How high will global sea levels rise?

Estimates of global sea level rise by 2100 have fluctuated wildly in recent decades – from more than two metres to as little as 31 centimetres.

The rubbery figures have been a source of ammunition for climate change sceptics and consternation for policy makers – undermining their ability to plan ahead. Most of the blame can be levelled at Antarctica. Its 30 million km3 ice sheet holds 90 per cent of the world’s fresh water. If it all melted, sea levels would rise 60 metres. By contrast, a melt of the Greenland ice sheet, the world’s second largest, would contribute six metres.

Predicting the rate of ice melt in Greenland is relatively straightforward; Antarctica’s melt is anything but.

The stability of Antarctica’s ice sheet depends on the floating ice shelves at its fringes. They act as plugs halting the movement of the ice sheet. It is the dynamics of that interaction that have been hard to fathom.

The most recent IPCC report (published in 2013) estimated Antarctic ice would contribute just 4 centimetres to global sea levels by the end of the century, leading to an overall rise of 70 centimetres by the end of the century under “business as usual” emissions scenarios.

That estimate, according to Nick Golledge, an Antarctic ice sheet modeller at Victoria University of Wellington, in New Zealand, was extremely conservative, because the report’s authors “just didn’t know enough about fast dynamics in ice sheets”.  Scientific understanding has moved on since then, confirming Antarctica will contribute way more than  4 centimetres by 2100. “The latest research is converging on a figure more like half a metre,” Golledge says.

Researchers discovered how unpredictable ice sheet dynamics could be in 2002 when a 3,500 km2 chunk of the Larsen B ice shelf disintegrated. Located on the Antarctic Peninsula, the continent’s most northerly and warmest point, it had appeared perfectly stable.

While the melting of Larsen B didn’t make any direct difference to sea levels (just as the melting of an ice block won’t raise the level of your drink), it was the canary in the coal mine.

Before the Larsen B event, scientists thought the ice sheet moved in a very steady fashion, says Matt King, who researches Antarctica’s contribution to sea level rise at the University of Tasmania. “You could kick it as much as you like and it didn’t really do much… Now we have a completely different view.” {%recommended 1336%}

What led to the collapse of Larson B was the rising summer temperatures in the Antarctic peninsula, with the mercury spending more and more time above zero in the years beforehand. As a result, vast pools of meltwater formed on top of the thinning ice, fracturing it and pouring into cracks that ultimately broke apart the whole chunk.

Summer surface melting is a well-understood process, and the dominant factor in Greenland’s melt – making it highly predictable. The process can explain what is happening in the northern tip of Antarctica, but it can’t account for the changes seen in the rest of the southern continent, where temperatures perpetually remain well below freezing.  Here the peril seems to come from below.

One “hotspot” in East Antarctica is the rapidly thinning ice shelf fringing the large Totten glacier. In a study published in Science Advances in December 2016, a CSIRO-led team confirmed warm water from the deep ocean is slipping up onto the Antarctic continental shelf and reaching Totten via deep canyons in the sea floor.

As the ice warms, thins and cracks, yet another feedback mechanism might come into play, according to modelling.

“If there’s one thing ice hates, it’s warm water – it’s tremendously efficient at melting ice,” King says.

That warm water is not just a threat to the floating ice shelves. In West Antarctica the ice sheet sits on bedrock that is below sea-level, raising the risk warmer water could stream in and undercut the ice sheet.

As the ice warms, thins and cracks, yet another feedback mechanism might come into play, according to modelling by Robert DeConto at the University of Massachusetts and David Pollard at Pennsylvania State University. Each time a piece of ice shelf breaks off, the remaining “ice cliffs” will be taller. “That’s inherently unstable,” says Tony Worby, who heads the Antarctic Climate and Ecosystems Cooperative Research Centre in Hobart. Sooner or later the cliffs will crumble under their own weight.

Understanding these processes is just the beginning of the ice modellers’ work. Predicting when, where and how rapidly they will occur, to forecast how much sea levels will rise, is quite another. So far the estimates of Antarctic contribution to sea level rise by 2100 remain highly variable. A key challenge is the lack of data to feed into the models. Huge sections of East Antarctica’s coastal zone remain effectively unmapped. “We don’t know where the bedrock is or where the warm water can flow,” King says.

In 2019, Australian researchers will take delivery of a new icebreaker able to map the seafloor on Antarctica’s fringe. The ship will carry an unmanned underwater vehicle capable of navigating under the ice shelves. “Very quickly we’ll start to build a picture of what the seafloor looks like around the continental shelf around Antarctica,” King says.

Within a decade, researchers should be more confident in their predictions. “Assuming,” King notes, “there’s not more unknown aspects of the ice sheet.”  We’ve been surprised before.