The polar ice sheets are losing ice and sea levels are rising globally. Of this, there can be no doubt. One of the key challenges facing scientists is to understand better how much and how fast these events are happening. The task of predicting the impacts of climate change largely falls to computer models but these are only as good as their ingredients. The most reliable way of testing models is to compare their projections with what has actually been observed to have happened.
When it comes to predicting how much and how fast the Antarctic and Greenland ice sheets are losing ice, it is only recently that it has become possible to accurately compare the findings of computer models with observations. In the early days, observations of ice sheet change could only be done by land or air, neither of which could offer a complete picture. In 2010, the European Space Agency launched CryoSat, a satellite project devised by a British-team of scientists at University College London. CryoSat is dedicated to measuring changes in the Earth’s polar ice. Critically, CryoSat gives scientists the capability to survey Antarctica and Greenland in their entirety, and how they are changing over time.
The ability to measure the volume of ice contained in the ice sheets means that scientists are able to determine both how much the Greenland and Antarctic ice sheets are melting, and how much they are contributing to global sea level rise (individually and collectively). The total volume of ice in the Antarctic ice sheet is equivalent to around 50-60m sea level rise globally. For the Greenland ice sheet, the figure is 5-6m. However, it is the latter which is melting faster, with Greenland contributing c. 12mm to sea level rise over the past 25 years, and Antarctica contributing c. 7mm. The disparity between the two is explained by the different drivers of ice loss: in Greenland, the effects of warmer air temperatures are widespread, whereas in Antarctica, it is the warming of surrounding ocean waters that is driving ice melt in isolated areas.
By making use of measurements from older satellite sensors that were not originally designed to survey the poles, scientists have been able to estimate that in the 1990s, combined ice sheet losses in Antarctica and Greenland contributed to around 10% of global sea level rise. Today, the contribution is around 30-40%. That is because ice sheet loss is accelerating, with approximately three times as much ice being lost now compared to twenty years ago.
The ability to observe these changes first-hand from space is proving to be important for assessing the accuracy of computer models that attempt to predict how fast the polar ice sheets are disappearing. For example, the United Nations Intergovernmental Panel on Climate Change (IPCC) has produced a series of scientific assessments of 21st Century sea level rise based on climate models since the 1990s. Strikingly, NERC’s Centre for Polar Observation and Monitoring has found using CryoSat data that in the earliest assessments, IPCC models were significantly underestimating how much the Antarctic ice sheet was melting (in fact it was believed to be growing).
However, as the models have improved in complexity, their projections have started to converge with actual observations. Indeed, the latest IPCC assessment in 2013 showed a surprising level of agreement between climate model predictions of sea level rise due to polar ice sheet losses and the satellite record. This agreement, lends confidence to projections of future sea level rise, as the models upon which they are based are now acknowledged to be reliable. Growing confidence in the models is critical for convincing policymakers to act.
Given the improvements to both modelling and observation of how they polar ice sheets are changing, scientists are increasingly in agreement on several points. Sea levels are rising globally, averaging around 3.5mm every year. Of that, around 1mm is attributed to polar ice sheet losses and associated acceleration of glaciers towards the sea. The rest is driven by other factors such as increased run-off into the oceans from inland waters, land subsidence and emergence, changes in ocean circulation, and thermal expansion of the oceans. At the current rate, sea levels can be expected to rise by a minimum of 35cm over the next century, but actually, much higher rates of rise are almost certain.
If all the ice covering Antarctica and Greenland were to disappear, sea level rise would be in excess of 60m, enough to bring the waters of the River Thames in line with the bottom of the clock face on Elizabeth Tower. While this is unlikely to happen for millennia, if even a fraction of the ice sheets is lost, the implications for sea levels globally are still significant enough to threaten coastal communities around the world.
Currently, in Antarctica, the acceleration towards the sea of ten glaciers that are part of the West Antarctic Ice Sheet are by far the main contributors to sea level rise. There, warmer water is being driven under the part of the ice sheet that floats on the sea, melting it from below. Once that floating ice sheet breaks away, the glaciers that it is buttressing will be free to accelerate towards the sea, speeding up the rate of sea level rise. The Amundsen Sea area, which contains both the Pine Island Glacier and the Thwaites glacier, and accounts for around 35% of the drain from the West Antarctic ice sheet, is thought to contain enough ice to contribute 1.1m sea level rise.
