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Greenland is the largest island in the world and on it lies the largest mass of ice in the Northern Hemisphere. If all that ice melted, the sea would rise over 7 meters.
But this won’t happen is it? Not very soon, but figuring out how much of the ice sheet could melt over the next century is a critical and urgent question that scientists are trying to address using sophisticated numerical models of how the ice sheet interacts with the rest of the climate system. The problem is that the models aren’t that good at reproducing recent observations and are limited by our poor knowledge of the detailed topography of subglacial terrain and the fjords over which ice flows.
One way around this is to see how the ice sheet has responded to climate change in the past and compare it to the model’s projections for the future for similar temperature changes. This is exactly what my colleagues and I have done in a new study now published in the journal Nature Communications.
We examined the three largest glaciers in Greenland and used historical aerial photographs combined with measurements the scientists had made directly over the years to reconstruct how the volume of these glaciers had changed over the period from 1880 to 2012. The approach is based on the idea that the past can help inform the future, not only in science but in all aspects of life. But just like other “classes” in history, the climate and the earth system in the future will not be a carbon copy of the past. However, if we understand exactly how sensitive the ice sheet has been to changes in temperature over the past century, this can provide useful guidance on how it will respond in the next century.
We found that the three largest glaciers were responsible for 8.1 mm of sea level rise, about 15% of the contribution of the entire ice sheet. In the period of our study the sea has risen globally by about 20 cm, about the height of an A5 booklet, and of this, about the width of a finger is entirely thanks to the melting of the ice from those three glaciers of Greenland.
Melt as usual
So what does it tell us about the future behavior of the ice sheet? In 2013, a modeling study by Faezeh Nick and colleagues also examined the same “three big” glaciers (Jakobshavn Isbrae to the west of the island and Helheim and Kangerlussuaq to the east) and predicted how they would react in different future climate scenarios. The most extreme of these scenarios is called RCP8.5 and assumes that economic growth will continue unabated throughout the 21st century, resulting in mean global warming of around 3.7 ° C above today’s temperatures (around 4.8 ° C above pre-industrial or from 1850).
This scenario has sometimes been referred to as Business As Usual (BAU) and there is an active debate among climate researchers about how plausible RCP8.5 is. Interestingly, however, according to a recent study by a group of US scientists, this may be the most appropriate scenario until at least 2050. Due to something called polar amplification, the Arctic is likely to warm up by more than twice the global average, with climate models indicating approximately 8.3 ° C warming over Greenland in the most extreme scenario, RCP8.5.
Despite this dramatic and terrifying rise in temperature, Faezeh’s modeling study predicted that the “big three” would contribute between 9 and 15mm to sea level rise by 2100, only slightly more than we got from a warming of 1.5 ° C in the 20th century. How can it be? Our conclusion is that the models are in error, even including the latest and most sophisticated available that are used to assess how the entire ice sheet will respond to the next century of climate change. These models appear to have a relatively weak link between climate change and ice melt, when our results suggest it is much stronger. Projections based on these models are therefore likely to underestimate how much the ice sheet will be affected. Other lines of evidence support this conclusion.
What does all this mean? If we continue along that very frightening RCP8.5 trajectory of increasing greenhouse gas emissions, it is very likely that the Greenland ice sheet will start melting at speeds we have not seen for at least 130,000 years, with disastrous consequences for sea level and the many millions of people living in low-lying coastal areas.
Jonathan Bamber, professor of physical geography, University of Bristol
This article was republished by The Conversation under a Creative Commons license. Read the original article.
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