tag:theconversation.com,2011:/fr/topics/continential-ice-sheet-7323/articlesContinential ice sheet – The Conversation2016-07-12T19:41:14Ztag:theconversation.com,2011:article/599962016-07-12T19:41:14Z2016-07-12T19:41:14ZCold and calculating: what the two different types of ice do to sea levels<figure><img src="https://images.theconversation.com/files/126463/original/image-20160614-12948-157moyq.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Knowing where the ice comes from can help work out what it will do to sea levels.</span> <span class="attribution"><span class="license">Author provided</span></span></figcaption></figure><p>It was back in 250ʙⅽ when Archimedes reportedly stepped into his bathtub and had the world’s first <a href="https://en.wikipedia.org/wiki/Eureka_(word)">Eureka</a> moment – realising that putting himself in the water made its level rise. </p>
<p>More than two millennia later, the <a href="http://arstechnica.com/science/2016/07/recent-antarctic-sea-ice-growth-boosted-by-la-ninas/?comments=1">comments sections of news stories</a> still routinely reveal confusion about how this same thing happens when polar ice melts and sea levels change. </p>
<p>This is in marked contrast to the confidence that scientists have in their collective understanding of what is happening to the ice sheets. Indeed, the <a href="https://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_Chapter04_FINAL.pdf">2014 Assessment Report of the Intergovernmental Panel on Climate Change</a> reported “very high confidence” that the Greenland Ice Sheet was melting and raising sea levels, with “high confidence” of the same for the Antarctic Ice Sheet. </p>
<p>Despite this, commenters below the line on news stories frequently wonder how it can be true that Antarctica is melting and contributing to sea-level rise, when satellite observations show Antarctic ice expanding.</p>
<p>Unravelling the confusion depends on appreciating the difference between the two different types of ice, which we can broadly term “land ice” and “sea ice” – although as we shall see, there’s a little bit more to it than that. The two different types of ice have very different roles in Earth’s climate, and behave in crucially different ways. </p>
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<img alt="" src="https://images.theconversation.com/files/126202/original/image-20160610-29209-17b9pep.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/126202/original/image-20160610-29209-17b9pep.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=372&fit=crop&dpr=1 600w, https://images.theconversation.com/files/126202/original/image-20160610-29209-17b9pep.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=372&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/126202/original/image-20160610-29209-17b9pep.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=372&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/126202/original/image-20160610-29209-17b9pep.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=467&fit=crop&dpr=1 754w, https://images.theconversation.com/files/126202/original/image-20160610-29209-17b9pep.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=467&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/126202/original/image-20160610-29209-17b9pep.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=467&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Sea levels rise when ice resting on land, <em>grounded ice</em>, melts (often after forming icebergs). Floating sea ice that melts has a very important role in other areas of our climate system.</span>
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<h2>Land ice</h2>
<p><a href="https://theconversation.com/au/topics/ice-sheets">Ice sheets</a> form by the gradual accumulation of snow on land over long periods of time. This “grounded” ice flows in glaciers to the ocean under the influence of gravity, and when it arrives it eventually melts. If the amount of ice flowing into the oceans is balanced by snowfall on land, the net change in global sea level due to this ice sheet is zero. </p>
<p>However, if the ice begins to flow more rapidly or snowfall declines, the ice sheet can be out of balance, resulting in a net rise in sea level. </p>
<p>But this influence on sea level is only really relevant for ice that is grounded on land. When the ice sheet starts to float on the ocean it is called an “ice shelf”. The contribution of ice shelves to sea-level rise is negligible because they are already in the sea (similar to an <a href="http://www.physlink.com/education/askexperts/ae389.cfm">ice cube in a glass of water</a>, although the ocean is salty unlike a glass of water). But they can nevertheless play an important role in sea-level rise, by governing the rate at which the grounded ice can discharge into the oceans, and therefore how fast it melts.</p>
<h2>Sea ice</h2>
<p>When viewed from space, all polar ice looks pretty much the same. But there is a second category of ice that has effectively nothing to do with the ice sheets themselves. </p>
