4.1 Introduction

The main components of the cryosphere are snow, river and lake ice, sea ice, glaciers and ice caps, ice shelves, ice sheets, and frozen ground (Figure 4.1). In terms of the ice mass and its heat capacity, the cryosphere is the second largest component of the climate system (after the ocean). Its relevance for climate variability and change is based on physical properties, such as its high surface reflectivity (albedo) and the latent heat associated with phase changes, which have a strong impact on the surface energy balance. The presence (absence) of snow or ice in polar regions is associated with an increased (decreased) meridional temperature difference, which affects winds and ocean currents. Because of the positive temperature-ice albedo feedback, some cryospheric components act to amplify both changes and variability. However, some, like glaciers and permafrost, act to average out short-term variability and so are sensitive indicators of climate change. Elements of the cryosphere are found at all latitudes, enabling a near-global assessment of cryosphere-related climate changes.

The cryosphere on land stores about 75% of the world’s freshwater. The volumes of the Greenland and Antarctic Ice Sheets are equivalent to approximately 7 m and 57 m of sea level rise, respectively. Changes in the ice mass on land have contributed to recent changes in sea level. On a regional scale, many glaciers and ice caps play a crucial role in freshwater availability.

Presently, ice permanently covers 10% of the land surface, of which only a tiny fraction lies in ice caps and glaciers outside Antarctica and Greenland (Table 4.1). Ice also covers approximately 7% of the oceans in the annual mean. In midwinter, snow covers approximately 49% of the land surface in the Northern Hemisphere (NH). Frozen ground has the largest area of any component of the cryosphere. Changes in the components of the cryosphere occur at different time scales, depending on their dynamic and thermodynamic characteristics (Figure 4.1). All parts of the cryosphere contribute to short-term climate changes, with permafrost, ice shelves and ice sheets also contributing to longer-term changes including the ice age cycles.

Figure 4.1. Components of the cryosphere and their time scales.

Seasonally, the area covered by snow in the NH ranges from a mean maximum in January of 45.2 × 10⁶ km² to a mean minimum in August of 1.9 × 10⁶ km² (1966–2004). Snow covers more than 33% of lands north of the equator from November to April, reaching 49% coverage in January. The role of snow in the climate system includes strong positive feedbacks related to albedo and other, weaker feedbacks related to moisture storage, latent heat and insulation of the underlying surface (M.P. Clark et al., 1999), which vary with latitude and season.

High-latitude rivers and lakes develop an ice cover in winter. Although the area and volume are small compared to other components of the cryosphere, this ice plays an important role in freshwater ecosystems, winter transportation, bridge and pipeline crossings, etc. Changes in the thickness and duration of these ice covers can therefore have consequences for both the natural environment and human activities. The breakup of river ice is often accompanied by ‘ice jams’ (blockages formed by accumulation of broken ice); these jams impede the flow of water and may lead to severe flooding.

At maximum extent arctic sea ice covers more than 15 × 10⁶ km², reducing to only 7 × 10⁶ km² in summer. Antarctic sea ice is considerably more seasonal, ranging from a winter maximum of over 19 × 10⁶ km² to a minimum extent of about 3 × 10⁶ km². Sea ice less than one year old is termed ‘first-year ice’ and that which survives more than one year is called ‘multi-year ice’. Most sea ice is part of the mobile ‘pack ice’, which circulates in the polar oceans, driven by winds and surface currents. This pack ice is extremely inhomogeneous, with differences in ice thicknesses and age, snow cover, open water distribution, etc. occurring at spatial scales from metres to hundreds of kilometres.

Glaciers and ice caps adapt to a change in climate conditions much more rapidly than does a large ice sheet, because they have a higher ratio between annual mass turnover and their total mass. Changes in glaciers and ice caps reflect climate variations, in many cases providing information in remote areas where no direct climate records are available, such as at high latitudes or on the high mountains that penetrate high into the middle troposphere. Glaciers and ice caps contribute to sea level changes and affect the freshwater availability in many mountains and surrounding regions. Formation of large and hazardous lakes is occurring as glacier termini retreat from prominent Little Ice Age moraines, especially in the steep Himalaya and Andes.

The ice sheets of Greenland and Antarctica are the main reservoirs capable of affecting sea level. Ice formed from snowfall spreads under gravity towards the coast, where it melts or calves into the ocean to form icebergs. Until recently (including IPCC, 2001) it was assumed that the spreading velocity would not change rapidly, so that impacts of climate change could be estimated primarily from expected changes in snowfall and surface melting. Observations of rapid ice flow changes since IPCC (2001) have complicated this picture, with strong indications that floating ice shelves ‘regulate’ the motion of tributary glaciers, which can accelerate manyfold following ice shelf breakup.

Frozen ground includes seasonally frozen ground and permafrost. The permafrost region occupies approximately 23 × 10⁶ km² or 24% of the land area in the NH. On average, the long-term maximum areal extent of the seasonally frozen ground, including the active layer over permafrost, is about 48 × 10⁶ km² or 51% of the land area in the NH. In terms of areal extent, frozen ground is the single largest cryospheric component. Permafrost also acts to record air temperature and snow cover variations, and under changing climate can be involved in feedbacks related to moisture and greenhouse gas exchange with the atmosphere.