Climate pioneer warns sea level could rise 9m

James Hansen (Reuters)
James Hansen (Reuters)

One of the world's leading experts on climate change has warned sea levels could rise "several metres" in the next 50 to 150 years.

The findings of Dr James Hansen's latest research paint a much gloomier picture than the United Nations' Intergovernmental Panel on Climate Change (IPCC).

While the IPCC predicts a sea level rise of up to 1m by 2100 unless greenhouse gas emissions are constrained, Dr Hansen says prehistory suggests it could be much worse.

The last time Earth was as warm as it is now was about 120,000 years ago, between ice ages. In scientific journal Atmospheric Chemistry and Physics, Dr Hansen and 18 colleagues say although it's not a direct analogue for our present climate, sea levels were between six and nine metres higher then.

Temperature differences across the world were much more pronounced, causing massive storms in the north Atlantic stronger than 2012's Hurricane Sandy.

What will accelerate the rise of the oceans, according to Dr Hansen, is melting of the ice caps from beneath by warmer water.

"Injection of fresh meltwater reduces the density of the upper ocean wind-stirred mixed layer, thus reducing the rate at which cold surface water sinks in winter at high latitudes," the study notes.

"The increased stratification due to freshwater injection causes heat to be retained at ocean depth, where it is available to melt ice shelves."

Much of the research was originally released last year, but wasn't peer-reviewed until now.

NASA's chief cryosphere (frozen regions of the Earth) scientist Tom Wagner welcomed it as a "provocative" study and "one of the few times when all of these different fields have been combined", but others have suggested its findings are "exponential" and perhaps extreme.

The latter view is one taken by local climate change sceptic and former leader of the National and ACT parties, Don Brash.

"James Hansen, the guy they quoted in this thing, is the leading climate change alarmist. He wasn't saying anything very new," Dr Brash said on the Paul Henry programme this morning.

"This still doesn't prove anything. It proves the climate is getting warmer; it doesn't establish what causes that."

But the study mentions "carbon" 42 times and its scientific notation "CO2" 142 times, and calls it the "principal determinant of Earth's climate state" and "the radiative control knob that sets global mean temperature".

Carbon concentration in the current atmosphere is about 400 parts per million. Previous interglacial periods have peaked at about 280ppm.

Chemical analysis of fossils has shown about 50 million years ago, when carbon levels peaked as high as 1000ppm due to natural causes, temperatures in Antarctica were around what they are in modern California.

Dr Brash isn't convinced, saying the Earth was warmer in the Middle Ages and Roman times.

"They were growing grapes in the UK at that point," he says.

But studies have shown this was only the case in lands bordering the north Atlantic, for example in Europe and the Mediterranean, and the result of changes to oceanic currents, not carbon and other greenhouse gas emissions.

Dr Hansen says 2degC of warming above pre-industrial levels -- the target the IPCC is aiming for -- is too high, and emissions will need to be cut by about 6 percent every year to avoid catastrophic change.

"Among the top experts there's agreement that this is very urgent -- we can't continue on this path hoping that emissions will go down; we have to take actions."

Dr Hansen was a pioneer of climate change awareness in the 1970s and 1980s, after studies on how Venus' atmosphere might have once been similar to Earth's, before a runaway greenhouse effect pushed its surface temperature above 450degC.