4.2.3 Cryospheric feedbacks

The most important feedback associated with the cryosphere is that due to the high albedo of snow and ice (see Table 1.3). If the temperature increases in response to a perturbation, snow and ice will tend to melt, leading to a reduction in the surface albedo (Fig. 4.12). As a consequence, the fraction of incoming solar radiation absorbed by the Earth will increase, leading to an greater warming. The quantification of this feedback, which is referred to as the snow-and-ice-albedo feedback or the temperature-albedo feedback, depends on the exact change in the surface albedo in response to the temperature change. The albedo can be influenced by the changes in the surface covered by ice or snow (and thus also by leads) or by modifications of the snow and ice surface properties caused by surface melting (in particular the formation of melt ponds at the sea ice surface).

The snow-and-ice-albedo feedback has a significant impact on a global scale, with an estimated value based on the recent model simulations performed in the fourth assessment report of the IPCC of $\lambda$ _$\alpha$ = 0.26 Wm^-2K^-1. The standard deviation of the results is 0.08 Wm^-2K^-1, i.e. the same magnitude as the standard deviation of the combined water-vapour and lapse-rate feedbacks. However, its influence is larger at high latitudes where it can be responsible for about half of the response to a doubling of the CO₂ concentration in the atmosphere. For snow over land, the impact is particularly large in spring when warming can produce fast disappearance of the snow cover leading to large albedo changes at a time when the incoming solar radiations are intense.

The snow-and-ice-albedo feedback is also important in producing the greater surface temperature changes at high latitude than at mid and low latitudes in response to a radiative perturbation (see Chapter 6). This polar amplification of climate change is a robust characteristic of climate model simulations. It is also strongly influenced by changes in the poleward energy transport (see section 2.1.5) and by water-vapour, lapse-rate and cloud feedbacks at high latitudes.

Another important cryospheric process is related to the insulation effect of sea ice. Sea ice has low thermal conductivity. It thus efficiently isolates the ocean from the atmosphere. If the temperature increases, the ice thickness will decrease. As a consequence, in winter, the heat flux from the relatively warm ocean to the cold atmosphere will increase, leading to winter warming of the atmosphere. This is not strictly a radiative feedback, but it can explain why temperature changes at high latitudes are large in winter although the ice-albedo feedback mainly operates in spring and summer when the incoming solar radiation is at a maximum.

On longer timescales, the formation of ice sheets is a formidable amplifier of climate changes and plays a large role in glacial-interglacial cycles (see section 5.4.2). If the snow does not completely melt in summer, it accumulates from year to year, eventually forming large masses of ice (as currently observed on Greenland and Antarctica, see section 1.4.1). When such an ice sheet is formed, it induces an increase in the planetary albedo because of the ice-albedo feedback. Because of the high elevation of the ice sheet, the surface is cold and not prone to melting, further stabilising the ice sheet. Both these effects tend to maintain the cold conditions once they have been initiated by the forcing.

---