Read more: “Climate change: What we do – and don’t – know“
If the melting of large ice sheets is a very slow process that takes thousands of years, there may be time to get the planet’s temperature back down before sea level rises by more than a few metres. If the ice sheets respond rapidly to warming, however, our descendants are going to live in a world with a dramatically altered coastline.
We have little clue how much room we have for manoeuvre. Past melting episodes provide little help. Melting can be rapid: as the last ice age ended, the disappearance of the ice sheet covering North America increased sea level by more than a metre per century at times. .
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To predict exactly how quickly sea level will rise, we would first need to know how much hotter the planet is going to get. As we have seen, we don’t. Next, we would need to know how much of this extra heat will get transferred to the ice sheets. Not so long ago, it was thought warmer air would be the main cause of melting, but now it seems .
That is bad news, because warm water melts ice much faster than warm air. Warm currents can melt the floating ice shelves that hold back ice on land. Worse still, in places like west Antarctica, ice sheets rest on land that is below sea level, and so could be exposed directly to warm water.
Until recently the IPCC was predicting that the vast ice sheets of Antarctica would grow over the 21st century as warming increased the water content of the atmosphere, leading to higher snowfall. The picture on the ground already looks rather different. Satellite measurements show that both Antarctica and Greenland are already losing large quantities of ice, and . If current trends continue – we don’t know if they will – the loss of ice from these sheets alone would raise sea level 0.5 metres by 2100. Extend that picture to the entire globe and many glaciologists now think seas could rise a metre or more by 2100. That would be bad news for many small-island nations, cities such as London, New York and Shanghai, and whole swathes of densely populated, low-lying land in places such as the Netherlands, Bangladesh and Florida.
The worst-case scenario is that strong positive feedbacks may kick in once melting gets under way. As ice sheets melt, for instance, their surfaces will get lower, exposing them to warmer air. If so, sea-level rise could accelerate rapidly.
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