The Cryosphere
Ice and snow in the seas, on the surface of the earth, and in the ground are collec- Northern Hemisphere
tively known as the cryosphere (see ‘The Cryosphere’, inside front cover). Snow, March
ice sheets and sea ice cover about 15 per cent of the Earth’s surface during the
peak period in March to April, and about 6 per cent in August to September. Per-
manently-frozen ground, or permafrost, is found in both polar and alpine areas
and covers about 20 per cent of Earth’s land areas. Ice and snow store more than
80 per cent of the fresh water on Earth, mainly in the big ice sheets in Greenland
and Antarctica with a combined volume of about 30 million cubic kilometres.
The various components of the cryosphere play strong but different roles within
the climate system.
Due to their large volumes and areas, the two continental ice sheets of Ant-
arctica and Greenland actively influence the global climate over time scales of
millennia to millions of years.
Snow and sea ice cover large areas too, but have relatively small volumes. They
vary in size over the seasons. Snow and sea ice are connected to key interac-
tions and feedbacks at global scales (albedo, ocean circulation). Permafrost
is another important feedback component in the climate system through the
methane cycle. Together with seasonal snow, permafrost influences soil water
Southern Hemisphere
content and vegetation over continental-scale northern areas.
Glaciers and ice caps, as well as seasonal ice on lakes, with their smaller areas
September
and volumes, react relatively quickly to climate effects, influencing ecosystems
and human activities on a local scale. They are good indicators of change, re-
flecting trends in a range of conditions and seasons, from winter lowlands
(lake ice) to summer alpine areas (mountain glaciers). Despite the total vol-
ume of glaciers being several orders of magnitude smaller than that of the two
major ice sheets, they currently contribute more to sea-level rise.
Seasonal variation in the extent of ice and snow cover is greatest in the Northern
Hemisphere. Imagine the Earth with white caps on the top and bottom (2.2). The
top cap increases by a factor of six from summer to winter, while the bottom cap
only doubles from summer to winter. This difference is due to snow cover: in the
Northern Hemisphere snow cover on land varies from less than 2 million km
2
in
the summer to 40 to 50 million km
2
in the winter
3
. There is little snow cover in the
Southern Hemisphere. In Antarctica, land ice covers about 14 million km
2
year-
round, with little change from summer to winter. Sea ice cover in the Arctic varies
between approximately 7 and 15 million km
2
seasonally, while sea ice cover in the
Figure 2.2: Ice and snow cover at peak pe-
Antarctic, though about the same extent at its peak, varies much more – from
riods in the annual cycles, Northern and
around 3 million km
2
during summer to 18 million km
2
in winter. This means that
Southern Hemispheres.
there is less multi-year sea ice in the Antarctic than in the Arctic, where much of
Source: Based on NASA Blue Marble NG, with
the sea ice is older than one year. data from the National Snow and Ice Data Centre
CHAPTER 2 WHY ARE ICE AND SNOW IMPORTANT TO US? 27