Yet uncertainty also remains, particularly when it comes to making projections about how quickly sea levels are likely to rise. While the contribution of factors such as thermal expansion and glaciers is well understood, changes in the ice sheets are much harder to predict. Since there are only two ice sheets, it is impossible to use statistics to inform judgements.
Furthermore, changes in ice sheets play out over very long periods of time. Monitoring by satellite over the last 30 years amounts to only a very short period of time in terms of ice sheet change. It is also difficult to predict how the complex interactions with the surrounding oceans are likely to unfold. Lastly, there is the possibility that high impact/low likelihood events in the next 100 years will drastically alter the pace of change. That has left many scientists to estimate that while the best estimate for sea-level rise may be between 30 and 80cm of sea level rise by 2100, there is a real possibility that it could exceed 1m.
Even though the ice sheets are highly unlikely to disappear anytime soon, the threat of almost a metre of sea level rise by the end of the century leaves London in a potentially precarious position. Incremental sea level rise over the coming years and decades is fairly straightforward to prepare for, but the Government also needs to consider how small rises in sea level rise can increase the threat posed by storm surges that magnify the risk of an extreme flooding event in the capital.
The Environment Agency has been planning for such an eventuality since 2002, when it set up the Thames Estuary 2100 (TE2100) project, partly using funds from the European Union, to manage flood risk across the Thames Estuary area. It was the first major project in the UK with climate change adaptation at its core. Those involved asked what could be done to manage the risk of flooding over the next century without underestimating the risk or exaggerating the threat. Whereas the best guess of scientists was that sea levels could rise by around 90cm by 2100, TE2100 considered it necessary to prepare for a worst-case scenario of a 4m storm surge event as well. That was eventually revised down to 2.7m after further consultations with ice sheet experts.
TE2100 set out three adaptation strategies that planners could move between depending on whether projections of future sea level rise and the risk of storm surges changed. Improvements to existing defences were likely to prove sufficient for dealing with a small level of sea level rise. Beyond that, the options were to maximise water storage in the Thames Estuary area, upgrade the existing system, or build a new Thames Barrier further downstream.
The TE2100 plan now underway has recommended the second or third options. Sea level rise appears to be inexorable, even in the case of a rapid transition globally to a low-carbon economy. The TE2100 project looked at long term implications identifying that even with a new barrier/barrage at a threshold of 5m sea level rise, the entire flow of the River Thames would need to be pumped to the sea. For now, TE2100 is focussed on maintaining existing flood defences over the next 15 years or so. In the 35 years after that, upgrades are likely to be required. From 2070 onwards, further improvements will be needed, including possibly the construction of a new Thames Barrier. Of course, those time horizons might change if new scientific evidence comes to light. For that reason, TE2100 is kept under regular review, with the next major review due in 2020.
So, it seems London is likely to remain safe for now, although similar plans for the rest of the United Kingdom remain underdeveloped. The Adaption sub-Committee of the Committee on Climate Change is beginning to consider what might be done across the rest of the country, but questions remain about whether the matter is being pursued with sufficient urgency.
Meanwhile, the UK Government is continuing to support research (including a joint project with the US National Science Foundation) into better understanding how much and how fast the Amundsen Sea area of the West Antarctic ice sheet is melting. The European Space Agency and the European Commission are considering whether to support a long-term continuation of the CryoSat mission. In 2019, the IPCC will publish a special report on the state of the cryosphere and the oceans.
This paper was prepared by Dr Duncan Depledge (Director, APPG for the Polar Regions Secretariat), and endorsed by James Gray MP (Chairman, APPG for the Polar Regions).
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This is not an official publication of the House of Commons or the House of Lords. It has not been approved by either House or its committees. The views expressed here are the author’s own and do not represent those of the All-Party Parliamentary Group for the Polar Regions.