About 10 percent of the Earth's land surface, 15 percent of the ocean surface during winter peaks, and substantial sub-surface volumes in high latitudes are made of ice. Collectively, these frozen parts of the Earth system are called the cryosphere — from the Greek "kryos" meaning cold or frost. The cryosphere comprises glaciers, ice sheets, ice shelves, sea ice, lake and river ice, snow cover, and permafrost. It holds about three-quarters of all the fresh water on the planet. It reflects substantial solar energy back to space, helping to regulate global temperature. It locks up enormous quantities of carbon and water that would otherwise be in the active climate system. And it is one of the most rapidly changing components of the Earth system under contemporary climate change. Understanding the cryosphere — its components, its dynamics, and its current trajectory — is central to understanding both the present climate and what the future climate is likely to look like.
The Components
The cryosphere has multiple distinct components that operate on different scales and timescales. The two great ice sheets — Antarctica and Greenland — contain about 99 percent of all glacier ice on Earth. The Antarctic ice sheet covers 14 million square kilometres and contains about 26.5 million cubic kilometres of ice. The Greenland ice sheet covers 1.7 million square kilometres and contains about 2.85 million cubic kilometres of ice. Smaller glaciers and ice caps in mountainous regions worldwide (the Alps, the Andes, the Himalayas, Alaska, Iceland, Patagonia, and many others) collectively cover about 700,000 square kilometres and contain about 158,000 cubic kilometres of ice. Sea ice is floating ice in the polar oceans — the Arctic sea ice extends over 14-15 million square kilometres in winter and shrinks to 4-5 million in summer; Antarctic sea ice has the opposite seasonality with a smaller seasonal range. Permafrost is permanently frozen ground covering about 23 million square kilometres of the Northern Hemisphere, primarily in Russia, Canada, and Alaska. Seasonal snow cover varies dramatically between hemispheres — peaking at about 47 million square kilometres in the Northern Hemisphere winter and only 1-2 million in the Southern.
Ice Sheets vs Glaciers
The distinction between ice sheets and glaciers is mostly about scale. Ice sheets are continent-scale (over 50,000 square kilometres by convention); the Antarctic and Greenland ice sheets are the only two on Earth today. Glaciers are smaller and typically valley- or mountain-based. The Antarctic ice sheet itself contains many distinct flow units called ice streams that behave somewhat independently — the West Antarctic Ice Sheet, for instance, drains through fast-flowing ice streams that have been retreating in recent decades. The Greenland ice sheet drains through outlet glaciers at its margins, including the famously productive Jakobshavn Isbrae which has been one of the fastest-moving glaciers in the world. Mountain glaciers occur in essentially every range that reaches sufficiently high altitude — even tropical mountain ranges like the Andes and the Rwenzori host glaciers, though these have been retreating rapidly.
Sea Ice: A Different Story
Sea ice differs from glacial ice in several important ways. It forms from freezing seawater rather than accumulated snow, so it is initially much saltier (though brine drains over time). It is much thinner — typically 1-5 metres compared to the kilometres of glacier ice. It is floating rather than grounded, so changes in sea ice volume do not directly contribute to sea level rise (the displaced water already accounted for the ice volume). Sea ice plays critical climate roles regardless. Its high albedo (reflectivity) sends solar energy back to space; when sea ice melts and exposes dark ocean water, more energy is absorbed, accelerating warming — a positive feedback called ice-albedo feedback. Sea ice also affects ocean circulation through its effects on salinity and stratification, and it provides essential habitat for polar species including polar bears, seals, walrus, and many fish and seabirds.
Permafrost
Permafrost — soil or rock that has remained at temperatures below 0°C for at least two consecutive years — underlies about 22-25 percent of the Northern Hemisphere's land surface. It can be up to 1,500 metres thick in central Siberia and Canada's northern islands. The active layer at the top thaws and refreezes seasonally; below that lies the permafrost proper. Permafrost holds an estimated 1,400-1,600 gigatonnes of carbon — about twice the carbon currently in the atmosphere. This carbon is in the form of frozen organic matter (peat, plant remains, animal remains) accumulated over thousands of years. As permafrost thaws under climate change, microbial decomposition of this organic matter releases CO₂ and methane back to the atmosphere, providing a major potential climate feedback. The permafrost carbon feedback is one of the largest sources of uncertainty in climate projections.