<p>“Sea ice” is formed when ocean water is frozen due to cooling by the air. Because it is floating in the ocean, sea ice does not (directly) affect sea level. </p>
<p>Sea ice is generally no more than a few metres thick, although it can grow to more than 10 metres thick if allowed to grow over many winters. Ice shelves, on the other hand, are hundreds of metres thick, <a href="https://www.youtube.com/watch?v=IxfORXWph2Q">as seen when an iceberg is created and rolls over</a>. </p>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/IxfORXWph2Q?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">A big breakup.</span></figcaption>
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<p>In the ocean around Antarctica, almost all the sea ice melts in the southern hemisphere spring. This means that every year an area of ocean twice the size of Australia freezes over and then melts – arguably the largest seasonal change on our planet. </p>
<p>So, while ice sheets change over decades and centuries, the time scale of sea ice variability is measured in months. </p>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/MLCfF7BLii4?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Antarctic sea ice grows and shrinks dramatically over the course of the year. These changes do not directly affect sea level. Land ice changes are slower but do affect sea levels, at least until the land ice becomes afloat.</span></figcaption>
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<p>The seasonal cycle of Arctic sea ice is much smaller. This is because the Arctic retains much more of its sea ice in the summer, and its winter extent is limited by land that surrounds the Arctic Ocean.</p>
<h2>What is happening to land ice?</h2>
<p>The two great ice sheets are in Greenland and Antarctica. Thanks to <a href="http://science.sciencemag.org/content/338/6111/1183">satellite measurements</a>, we now know that since the early 1990s both have been contributing to sea-level rise.</p>
<p>It is thought that most of the Antarctic changes are caused by seawater melting the ice shelves faster, causing the land ice to flow faster and hence leading to sea-level rise as the ice sheet is tipped out of balance.</p>
<p>In Greenland, both surface and ocean melting play important roles in driving the accelerated contribution to sea levels. </p>
<h2>What about sea ice?</h2>
<p>Over the last four decades of satellite measurements, there has been a rapid <a href="http://www.the-cryosphere.net/9/269/2015/tc-9-269-2015.html">decrease and thinning</a> of summer Arctic sea ice. This is due to human activity warming the atmosphere and ocean. </p>
<p>In the Antarctic there has been a modest <a href="http://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_Chapter04_FINAL.pdf">increase</a> in total sea ice cover, but with a complex pattern of localised increases and decreases that are related to changes in winds and ocean currents. What’s more, satellite measurement of changes in sea ice thickness is much more difficult in the Antarctic than in the Arctic mainly because Antarctic sea ice has a lot of poorly measured snow resting on it.</p>
<p>The Southern Ocean is arguably a much more complex system than the Arctic Ocean, and determining humans’ influence on these trends and projecting future change is challenging. </p>
<p>Observations of the changes happening in the Arctic and Antarctic reveal complex stories that vary from place to place and over time. </p>
<p>These changes require ongoing monitoring and greater understanding of the causes of the observed changes. And public confusion can be avoided through careful use of the different terms describing ice in the global climate system. It pays to know your ice sheets from your sea ice.</p><img src="https://counter.theconversation.com/content/59996/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Matt King receives funding from the Australian Research Council and the Department of Environment.</span></em></p><p class="fine-print"><em><span>Ben Galton-Fenzi works for the Australian Antarctic Division. He receives funding from the Department of the Environment.</span></em></p><p class="fine-print"><em><span>Will Hobbs is employed by the Antarctic Climate and Ecosystems Cooperative Research Centre, and receives funding from the Australian Research Council. </span></em></p>Polar ice isn’t all the same - it can be divided roughly into “land ice” and “sea ice”. What matters most for sea levels is how much ice slides off the land and melts in the sea.Matt King, Professor, Surveying & Spatial Sciences, School of Land and Food, University of TasmaniaBen Galton-Fenzi, Senior Scientist, Australian Antarctic DivisionWill Hobbs, Physical Oceanographer, Antarctic Climate and Ecosystems Cooperative Research Centre, University of TasmaniaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/280122014-06-13T16:10:17Z2014-06-13T16:10:17ZVolcanic rift valley under Antarctica hotter than expected<figure><img src="https://images.theconversation.com/files/51074/original/3qdnnkt7-1402670488.