Tuesday, November 22, 2011

Hothouse Earth Is On The Horizon

An era of ice that has gripped Earth's poles for 35 million years could come to an end as extreme global warming really begins to bite. Previously unknown sources of positive feedback — including "hyperwarming" that was last seen on Earth half a billion years ago— may push global temperatures high enough to send Earth into a hothouse state with tropical forests growing close to the poles.
Climate scientists typically limit themselves to the 21st century when predicting how human activity will affect global temperatures. The latest predictions are bolder, though: the first systematic forecasts through to 2300 are beginning to arrive. They follow four possible futures, including one in which we rapidly cut emissions and another in which we burn fossil fuels into the 22nd century (Climatic Change, DO!: 10.1007/ s1o584-011-0157-y).
Chris Jones of the UK Met Office in Exeter says that unpublished results suggest the "burn everything" scenario could see atmospheric carbon dioxide levels reach 2000 parts per million—the figure today is 388 ppm. That pulse of CG, could lead to a global temperature rise of 10°C. Temperatures this high were last seen in the Eocene, 34 million years ago, says Paul Pearson of Cardiff University in the UK. Conditions were so different back then that the Canadian High Arctic was populated by plants that are now found in the south-eastern US (Proceedings of the Royal Society B, DO!: 10.1098/ rspb.2011.1704).
The Eocene marked the end of a hothouse that had begun in the Cretaceous (see chart). Throughout this time there was no ice at the poles; Antarctica was once populated by dinosaurs. Might the predicted rise in temperature be enough to see a return to an ice-free world?
The poles will warm much more than the tropics, says Tim Lenton of the University of Exeter, UK, so the Arctic could well lose all its ice. But Antarctic ice would probably survive, thinks Andrew Watson of the University of East Anglia, in Norwich, UK, because Antarctica is isolated from the rest of the continents.
In fact, Antarctica may have gained its ice when it became cut off from Australia during the Eocene and lost the warming influence of equatorial currents. Plate tectonic models predict that Antarctica will remain isolated from the other continents for at least the next 250 million years (New Scientist, 17 September, p 16). Even so, Antarctica's icy future may not be secure. The long-term climate models that go up to the year 2300 are missing key positive feedbacks that could send global temperatures towards levels high enough to melt even an isolated Antarctica.
In particular, the release of methane from melting Arctic permafrost has not yet been factored in. Methane is a potent greenhouse gas, but remains in the atmosphere for only to years on average before it reacts with hydroxyl radicals in the air to form CO,. However, a large release of methane from melting permafrost could swamp the hydroxyl supply, allowing the methane to linger in the atmosphere for 15 years or more, further amplifying the warming (Global Biogeochemical Cycles, DOI: to.1029/2010GB003845). Some feedbacks never before considered might also come into play. Pearson says that in the future oceans may store less carbon. Normally some atmospheric carbon is lost at sea, buried in the carcasses of tiny marine animals. But sediment from the Eocene contains little carbon, suggesting that this process failed during the last hothouse (Paleoceanography, DOT: 10.1029/2005PA0o123o).
To work out why, Pearson looked at fossils of foraminifera, microscopic shelled marine animals. The tiny shells contain a chemical record of the position the animals occupied in the water column when they were alive. He found that Eocene foraminifera lived closer to the ocean surface than they do today, suggesting there was little food to sustain deeper-dwelling species.
Pearson thinks the warmer temperatures allowed bacteria at the ocean surface to metabolise faster, recycling carbon before it could sink and feed foraminifera living at depth. "if we warm the planet now, we switch on our bacteria," he said last month at a Royal Society discussion meeting in London.
A warming climate will also see trees and other large plants spreading north into the Arctic, says Bette Otto-Bliesner of the US National Center for Atmospheric  Research in Boulder, Colorado, who also attended last month's Royal Society event. Plants are darker than snow, so they absorb more of the sun's radiation. When Otto-Bliesner plugged the effect into a climate model of the Arctic, it got 3°C warmer.
Then there's hypenvarming. Ed Landing of the New York State Museum in Albany coined the term to describe the spiralling temperatures seen during the Cambrian period as a result of rising sea levels. Vast areas of the continents were covered with shallow seas during the Cambrian, which began 542 million years ago, because sea levels were sometimes tens of metres higher than today. Sea water absorbs more of the sun's heat than land, so swamping the continents caused the planet to warm up even more. Sea temperatures reached 40°C and oxygen levels in the water crashed (Palaeogeography, Palaeoclimatology, Pa laeoecology, DOI: to.1p16/j.palaeo.2o11.o9.005).
Something similar could happen again today. "These effects will operate as sea level rises to an appreciable degree and floods continental areas," agrees Thomas Algeo of the University of Cincinnati in Ohio. However, the effect today may not be as strong as it was in the Cambrian, says Lee Kump of Pennsylvania State University in University Park. There were no land plants back then, so the continents were more reflective and flooding them had a bigger effect.
Pearson and Landing's processes have not yet been plugged into any climate models so we do not know how significant they will be to our future. Pearson emphasises that hothouse Earth is far from inevitable. "We can prevent this happening," he says. But as researchers dig deeper into the factors that influence global climate, it is becoming increasingly clear that global warming might be about to get much more extreme.


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