The Cryosphere and Sea Level
The cryosphere is the primary control on global sea level. During the last glacial maximum about 20,000 years ago, with vastly more ice on land than today, sea level was about 120 metres lower than now. Since the end of the last ice age, melting land-based ice has raised sea level by approximately that 120 metres. Currently, the contributions to ongoing sea level rise come from melting glaciers and ice sheets (about 50 percent), thermal expansion of warming ocean water (about 40 percent), and changes in land water storage (the remaining 10 percent). Greenland alone is contributing about 0.7-0.8 millimetres per year to sea level — accelerating. Antarctica is contributing roughly 0.4 mm/year — also accelerating. Mountain glaciers collectively contribute another 0.5 mm/year or so. The total rate of sea level rise is currently about 4-5 mm/year — substantially faster than the 20th-century average — and projected to accelerate further through the 21st century.
Glacial Mass Balance
The "mass balance" of a glacier or ice sheet is the difference between accumulation (snow falling and compacting into ice in the upper parts) and ablation (melting and iceberg calving in the lower parts). When accumulation exceeds ablation, the glacier grows. When ablation exceeds accumulation, the glacier shrinks. Most of the world's glaciers are currently in negative mass balance — they are losing more ice than they are gaining. This has been true since the early 20th century in most regions and has accelerated since around 1990. Some glaciers, particularly in regions like the Karakoram, have been more stable or even gained mass in recent decades, but these are exceptions to the broader pattern of widespread retreat. The cumulative loss of mountain glacier ice since 1960 has been about 9,000-10,000 cubic kilometres.
Ice Sheet Dynamics
Ice sheets are not static blocks of ice — they flow under their own weight, draining toward their margins. The Greenland ice sheet flow speeds range from a few metres per year in the interior to several kilometres per year in the fastest outlet glaciers. Antarctic ice flows similarly. Flow happens through three main mechanisms: internal deformation of the ice (slow), sliding along the bed (fast where water is present at the bed), and deformation of soft sediment beneath the ice (variable). One of the major concerns about Antarctic ice sheet stability is the potential for marine ice sheet instability — the West Antarctic Ice Sheet sits on bedrock that is below sea level and slopes inward, so once retreat begins it could continue unstably. The recent acceleration of glaciers like Pine Island and Thwaites in West Antarctica has been a major focus of glaciological research because of these implications.
Changes in the 21st Century
The cryosphere is changing rapidly under contemporary climate change. Arctic sea ice extent has declined about 13 percent per decade in September since 1979. Greenland and Antarctica are losing mass at accelerating rates. Mountain glaciers worldwide are retreating, with some regions (notably the European Alps and tropical Andes) projected to lose most of their ice by 2100. Permafrost is warming and the active layer is deepening. Snow cover is decreasing in the Northern Hemisphere spring. The pace of change varies regionally and has substantial year-to-year variability, but the long-term trends are unambiguous across essentially every component of the cryosphere. The IPCC's most recent assessment concluded that the cryosphere will continue to lose mass under almost every climate scenario through the 21st century, with substantial sea level rise consequences regardless of mitigation actions taken now.
The Cryosphere in the Climate System
The cryosphere influences climate through multiple mechanisms beyond ice-albedo feedback. The cold dense water formed near the ice edges drives global thermohaline circulation. The fresh water released by melting ice can alter ocean stratification and circulation patterns. Greenhouse gases released from thawing permafrost amplify warming. The deep ice cores from Greenland and Antarctica provide some of the most important climate records — air bubbles trapped in the ice preserve samples of atmospheric composition extending back hundreds of thousands of years. Studying the cryosphere is therefore central not just to understanding present climate change but to understanding the longer history and future trajectory of Earth's climate system. The continued integrity of the cryosphere is also a question that will substantially shape human geography over the coming century — sea level, freshwater availability, ecosystem services, and climate stability all depend on what happens to the world's ice.