UH research: feedbacks to lead to unpredicted warming?

A massive pulse of carbon dioxide entered the world's atmosphere 55 million years ago, in association with a significant rise in global temperature.

But it seems to have gotten a lot hotter than it should have.

(Image: The scientific drilling ship JOIDES Resolution, seen off Diamond Head, conducted ancient sediment samples during the Integrated Ocean Drilling Program, which provided the ship picture. In the inset are some of the deep sea sediment cores collected. The dark red/brown color is a clay section amid lighter calcium carbonate deposits. The clay layer represents the beginning of a period of global warming and ocean acidification 55 million years ago. Inset credit: J.C. Zachos).

University of Hawai'i researchers who studied the ancient climate event say there are major puzzles about how the atmosphere responded to the input of carbon—puzzles that may be important in the Earth's current human-caused pulse of carbon dioxide.

One issue is understanding feedback mechanisms. You can calculate how much warmer the atmosphere ought to get by adding carbon dioxide to it. But back in the Palaeocene-Eocene Thermal Maximum (PETM), 55 million years ago, the temperature got significantly warmer than the amount of new carbon dioxide justified.

Something else must have come into play.

If you're in a soapbox cart, and you release the brake, you slowly start rolling forward. But if your friends see you release the brake, they step up to give you a shove, and you end up going faster. The response of the friends acts as a feedback mechanism—increasing the impact of simply releasing the brake.

What was the feedback mechanism during the Palaeocene-Eocene Thermal Maximum? During that period temperatures rose 5 to 9 degrees Celsius. But there wasn't enough carbon dioxide to justify that much temperature rise, said Richard Zeebe, the University of Hawai'i oceanographer who led a team that looked into the question.

Their study is published in the journal Nature Geosciences. (Zeebe, R. E., Zachos, J. C., and Dickens, G. R. Carbon dioxide forcing alone insufficient to explain Paleocene-Eocene Thermal Maximum warming. Nature Geoscience, Advance Online Publication, July 13, 2009.)

The team collected deep sea sediment cores that date back to the Palaeocene-Eocene period.
The implication for modern humans is whether, if we keep dumping carbon dioxide into the air, some feedback mechanism will kick in, causing rapid climate warming—and associated issues with sea level, rainfall, storms and so forth.

The initial source of the Palaeocene-Eocene carbon isn't entirely clear. It came from some natural carbon reservoir, but “the source remains an open issue,” Zeebe's paper says.

Other researchers have recreated what temperatures globally were doing that period, and they report the 5 to 9 degree Celsius increase. That's 9 to 16 degrees Fahrenheit—a lot.

“If the temperature reconstructions are correct, then feedbacks and/or forcings other than atmospheric CO2 caused a major portion of the PETM warming. The origin of this extra warming is unknown at present,” the Zeebe paper says.

One possibility is that the warming caused the release of other greenhouse gases, which in turn prompted more warming.

Clearly, things were different on the planet 55 million years ago, but the possibility of a big, unexpected hike in temperatures is something we need to know about.

“This gap needs to be filled to confidently predict future climate change,” the authors write.


© Jan TenBruggencate 2009