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Mount Erebus is Antarctica's obvious volcano, but there is more below the ice.</span> <span class="attribution"><a class="source" href="http://commons.wikimedia.org/wiki/File:Mount_Erebus_craters,_Ross_Island,_Antarctica_(aerial_view,_18_December_2000).jpg">Josh Landis/USAP</a></span></figcaption></figure><p>The Thwaites glacier is one of the most rapidly changing in Antarctica. It’s been the focus of considerable attention in recent weeks, after scientists suggested that this sector of the huge West Antarctic Ice Sheet was <a href="http://www.sciencemag.org/content/344/6185/735.abstract">already on route towards collapse</a> due to warming ocean temperatures. A major collapse of this part of the ice sheet could have dire consequences worldwide, with a global sea level rise of potentially up to 1m. Some models suggest this could take place comparatively rapidly, within a few centuries.</p>
<p>But hidden beneath the kilometres of ice in this rapidly changing part of the continent is a largely unexplored geological feature: the <a href="http://www.awi.de/en/research/research_divisions/geosciences/geophysics/projects_marine_geophysics/west_antarctic_and_southern_pacific_region/">West Antarctic Rift System</a>, a strip of volcanic activity thought to extend for more than 3,000km across the Antarctic continent, provides further heat to melt the ice sheet from below.</p>
<p>The <a href="http://education.nationalgeographic.com/education/encyclopedia/rift-valley/?ar_a=1">rift</a> is where the Earth’s crust has in the past been stretched, pushing magma near to the surface and causing widespread volcanic activity. It’s important to establish the amount of volcanic heat from the rift in order to more accurately predict the response of the Thwaites glacier and entire West Antarctic Ice Sheet to the effects of a warming climate and ocean. </p>
<p>Direct measurements of the geothermal heat flux from the rift are however difficult and expensive to obtain – the overlying ice sheet is in places 4km thick. Estimates of geothermal heat flux available so far are derived mainly from satellite magnetic data or seismology that struggle to resolve the regional details required to understand what effects the heat would have on the ice sheet. </p>
<p>In a paper <a href="http://www.pnas.org/content/early/2014/06/04/1405184111.full.pdf+html">published</a> in Proceedings of the National Academy of Sciences of the US, researchers from the <a href="http://www.ig.utexas.edu/">Institute for Geophysics</a> at The University of Texas, Austin, report a new method of estimating the geothermal heat flux beneath Thwaites glacier. Using radar data to map how water flows under the ice sheet and estimate ice melting rates, they have identified significant sources of high geothermal heat.</p>
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<a href="https://images.theconversation.com/files/51067/original/ywb6p4yk-1402664609.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/51067/original/ywb6p4yk-1402664609.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/51067/original/ywb6p4yk-1402664609.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=659&fit=crop&dpr=1 600w, https://images.theconversation.com/files/51067/original/ywb6p4yk-1402664609.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=659&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/51067/original/ywb6p4yk-1402664609.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=659&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/51067/original/ywb6p4yk-1402664609.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=828&fit=crop&dpr=1 754w, https://images.theconversation.com/files/51067/original/ywb6p4yk-1402664609.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=828&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/51067/original/ywb6p4yk-1402664609.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=828&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Radar-detected geothermal flow underneath Thwaites, with areas where flow exceeds 150 milliwatts per sq m (dark triangles) and 200 milliwatts per sq m (light triangles). Letters show high melt areas, in the western-most tributary (C), adjacent to the Crary mountains (D), and in the upper central tributaries (E).</span>
<span class="attribution"><a class="source" href="http://www.jsg.utexas.edu/news/2014/06/researchers-find-major-west-antarctic-glacier-melting-from-geothermal-sources/">Schroeder/Blankenship/Young</a></span>
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<p>According to lead author <a href="http://dustinmschroeder.com/">Dustin Shroeder</a> (who now works at NASA), these appear to be distributed over a much wider area than previously thought. The minimum average geothermal heat flow beneath Thwaites glacier is about 100 milliwatts per square metre, with some hotspots reaching 200 milliwatts per square metre. This is considerably higher than the average heat flow of the Earth’s continents, at less than 65 milliwatts per square metre. </p>
