Box 7.1: Sea ice and climate change.

Sea-ice processes:
Sea ice in the Arctic and around Antarctica responds directly to climate change and may, if properly monitored, become increas-ingly important for detecting climate change. Although sea ice covers only about 5% of the Earth’s surface, its extent and thickness have important influences in the coupled atmosphere-ocean system. Increasing the understanding of these processes and representing them more realistically in climate models is important for making more reliable climate change projections.
Several processes associated with sea ice are climatically relevant. The sea-ice albedo effect is an important contributor to the amplification of projected warming at high latitudes. Albedo decreases if the extent of sea ice is reduced and more ocean surface is exposed, resulting in increased heat absorption and hence warming. Melting of snow and the formation of melt-water ponds also reduces albedo and alters the radiation balance. Changes in sea-ice thickness and lead (open water) fraction modify the heat transfer from the ocean: thinner sea ice and more leads result in enhanced heat loss from the exposed ocean thus further warming the atmosphere. Changes in cloud cover may influence how large this effect really is. A principal mechanism for dense water formation in the ocean around Antarctica and in shelf regions of the Arctic is the rejection of brine as sea water freezes.
Changes in sea-ice formation alter the properties and formation rates of ocean deep water and therefore have an influence on the water mass structure that reaches far beyond the area of sea ice. Finally, ice export from the Arctic represents an important southward flux of fresh water which influences the density structure of the upper ocean in the Nordic, Labrador and Irminger Seas.

Observations of sea ice:
Observations of sea-ice extent and concentration (the fraction of local area covered by ice) are based primarily on satellite data available since the late 1970s. Sea-ice thickness is also important in assessing possible changes in the amount of sea ice; however, thickness observations are more difficult to make. For the Arctic, thickness data come primarily from sonar measure-ments from submarines and a few oceanographic moorings. Although limited, the observations indicate statistically significant decreases in ice extent and thickness over the past few decades, with Arctic sea-ice extent declining at a rate of about 3% per decade since the late 1970s. Sea-ice retreat in the Arctic spring and summer over the last few decades is consistent with an increase in spring temperature, and to a lesser extent, summer temperatures at high latitudes. Thickness data show a near 40% decrease in the summer minimum thickness of Arctic sea ice over approximately the last 30 years. Estimates using independent methods for the winter, but over a much shorter period, also suggest thickness reductions, but at a markedly slower rate.
However, due to limited sampling, uncertainties are difficult to estimate, and the influence of decadal to multi-decadal variability cannot yet be assessed.

While Arctic sea-ice extent and thickness have clearly decreased in the last 20 years, changes in Antarctic sea-ice extent have been insignificant. The earlier part of the data set indicates somewhat greater ice extents in the early 1970s, and indirect evidence from historical records also points to more northerly sea-ice margins in the 1930s and 1940s. Warming over much of
Antarctica has only been about 0.5°C over the last 50 years with the notable exception of the Antarctic Peninsula where temper-atures have increased by about 2°C for reasons that remain unclear.

Sea-ice modelling and projection:
Sea ice is particularly difficult to simulate in climate models because it is influenced directly by both the atmosphere (temper-ature, radiation, wind) and the ocean (heat transport and mixing, and surface currents), and because many of the relevant processes require high grid resolution or must be parametrized. Recent coupled climate models include a sea-ice component that incorporates openings in the ice, often in conjunction with ice dynamics (motion and deformation). Furthermore, updated parametrizations of snow ageing and associated albedo changes and multi-layer formulations of heat conduction through the ice and overlying snow cover are being implemented in some models. Although many thermodynamic processes are crudely approximated, it is unclear how these approximations contribute to errors in climate model simulations. Sea-ice dynamics is important in determining local ice thickness and export of sea ice from the formation areas, but despite the rather mature status of physically based sea-ice dynamics models, only a few of the current coupled climate models include such a component. Coupled model simulations of the seasonal cycle of sea-ice coverage in both hemispheres exhibit large deviations from the limited observational data base, as illustrated in Chapter 8, and current research is aimed at improving model performance.

Coupled model projections of the future distribution of sea ice differ quantitatively from one to another as shown in Chapter 9. However, they agree that sea-ice extent and thickness will decline over the 21st century as the climate warms. Box 7.1, Figure 1 illustrates this with annual mean Arctic ice extent results from two coupled models. The simulations of ice extent decline over the past 30 years are in good agreement with the observations, lending confidence to the subsequent projections which show a substantial decrease of Arctic sea-ice cover leading to roughly 20% reduction in annual mean Arctic sea-ice extent by the year 2050.