通过海洋的过去和未来研究地球

RENEE MONTAGNE, HOST: One of the most powerful ways to figure out how the Earth will respond to all the carbon dioxide we're putting into the atmosphere is to look back at Earth's history. Paleontologists have spent a lot of time trying to understand a time, more than 50 million years ago, when the planet was much hotter than it is today.

NPR's Richard Harris reports that the news is not all bad, when you take the long view.

RICHARD HARRIS, BYLINE: About 10 million years after the dinosaurs died out, the Earth suffered another huge ecological shock. Carbon dioxide levels in the air soared. The oceans turned more acidic, as carbon dioxide dissolved in the water and turned into carbonic acid. And that's happening again, thanks to our use of fossil fuels.

Dick Norris, a paleontologist at the Scripps Institution of Oceanography, has been trying to figure out what this means for the near future.

DICK NORRIS: Looking in the geologic past - and also, of course, using models to forecast the future - is a little bit like looking into a house through dirty windows, or something. You don't get all the details of what's going on there, but you get sort of the gist.

HARRIS: He and some colleagues review that deep history in today's issue of Science magazine. The bad news from the greenhouse event 56 million years ago is a lot of life in the deep ocean simply went extinct, either from getting too warm, not having enough oxygen, or both. Coral also took a hit, but not as bad.

NORRIS: The reefs disappear, but we don't think it was an extinction of reef corals. Rather, it was the reef corals being outcompeted by other kinds of organisms during, you know, this much warmer period of Earth history.

HARRIS: Algae that didn't mind the heat and acidity may simply have taken over the reefs, and left them flat and boring ecosystems. But the coral laid low for millions of years, until conditions improved again for the majestic branching coral we see today.

Paul Falkowski at Rutgers University is also fascinated with this period of Earth history. He notes that the prehistoric hot spell prevented ocean waters at the surface from circulating into the deep. And that's something he worries could happen again in a sharply warmer world.

PAUL FALKOWSKI: That would lead to a major change in the cycling of many elements on the planet, such as nitrogen, sulfur, carbon and so on.

HARRIS: Those nutrient cycles don't just affect what happens in the ocean. They affect life on land in surprising and uNPRedictable ways, as gases flow from the sea into the air. For example, it was during this ancient hot spell that grasses spread across the Earth and, along with that, the grazing animals that depend on them.

FALKOWSKI: It's hard to believe that a horse could be the size of a Chihuahua. But horses started out about that size, and evolved to be - because of the rise of grasses - the animals that we commonly associate them to be today, large animals.

HARRIS: In short, it was a time of upheaval, with both winners and losers. The wildcard now is that these changes are happening so fast, it's likely to be harder for species to adapt or evolve. And Dick Norris at Scripps says that the pace and severity of the change depends on how long we continue to burn fossil fuels, and allow all that carbon dioxide to flood into the air and the ocean.

NORRIS: If we wait - even a fairly short period of time, like 80 years - to deal with greenhouse gases and the global change problem, then we end up with these really remarkably long ecological impacts on Earth, where it takes, you know, 20,000 years to sort of start to readjust back to something we would be familiar with now.

HARRIS: But he says if we switch quickly to cleaner sources of energy, the world could remain much more like the planet we depend upon today.

Richard Harris, NPR News.