A tundra climate is a very cold climate characterised by low temperatures, a short cool growing season, limited precipitation, and vegetation dominated by low growing plants rather than forests. The defining climatic feature is that temperatures remain too low for trees to grow naturally over most of the landscape. In the widely used Koppen climate classification, tundra climate is designated ET. Its warmest month has a mean temperature above 0 degrees Celsius but below 10 degrees Celsius.
This distinguishes tundra from polar ice cap climate, in which even the warmest month has a mean temperature at or below 0 degrees Celsius. Tundra climate therefore represents an environment that is extremely cold but still experiences at least a brief period when surface temperatures rise sufficiently above freezing to support seasonal plant growth.
Tundra climates occur primarily at high latitudes in the Northern Hemisphere, particularly around the Arctic margins of North America, Greenland, northern Europe and northern Asia. Examples occur across northern Alaska, northern Canada, coastal Greenland, northern Scandinavia and Siberia. A related form occurs at high elevations where temperature decreases sufficiently with altitude to produce tundra like conditions, although high altitude environments are generally classified separately as highland climates when using broad climate systems.
Antarctic coastal areas also contain tundra like environments in places where temperatures permit some vegetation, but the Antarctic interior is predominantly classified as ice cap climate. The geographical distribution of tundra therefore reflects the combined effects of latitude, altitude, ocean influence, atmospheric circulation and local topography.
Tundra climates are associated with distinctive seasonal patterns. Winters are generally long, cold and dark, while summers are short and cool. At high Arctic latitudes, the Sun may remain above the horizon continuously for part of summer and below the horizon continuously during part of winter. Despite continuous summer daylight, temperatures remain limited because the Sun remains relatively low in the sky and the ground may remain cold or frozen. The annual temperature range can vary considerably depending on proximity to the ocean. Coastal tundra often has relatively moderated winters and cool summers, whereas continental tundra can experience much larger seasonal temperature contrasts.
Precipitation in tundra climates is generally low, often comparable to that of some deserts, although the landscape may remain wet because evaporation is also extremely limited. Much precipitation falls as snow during the long cold season. Snow cover can persist for many months and provides insulation for soils and organisms beneath it. In summer, melting snow and frozen ground can produce extensive surface water because drainage is restricted. The amount of precipitation therefore does not alone determine how wet tundra appears. Low evaporation, frozen soil, shallow active layers and poor drainage can produce ponds, wetlands and saturated ground despite relatively modest annual precipitation.
Permafrost is a major characteristic of many Arctic tundra regions. Permafrost is ground that remains at or below 0 degrees Celsius for at least two consecutive years, although the exact physical definition concerns temperature rather than whether the ground contains ice. The upper layer above permafrost can thaw seasonally and is called the active layer. Plant roots and microbial activity are concentrated largely within this seasonally thawed layer. The depth of thaw varies according to climate, vegetation, snow cover, soil properties and local conditions. Permafrost strongly influences drainage, construction, carbon storage and ecosystems. Where it is continuous, water movement through the ground is severely restricted, contributing to surface wetness during summer.
Tundra vegetation is adapted to cold temperatures, short growing seasons, wind exposure and nutrient limitations. Common plants include mosses, lichens, grasses, sedges, dwarf shrubs and small flowering plants. Trees are generally absent because the growing season is too short and soils and roots may be affected by permafrost, although isolated shrubs and stunted woody plants can occur near the climatic boundary of the tundra. Plants often grow close to the ground, where they can benefit from greater shelter and slightly warmer conditions. Many species have shallow root systems, slow growth rates and adaptations that allow them to complete reproduction during a brief summer period.
Tundra climate has major ecological importance and supports animals adapted to extreme seasonal conditions. Arctic tundra may support reindeer and caribou, musk oxen, Arctic foxes, Arctic hares, lemmings, migratory birds and numerous insects during summer. Coastal tundra can also interact strongly with marine ecosystems. Seasonal productivity can be surprisingly high during the brief growing period because prolonged daylight allows rapid plant growth when temperatures become suitable. However, the ecosystem is vulnerable to disturbances because biological recovery is slow. Trampling, vehicle traffic, industrial development and changes in permafrost can produce effects that persist for decades or longer.
Tundra climate is particularly important in discussions of climate change because Arctic land areas are warming rapidly and permafrost stores large quantities of organic carbon. When frozen ground thaws, previously protected organic material can become available for microbial decomposition, potentially releasing carbon dioxide and methane. Changes in snow cover, vegetation, wildfire, hydrology and surface albedo can further alter the energy balance of tundra landscapes. Tundra should not therefore be defined simply as a place that is cold and treeless. It is a specific climatic and ecological system produced by low temperatures, limited growing seasons and interactions among atmosphere, soil, vegetation, snow, ice and water.