<p>Detecting such high geothermal heat flux within this part of West Antarctic Rift means that there is likely to be much more water beneath Thwaites glacier. The presence of water can lubricate and speed up the flow of glaciers, even in the deep interior of the ice sheet kilometres beneath the surface. Several models used to simulate the present ice sheet assume much lower geothermal heat for the region, or less variability in heat flow than what the study team has proposed. As Schoreder said, the combination of heat and water interacting with the glacier’s base could “threaten the stability of Thwaites glacier in ways that we never before imagined.”</p>
<p>We need to carry out further geophysical research, with other methods, to validate the predictions of such high geothermal heat that the team has derived from radar data analysis alone. <a href="http://onlinelibrary.wiley.com/doi/10.1029/JB088iB03p02316/abstract">Magnetic</a> and gravity methods have been used, for example, to study rifts and geothermal heat flux patterns in many regions worldwide, and could be applied here to obtain an independent perspective.</p>
<p>We also need more data and further computer modelling to try to understand more fully what impact this high geothermal heat has on the flow of water underneath glaciers, how this affects ice sheet dynamics, and ultimately how this will further our understanding of how Antarctica is responding to a warming world.</p>
<p>This remarkable study is focused on the variability of geothermal heat beneath the Thwaites glacier that may have a bearing on ice dynamics in this vulnerable part of Antarctica. The level of heat in the rift system inferred from radar probe data doesn’t imply that ocean warming driven by global warming is not a significant contributor to the ice loss seen in this part of West Antarctica. </p>
<p>This study does not address ice sheet stability directly at all – it neither supports nor refutes the recent studies’ conclusions that the Thwaites glacier is already on route to collapse. But better understanding of how geothermal heat affects the flow of water underneath glaciers will allow us to develop improved models to better predict ice sheet behaviour, and ultimately how Antarctica is responding to a warming world.</p><img src="https://counter.theconversation.com/content/28012/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Fausto Ferraccioli receives funding from NERC/BAS and as head of airborne geophysics in BAS works in related fields.</span></em></p>The Thwaites glacier is one of the most rapidly changing in Antarctica. It’s been the focus of considerable attention in recent weeks, after scientists suggested that this sector of the huge West Antarctic…Fausto Ferraccioli, Airborne Geophysics Group Leader, British Antarctic SurveyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/267322014-05-15T14:31:08Z2014-05-15T14:31:08ZMelting glaciers deform the Earth’s mantle and crust<figure><img src="https://images.theconversation.com/files/48521/original/3xsy9xrr-1400087323.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Going, going, gone: Antartica's Larssen ice shelf.</span> <span class="attribution"><a class="source" href="http://www.fotopedia.com/items/flickr-5973798247">MODIS/NASA</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>The significant retreat of elements of the West Antarctic ice sheet such as the Thwaites glacier <a href="http://www.sciencemag.org/content/early/2014/05/12/science.1249055">recently reported</a> in the journal Science suggests a possible sea level rise of 3 to 3.7m. This is a huge increase in sea level that cannot fail to affect us all.</p>
<p>With so much of the world’s population <a href="http://ngm.nationalgeographic.com/2013/09/rising-seas/if-ice-melted-map">living in low-lying cities</a>, it seems more important now than ever that we’re able to make accurate predictions of sea level change. However, to do this we need to know not only how much ice is being lost from Antarctica and other frozen regions, but how the Earth’s surface deforms in response to that loss. </p>
<p>The trillions of tonnes of ice locked in ice sheets weighs heavily on the bedrock underneath it. As the mass of ice grows and shrinks, over geological time the Earth is compressed by the weight, or springs back as it melts, which in turn displaces more or less of the oceans and affects the extent of sea level rise.</p>
<p>This is tricky to calculate because the Earth deforms in two ways. There is an <a href="http://www.antarcticglaciers.org/glaciers-and-climate/sea-level-rise-2/recovering-from-an-ice-age/">instantaneous elastic rebound</a> of the crust, like a spring bouncing back after a weight has been taken off. This is followed by a very slow uplift due to movement of the mantle, the layer of semi-molten rock underlying the roughly 100 mile-thick hard outer crust. </p>
