Climate zones and biomes

Weather is what the atmosphere is doing at a particular moment: today's rain or tomorrow's heat. Climate is the long-term pattern, usually averaged over at least 30 years. A useful saying captures the difference: climate is what you expect, weather is what you get.

Climate depends mainly on temperature and precipitation and how they change through the year. Those in turn depend on latitude, altitude, distance from the sea, ocean currents, prevailing winds and mountains.

Near the equator the sun is high in the sky all year, so its energy is concentrated on a small area and temperatures stay warm. Toward the poles, sunlight arrives at a low angle and spreads across a larger area, and seasons become extreme, with long summer days and long winter nights.

Earth's tilt of about 23.4 degrees causes the seasons. When the Northern Hemisphere tilts toward the sun it has summer, while the Southern Hemisphere has winter, and six months later the situation reverses.

• A, tropical: warm all year, such as rainforests and monsoon regions.
• B, dry: evaporation exceeds precipitation, including deserts and semi-arid steppes.
• C, temperate: mild winters, such as the Mediterranean, humid subtropical and oceanic climates.
• D, continental: warm summers and cold, snowy winters, found in the interiors of North America and Eurasia.
• E, polar: cold all year, including tundra and ice caps.

A biome is a large community of plants and animals shaped by a similar climate. Because climate largely decides which plants can grow, similar biomes appear on different continents even when their species are unrelated. The dry grasslands of Argentina, Ukraine and the North American Great Plains look alike for this reason.

Moving from the equator toward the poles, you pass broadly through tropical rainforest, savanna, desert, temperate grassland and forest, the vast coniferous boreal forest or taiga, and finally the treeless tundra. Climbing a high mountain produces a similar sequence over a much shorter distance.

Tropical savannas, such as the Serengeti, are grasslands with scattered trees and a pronounced wet and dry season, supporting great herds of grazing animals. Temperate deciduous forests, found in much of Europe, eastern North America and East Asia, have trees that drop their leaves in autumn to survive winter.

The taiga is the largest land biome, stretching across Canada, Scandinavia and Russia. Its conifers, such as spruce, fir and pine, have needle-shaped leaves and conical shapes that shed snow. Further north, the tundra has permanently frozen ground called permafrost, a short growing season, and low plants such as mosses, lichens and dwarf shrubs.

Climate zones and biomes are not fixed. During the last ice age, ice sheets covered much of North America and northern Europe, and the Sahara has swung between desert and green grassland with lakes and hippos over the past several thousand years because of changes in Earth's orbit.

Today, human-caused warming is moving climate zones toward the poles and up mountainsides. Trees are advancing into tundra in some areas, permafrost is thawing and releasing greenhouse gases, and species that cannot move or adapt quickly enough face a higher risk of extinction.