Ice thaws will bring short-lived cooling (Image: Jeremy Walker/naturepl.com)

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 planet falls by a degree or two. This is according to unpublished work by climate scientist James Hansen of NASA’s Goddard Institute for Space Studies in New York City.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, by the time this occurs the temperature differences produced by the “iceberg cooling effect” could lead to even more climate chaos in a world already being 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. The planet would still be accumulating heat because of higher carbon dioxide levels – it is just that vast amounts of heat energy would go 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.


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 heat energy will go into defrosting the freezer rather than warming the air.

There are two ways in which disintegrating ice sheets can cool surface temperatures. Melting ice “soaks up” heat energy without raising the water temperature and an injection of fresh water can change ocean circulation patterns and transfer heat from the surface to deeper waters.

Most climate scientists think the freezer door will remain firmly shut this century, but not Hansen. He has long 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 that a full paper is being prepared for publication, but would not discuss the details.

Assuming a 0.6-metre sea level rise from 2010 levels by 2065, which could inundate many coastal cities, Hansen’s model suggests that the average global surface temperature would be just 1.5 °C warmer than in preindustrial times, compared with 1.9 °C without the iceberg cooling effect. With a massive 1.4-metre sea level rise by 2080, the surface temperature would fall to 0.9 °C above preindustrial levels, instead of rising to 2.2 °C above. 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.

Climate modellers have long done experiments looking at the complex effects of melting ice sheets, says Sigman. These experiments typically show regional cooling, but in Hansen’s simulation the effect is much greater. The likely reason is that his simulation assumes a more rapid acceleration of ice loss, doubling every 10 years.

Most other climate scientists think the ice sheets will melt slowly – this is what happened at the end of past ice ages. Hansen thinks this logic is flawed. The reason that sea level rose slowly in the past, he writes, is because the planet warmed slowly. After the last ice age, for example, it took 10,000 years for the average global temperature to rise about 4 °C.

Now the world is on course to warm this much in fewer than 200 years.