Sea level rise could lead to a cooler, stormier world

A catastrophic rise in sea level before the end of the century could have a hitherto-unforeseen side effect. Melting icebergs might cool the seas around Greenland and Antarctica so much that the average surface temperature of the entire planet falls by a few degrees, according to unpublished work by climate scientist James Hansen of NASA's Goddard Institute for Space Studies in New York City.

While it might sound welcome, the temperature differences produced by the "iceberg cooling effect" could lead to even more climate chaos in a world already devastated by extreme weather. Winter storms, for instance, are powered by the temperature differences between the poles and the equator, so there might be storms of unprecedented ferocity.

And the temporary cooling would be deceptive. Due to the greenhouse effect, the planet as a whole would still be accumulating heat - it's just that vast amounts of heat would be going into melting ice and warming water. "It's a redistribution of heat energy," says Daniel Sigman of Princeton University, who studies the end of the last ice age and was not involved in Hansen's work.

Freezer door

To visualise the cooling effect, imagine being shut in a stiflingly hot kitchen. You could cool the air by flinging open the freezer and letting the food defrost. The kitchen as a whole will not lose heat if there is nowhere for it to escape to, but some of the heat energy will go into defrosting the freezer rather than warming the air.

Most climate scientists think the "freezer door" will remain firmly shut this century, but not Hansen. He has longed warned that there could be a huge rise in sea level this century and, with colleagues Makiko Sato and Reto Ruedy, he recently simulated the possible effects. Hansen included a brief summary of some of the results in an analysis of Greenland ice loss released in December. He told New Scientist a full paper is being prepared for publication, but would not discuss the details.

Assuming a disastrous 0.6-metre sea level rise by 2065, Hansen's model suggests the average global surface temperature would be just 1.5??C warmer than in preindustrial times, compared to 1.9??C without the iceberg cooling effect. With a massive 1.4-metre rise by 2080, the surface temperature would fall by 0.9??C, instead of rising by 2.2??C. Although most of the world would remain much warmer than now, northern Europe might cool to preindustrial levels and the UK might actually be chillier.

Other climate scientists are reluctant to comment before seeing the full details, but Sigman points out that climate modellers have long done experiments looking at the complex effects of melting ice sheets. These experiments also typically show regional cooling, but in Hansen's simulation the effect is much greater. The likely reason for the difference is because his simulation assumes a much more rapid acceleration of ice loss, doubling every 10 years.

Most other climate scientists think the ice sheets will only melt slowly, largely because this is what happened at the end of past ice ages. Hansen, however, thinks this logic is flawed. The reason that sea level only rose slowly in the past, he writes, is because the planet only warmed slowly. After the last ice age, for instance, it took 10,000 years for the average global temperature to rise around 4??C. Now the world is on course to warm this much in less than 200 years.

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