<p>Although we think of the mantle as rock, it flows like a highly viscous fluid when subject to changes in surface load. This typically occurs over many thousands of years, but the timescale depends on the viscosity of the mantle and how easily it can flow. As it is many miles below the surface and exists at temperatures of up to 1,000°C, we cannot directly measure its properties – so we have to infer them indirectly.</p>
<h2>The big rebound</h2>
<p>Our <a href="http://www.sciencedirect.com/science/article/pii/S0012821X14002519">recent study</a> of the <a href="http://www.antarcticglaciers.org/glaciers-and-climate/glacier-recession/recent-change/">Northern Antarctic Peninsula</a> published in the journal Earth and Planetary Science Letters was a sort of fast-forward version of this problem. The mantle in this region is commonly thought to be less viscous and more mobile than in other parts of Antarctica. This means that it responds much more quickly to changes in pressure as ice is unloaded at the surface. We combined records of the significant loss of ice that has taken place here in the past ten to 15 years with GPS measurements of the Earth’s rebounding motion to unravel this problem on a regional scale.</p>
<p>Over the past few decades, several ice shelves at the very tip of the Antarctic Peninsula have collapsed, with the result that glaciers flowing into where the shelves once were now deliver more ice into the sea. Satellites record the glaciers’ elevation frequently, so scientists can determine the rate of thinning and how much ice is lost from the land into the sea.</p>
<p><a href="http://nora.nerc.ac.uk/506925/">GPS transmitting stations</a> fixed to the Antarctic bedrock record deformation of the Earth – and several sites have been installed in the Northern Antarctic Peninsula to measure the rebounding due to ice loss in this area. What we have discovered from this data is how quickly the Earth is moving 250 miles below the surface.</p>
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<img alt="" src="https://images.theconversation.com/files/48599/original/zswmw3rf-1400147815.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/48599/original/zswmw3rf-1400147815.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/48599/original/zswmw3rf-1400147815.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/48599/original/zswmw3rf-1400147815.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/48599/original/zswmw3rf-1400147815.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=501&fit=crop&dpr=1 754w, https://images.theconversation.com/files/48599/original/zswmw3rf-1400147815.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=501&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/48599/original/zswmw3rf-1400147815.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=501&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">GPS transmitting stations, tracking Antarctica’s rise and fall, and rise.</span>
<span class="attribution"><span class="source">Matt King</span></span>
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<p>The instantaneous elastic rebound expected from the ice loss did not tally with the GPS records, leaving an unexplained and rapid rise of up to 15mm per year. This means that the remaining rebound has to come from the flow of the mantle pushing underneath. </p>
<p>We used the measurements to determine that the mantle viscosity had to be around ten times lower than previously thought for this region – and consequently has responded much more quickly to the lightening load at surface level – in scales of years rather than centuries. We can almost see it happening and being able to take accurate GPS measurements is incredibly useful. It also enables us to predict the amount of rebound much closer to the thinning glaciers than our current GPS sites, and based on what we know, this could be as much as 47mm per year.</p>
<p>This has really significant implications for the interpretation of GPS records, which are often used to verify models used in the prediction of sea-level rise and for measurements of the Earth’s gravitational field, which are used to detect ongoing changes in the mass of ice sheets.</p>
<p>So the glacier retreat reported in West Antarctica will also be driving further, rapid bedrock uplift. This could be as much as 5cm (2 inches) per year, but the historical GPS record in this region is still too short to provide detailed answers. As Antarctic glaciers continue to thin, the unloading of ice will provide an opportunity to probe the Earth in other parts of Antarctica. </p>
<p>While this is fortunate for earth scientists to pursue their interests, given that sea level rise affects everyone it’s not something we really welcome.</p><img src="https://counter.theconversation.com/content/26732/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Grace Nield receives funding from NERC. She studies at Newcastle University.</span></em></p>The significant retreat of elements of the West Antarctic ice sheet such as the Thwaites glacier recently reported in the journal Science suggests a possible sea level rise of 3 to 3.7m. This is a huge…Grace Nield, PhD student, Newcastle UniversityLicensed as Creative Commons – attribution, no derivatives.