PHILADELPHIA — For years, climate researchers puzzled over a long hot spell around 90 million years ago, when tropical breadfruit trees flourished in Greenland, crocodiles slithered above the Arctic Circle and at least a few dinosaurs roamed Antarctica.
It’s a serious concern because the warmth came at least in part from greenhouse gases, and climate scientists need to understand that period, they say, to better forecast how rising carbon dioxide levels will change our climate in the future. Computer models scientists rely on for those predictions can’t quite account for how steamy it got during that ancient period, says Lee Kump, a geoscience professor at Pennsylvania State University.
In a recent issue of the journal Science he and colleague David Pollard propose a possible solution: a radical change in the number of microorganisms in the ocean that in turn altered the Earth’s cloud cover.
Part of the warming is explained by volcanic eruptions, whose products appear in the geologic record, Kump said. Those would have spewed lots of heat-trapping carbon dioxide into the atmosphere — just not enough to allow crocodiles to avoid freezing in the Arctic.
Their climate models did much better, he said, when he factored in a drop in marine algae. The algae influence the climate by excreting a gas known as dimethylsulfide, which reacts to form other compounds that create cloud-seeding droplets. Fewer algae mean fewer and thinner clouds, allowing more sunlight to reach Earth and add more heat.
If such a die-off happened in the coming decades it could make our global-warming situation worse, he said.
“It’s this lurking amplifier of climate change, and so I think it applies to any period,” Kump said.
The ancient heating in question happened during the geological period known as the Cretaceous, which followed the better known Jurassic. The Cretaceous — from 145 million to 65 million years ago — marks the final period of the reign of the dinosaurs.
Today’s familiar continents had separated by then, and had drifted close to their current positions. Antarctica was at the bottom of the world and Greenland near the top, but they had yet to accumulate their current ice caps.
During the warmest stretch — between 100 million and 83 million years ago — the Earth’s temperature was not just hotter, it was much more uniform from equator to poles.
Could the volcanoes plus a change in aquatic life have led to all that? “It’s a novel idea that can explain a lot of observations,” said Richard Norris, a professor of paleobiology at the Scripps Institution of Oceanography. But he warned that the idea might prove hard to test.
Norris said it’s also possible that even the most respected predictions underestimate the warming we can expect from a given increase in carbon dioxide — an issue that came up at a recent climate meeting at Harvard University. “Most of us at that conference agreed that the likelihood is we’ve been lowballing the sensitivity of climate to carbon dioxide forcing,” he said.
To make things more confusing, Norris said, the volcanoes in question may have erupted too early to account for the Cretaceous warming. “But that makes it all the more mysterious why we had hot-tub temperatures at the tropics and breadfruit trees at high latitudes,” he said. “We’re missing something fundamental in our climate models. Everyone thinks clouds have something to do with the problem.” Others expressed skepticism of Kump’s idea but not of the importance of understanding the distant past. “Studying these past warm climates is the only tool we have for evaluating if these models are any good when we tell them to predict what the future looks like,” said Karen Bice, a climatologist at Woods Hole Oceanographic Institution. But she said there’s no evidence for a drop in the ocean’s microorganism populations as Kump proposes.
“In order for their mechanism to explain the tens of millions of years of warmth we’d need a stressed biosphere during that entire time,” she said — meaning a sudden enough change in conditions to kill lots of algae. To the contrary, she said, the fossil record shows that sulfide-expelling marine organisms adapted to the increased heat and were doing just fine during the Cretaceous.
She said it’s possible that ocean and atmospheric currents at the time distributed heat differently from today, but that idea is hard to test.
Carbon dioxide-based warming can bring on changes that accelerate the process, she said, as appears to have happened in a subsequent and more sudden heat wave in the Eocene period around 55 million years ago. Slow warming from volcanic CO2 started melting sea-floor sediments called methane hydrates. That allowed lots of heat-trapping methane to bubble up from the oceans and into the atmosphere.
Everything from the changes in the Earth’s rotation axis to its orbit around the sun have been connected to past climate fluctuations, but Bice said the really long-term changes like the one seen in the Cretaceous period come from changes in the inorganic carbon cycle. The researchers estimated that carbon dioxide reached about three times present levels back then — something they can’t rule out happening in the next century if fuel burning keeps increasing.
In just the last 100 years we’ve increased the atmospheric carbon dioxide from 280 to 385 parts per million, Bice said. “We’re reaching into the Earth and taking out carbon dioxide by pumping oil and burning it.” Scripps’ Norris said that while scientists can’t yet predict exactly what that will do to various regions of the planet, they do know we’ve already changed the atmosphere radically, and for all practical purposes, irreversibly.
“We’re going to a world that’s not like today,” he said. And if it’s true that the predictions underestimate the potency of carbon dioxide to change the climate, he said, “then as George Bush Senior once said, we are in deep doo-doo.”