The cryosphere is the frozen part of the Earth System, comprised of both land and sea ice. Sea ice forms in the high latitudes where the atmosphere is cold enough to allow the salty ocean to freeze. The distribution of sea ice is constantly fluctuating from season to season. In the winter months polar regions experience up to 24 hours of darkness, temperatures stay below zero for months and extensive sea ice growth occurs. In contrast, during summer the Arctic experiences 24 hours of daylight, temperatures warm allowing some of the winter sea ice to melt.

The cycle of sea ice growth and retreat is very sensitive to changes in the climate. Warming in the winter will reduce the amount of ice that freezes and warming in the spring and summer increases the length of the melt season (when temperatures are above 0°C). Satellite observations of September Arctic sea ice cover (the month when Arctic sea ice reaches its minimum amount) reveal an average decrease in Arctic sea ice extent of ~3% per decade since 1979, as shown in the graph (Figure 1). To contrast, Antarctic sea ice is currently growing by ~1% per decade over the same time period as the Antarctic is partly protected from rising sea surface temperatures by the presence of the circumpolar current, a cold water current that orbits the continent and prevents warmer water from the equator from reaching the point of ice extent. Satellite observations also show a significant decrease in the multi-year ice (ice that remains frozen all year round, currently about 30% of the total volume). Maintaining healthy levels of multi-year ice is particularly important as it will be built up year on year becoming thicker and less likely to be lost entirely the next summer.

Figure 1. Graph showing the decline in Arctic sea ice extent since satellite observations began. Source:

While melting sea ice will not impact sea level, as the ice displaces surrounding water and so does not affect the total volume of the ocean as it melts, the distribution of sea ice can affect the Earth System in other ways. For example, sea ice regulates heat transfer between the ocean and atmosphere, acting as an insulator to heat loss in winter and limiting the absorption of solar radiation in summer. It also acts as a barrier to the exchange of water, gases and aerosols.

Melting sea ice does result in a significant freshening of the surface Arctic Ocean, affecting the density of the surface water. Along with affecting the mixing of surface waters, this freshening may also have implications for the formation of deepwater in the Arctic and the “thermohaline circulation”: the global conveyer belt of ocean currents where cold, salty water sinks to the ocean bottom in the Arctic before spreading southwards and is replaced at the surface by northward flowing warm, salty water from the equatorial regions. Ocean currents and the heat they transport have an important influence on regional climates and weather patterns, changes to the ocean circulation as a result of melting sea ice may therefore have a direct impact on the lives of humans.

Perhaps the greatest influence melting sea ice will have on Earth’s climate is through the positive feedback loop known as the sea ice albedo feedback (albedo refers to the reflectivity of a given surface or the proportion of incoming solar radiation that is reflected by that 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 sea ice albedo feedback is an example of a positive feedback. Surfaces such as ocean or forests have a low albedo (they are relatively dark) and they reflect very little of the incoming solar radiation. Sea ice on the other hand, has a high albedo, reflecting 50-70% of the incoming solar radiation that hits it. This means that the albedo of the Earth’s surface is reduced as sea ice turns into open ocean, leading to more incoming solar radiation penetrating into the ocean and warming the waters. This warming encourages further sea ice melt and a positive feedback loop is created. As sea ice is a key indicator of global climate change, monitoring changes in ice extent and thickness are extremely important. Such observations also help us ensure our Earth system models are accurate, giving us more confidence in future climate change projections made with these models.

Read more about Cryosphere sea ice in UKESM1