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The Independent says we’re headed for “catastrophic" sea level rise: Here’s what you should know about melting ice sheets

• 07 Jan 2013, 17:15
• Roz Pidcock

Catastrophic?

Based on a new study, today's Independent claims scientific experts believe melting ice sheets contribute more to sea level rise than previously thought. But the idea that past sea level rise predictions have underestimated melting ice sheets is already well known in the scientific community. Here's your guide to how scientists' understanding of the ice sheets has improved in recent years and where the new study fits in.

According to today's Independent:

"Glaciologists fear they may have seriously underestimated the potential for melting ice sheets to contribute to catastrophic sea level rise in coming decades".

The study doesn't use the term "catastrophic" - so the Independent has rather sensationalised the paper's findings. But that point aside, let's look at the research that sparked the piece.

Expert judgement

Earth scientists from the University of Bristol carried out a survey of 26 leading climate scientists, asking them to give detailed judgements on how rising global temperatures will affect the Greenland and Antarctic ice sheets in the coming century.

The study, published in the journal Nature Climate Change, uses a technique that "mathematically pools" expert opinion. It's been used before in engineering medicine and the natural sciences - but not in assessing the weight of opinion on earth's melting ice sheets.

The collective opinion of the scientists surveyed is that melting ice sheets are likely to contribute an average of 29 cm to sea level rise by 2100. Some predict even more, with a five per cent chance it could be as much as 85 cm. Together with other factors causing sea level rise, such as the expansion of water as it warms, the experts suggest this could conceivably push total sea level rise to over a metre by 2100.

The Greenland and Antarctic ice sheets hold 99.5 per cent of the earth's glacier ice and are the largest potential source of future sea level rise.

As lead author of the new research, Professor Jonathan Bamber, explains:

"[Our research] shows glaciologists believe there is a one in 20 chance of sea levels rising by a metre or more by 2100, and a metre in sea level rise is really very serious."

Were past estimates too low?

Every five or six years, the Intergovernmental Panel on Climate Change (IPCC) produces an in-depth assessment of the state of research in all areas of climate science, including sea level rise. The IPCC published its last assessment in 2007 (AR4) and the next one (AR5) is due early next year.

Scientists know that rising global temperatures are causing ice sheets to melt, but estimating their contribution to sea level rise is more difficult. At the time of AR4, scientists were uncertain about the mechanisms causing ice loss - so much so that they couldn't be sure whether the Antarctic was gaining or losing ice overall.

But with several years' more data and substantial technical improvements, scientific knowledge has moved on. A recent analysis of 19 years of satellite data shows that melting ice sheets have caused sea level to rise by about 11 millimetres since 1992, which is towards the upper end of the IPCC's 2007 predictions.

Satellite measurements show both ice sheets together contributed a total of 11 mm to sea level rise between 1992 and 2011. Source: Shepherd et al., (2012)

Weather or climate change?

But this isn't the end of the story. While there's no doubt that rising global temperatures caused by human activity are contributing to melting ice sheets, scientists are still uncertain about how much of the recent changes could be due to natural fluctuations in the climate system, as the new paper notes.

What's more, the physical processes that influence ice loss differ across the ice sheets. For example, Western Antarctica and the Antarctic Peninsula have both lost ice since 1992, whereas Eastern Antarctica gained ice because of increased snowfall.

In the face of such uncertainty, it appears the AR5 draft takes a conservative position. It estimates that, on average, ice sheets will contribute 11 cm to sea level by 2100 - less than the 29 cm suggested by the group of experts in the new study. The AR5 report says:
"Larger values cannot be excluded, but current scientific understanding is insufficient for evaluating their probability"

Extra information

So does the new survey of climate scientists add anything new? It provides an analysis of expert judgement rather than predictions of sea level rise based on new data - but it gives us some useful extra information. As Bamber says in the paper:

"[This study] is not a substitute for improved process understanding; nor is it intended to remove uncertainty, but rather to quantify it, given limitations in available information."

It's not news within the scientific community that previous IPCC projections of sea level rise were probably too low. At the time, the AR4 report recognised ice sheet melt as the most significant remaining uncertainty. Nevertheless, the new study highlights how far the science has progressed and is another way of evaluating where uncertainties still remain in a rapidly developing, but still complex, field.