Antarctica. The troubling undercurrent to the puzzle was: Could that factor also be affecting future climate change?

The past holds the answer to this problem.

"Past climates are the case studies we can use to understand past and future climate," Poulsen said.

Why weren't the dinosaurs frozen?

Poulsen and his colleagues found that there was indeed a factor that warmed the Cenomanian climate: oxygen.

Their findings, published yesterday in Science, are surprising. Carbon dioxide is well-known as a greenhouse gas, as are methane, water vapor, nitrous oxide and more. But oxygen has not been implicated in warming the planet until now.

The study finds that when oxygen levels in the atmosphere drop, global temperatures get hotter, and vice versa.

Changes in the oxygen content of our atmosphere play out over millions of years, which means it is irrelevant on human timescales. So, although oxygen levels at present are decreasing, the effects would not be felt for many millenia.

Less oxygen warmed them up

Oxygen cools the planet because it scatters the incoming light of the sun back into space.

The atmosphere today contains 21 percent oxygen, which scatters 23 percent of incoming solar radiation back to space.

If the oxygen levels decreased to 10 percent, only 18 percent of the incoming solar radiation would be reflected back to space.

Cenomanian, it is thought, contained only 10 percent oxygen, and levels of CO2 were a steep 1,120 parts per million (present-day CO2 levels are at 403 ppm). Poulsen and his colleagues modeled this scenario using global climate models.

Adding in oxygen brought the models in closer agreement with temperatures reconstructed using proxy records. At the poles, temperatures warmed by an additional 3 to 5 degrees Celsius, allowing forests — and dinosaurs — to exist there.

Oxygen did not account for all of the discrepancy, however. Additional factors may have been in play, Poulsen said.

Other periods of Earth history contain similar discrepancies between models and proxies. Oxygen should be added into climate models to correct for this, Daniel Peppe, a paleontologist at Baylor University, and Dana Royer, a paleontologist at Wesleyan University, wrote in commentary in Science.

This could be challenging since available records for oxygen content over Earth's history are patchy. Scientists should fix this issue, Peppe and Royer wrote.

The study "provides a convincing argument for the importance of developing a robust history of atmospheric oxygen," they wrote.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500