The cryosphere is the frozen part of the Earth System, comprised of both land and sea ice. Land ice includes all sizes of terrestrial ice, from glaciers to ice sheets. Ice sheets have the potential to have a very significant impact on our global sea level, mainly due to their massive size. Ice sheets are accumulations of glacial ice, built up over thousands of years and covering areas of over 50,000 km². Today, the distribution of ice sheets is relatively small compared to in the past. 24,000 years ago, at the height of the Last Glacial Maximum, large ice sheets covered most of North America, Northern Europe, Scandinavia, and Patagonia. Unlike sea ice which displaces the surrounding seawater and therefore does not affect the total volume when melted, land ice like ice sheets will add extra water to the oceans as they melt. In the present, the Earth’s ice sheets together make up 99% of the world’s freshwater ice – a fantastic volume that if melted could increase global sea level by 68.3 m, submerging large areas of land including several of the world’s major cities. Whilst this level of change will not happen within our lifetimes, it is still very important to monitor changes in ice sheet coverage as they play a more extensive role within the wider Earth System and are vital to understanding future changes to our planet.
Ice sheets advance and retreat according to the glacial mass balance, which is the difference between ablation (melting) and accumulation (addition of snow). If accumulation is greater than ablation then the ice sheet will advance and cover a wider area, but if ablation outweighs accumulation, then more ice is melting than is being replaced by precipitation and the ice sheet will retreat. As our global temperature increases, two things will occur: there will be slightly more precipitation as the warmer air allows more evaporation and cloud formation; and there will be much greater melting of ice, particularly at low altitudes. This means that our ice sheets are destined to retreat in future.
Meanwhile, this loss of ice may trigger a positive feedback cycle due to the influence of albedo (proportion of incoming solar radiation that is then reflected by a surface). Feedback loops are self-perpetuating cycles with the outputs becoming the new inputs. In a positive feedback loop, A increases B which in turn increases A again so the magnitude increases with each cycle. A negative feedback is the opposite with A increasing B which decreases A again. The ice-ocean feedback loop is an example of positive feedback. Surfaces such as ocean or forest have a low albedo as they reflect very little of the incoming solar radiation. Ice on the other hand, has a high albedo and reflects 50-70% of the incoming solar radiation that hits it. The positive feedback loop begins as our ice sheets melt, causing the albedo of the land surface to decrease and more solar radiation to be absorbed, warming the atmosphere even further. This will cause further melting and the positive feedback cycle will continue to exacerbate the change to Earth’s climate.
Moreover, ice sheets also have an impact on the oceans. Melting land ice will add freshwater into the salty polar oceans affecting the salinity, and consequently the density of this ocean water. This could have an impact on the formation of deepwater and the circulation of the thermohaline circulation: the global conveyer belt of ocean currents where cold, salty water sinks at the poles to form deep water and brings warmer water from the equator to higher latitudes. Ocean currents are very important to regional climates and weather patterns meaning changes to the ocean circulation as a result of melting ice sheets would have a direct impact on the lives of humans.
In the past, the significance of the role of ice sheets in the Earth system has been overlooked in the making of climate models. UKESM aims to change that, integrating the cryosphere into the larger Earth System Model, in order to more accurately predict the future changes to the Earth’s climate.