Climate feedbacks: Ice and snow
You need to have Flash Player 10 or above installed to view this video
The amount of sunlight reflected by a surface is known as its albedo. Ice and snow are bright surfaces with high albedo, reflecting about 60–80% of the sunlight falling on them. So ice on water or snow on land keeps the Earth cooler than it would otherwise be. An initial change such as a rise in temperature can trigger a feedback – melting some snow and ice leads to the exposure of low-albedo land or water surfaces, which increases the amount of sunlight absorbed by the Earth’s surface, leading to a further rise in temperature as a result. This is known as the ice-albedo feedback.
The ice-albedo feedback amplifies temperature changes in regions where ice or snow cover is present. A rise in temperature, leading to ice or snow melt, uncovers water in the case of sea ice, or land and vegetation in the case of land-based ice and snow. These surfaces are darker than the ice and snow and so have lower albedo, meaning that less sunlight is reflected and more is absorbed. In turn this raises temperatures further, resulting in a positive feedback effect. This amplification also works in reverse if temperatures drop, leading to ice or snow formation, higher local albedo and a further decrease in temperature. Scientists estimate that, globally, the ice-albedo feedback adds about 50% to an initial temperature change, turning a rise of 1 °C into 1.5 °C of warming, or a drop of 1 °C into 1.5 °C of cooling.
When ice melts, much of the resulting meltwater flows directly into the surrounding sea or onto the surrounding land. However, in the case of a large expanse of ice, the meltwater sometimes has no immediate path by which it can flow off. This can lead to pools of meltwater accumulating on the ice surface. Water is darker and less reflective than ice, so these pools have lower albedo than the ice surface that they cover. As a result, the meltwater pools are heated by their absorption of sunlight and this heats the ice underneath the pools as well, accelerating the melting. Conversely, if temperatures drop and some of the meltwater refreezes, the increased albedo leads to further cooling and accelerates the freezing process.
Meltwater on the surface of a glacier can simply accumulate, sometimes forming large pools known as proglacial lakes. But this isn’t always the case. Meltwater pools have lower albedo than the ice, absorbing more sunlight, which heats both the water and the ice below it. This causes accelerated melting of the ice directly beneath the pool, which can lead to the formation of a tunnel – known as a moulin – descending into the glacier’s interior. If sufficient meltwater is produced at the surface, the moulin can flow all the way to the base of the glacier and accumulate between the ice and the land beneath it. In the case of a moving glacier, this water can act as a lubricator, decreasing the friction between the ice and the land and increasing the glacier’s speed.
In summer the reflection of sunlight by sea ice prevents the ocean below the ice from absorbing solar energy, keeping the ocean cooler. But in winter, sea ice acts as an insulating blanket, preventing heat loss from the relatively warm ocean to the colder atmosphere. A change in ice cover changes this insulation. For example, reduced sea ice cover leaves more of the ocean surface exposed. In winter this causes more heat to be transferred from the ocean to the atmosphere, causing a rise in air temperatures. However, increased air temperatures also increase the amount of heat lost to space. As a result, scientists think changes in sea ice cover may produce some negative as well as positive feedbacks, though the overall effect of changing sea ice acts as a positive climate feedback.