2.4 Climate and biomesIB Environmental Systems and Societies HL: Revision notes

Climate describes atmospheric conditions over relatively long periods (commonly 30 years or more); weather describes conditions over a short period such as a day.

A biome is a group of comparable ecosystems that developed in similar climatic conditions, wherever they occur. Abiotic factors, mainly temperature and precipitation, are the determinants of terrestrial biome distribution.

Biomes can be grouped as freshwater, marine, forest, grassland, desert and tundra. Each has characteristic abiotic limiting factors, productivity and diversity: tropical rainforest has high temperature and rainfall, very high productivity and high diversity; desert is limited by water, with low productivity; tundra is limited by low temperature and a short growing season.

The tricellular model explains the behaviour of atmospheric systems and the distribution of precipitation and temperature at different latitudes. Each hemisphere has three cells: Hadley, Ferrel and polar.

• At the equator intense solar heating makes moist air rise, cool and condense: a low-pressure zone with heavy rain, supporting tropical rainforest.
• At about 30° N and S the air descends, warms and is dry: a high-pressure zone, giving the great deserts such as the Sahara.
• At about 60° N and S warm air rises over cold polar air, giving a second low-pressure zone with moderate rain.
• At the poles cold air descends, giving dry, cold conditions.

The oceans absorb solar radiation and ocean currents distribute the heat around the world, moving warm water from the tropics towards the poles and cold water towards the equator. This moderates climates, for example the warm North Atlantic Drift keeps north-western Europe milder than other places at the same latitude.

Global warming is leading to changing climates and shifts in biomes. As temperatures rise, climate zones move towards the poles and to higher altitudes. Example: the treeline in Alaska and Siberia is moving northwards, so boreal forest replaces tundra.

(HL) There are three general patterns of climate types connected to biome types, broadly related to latitude and altitude: warm and wet tropical, seasonal temperate, and cold polar or alpine climates, each with characteristic biomes.

(HL) The biome predicted by a given temperature and rainfall pattern may not develop because of secondary influences (soil type, altitude, drainage, fire, grazing, volcanic activity, disturbance by storms) or human interventions such as clearing forest for farming. Example: much of the Philippines would naturally support tropical rainforest, but much has been cleared for rice, coconut plantations and settlement.

(HL) The El Niño Southern Oscillation (ENSO) cycle is the fluctuation in wind and sea surface temperatures that characterises conditions in the tropical Pacific Ocean.

• In a normal year the east–west Walker circulation blows trade winds westwards, pushing warm surface water west and allowing upwelling of cold, nutrient-rich water off north-western South America.
• El Niño is due to a weakening or reversal of the Walker circulation. This increases surface stratification and decreases upwelling of cold, nutrient-rich water near the coast of north-western South America, so productivity and fisheries such as the Peruvian anchoveta fall.
• La Niña strengthens the normal Walker circulation, increasing upwelling.

(HL) Tropical cyclones are rapidly circulating storm systems with a low-pressure centre that originate in the tropics and are characterised by strong winds. They form over warm ocean water, where warm moist air rises, and condensation releases energy that fuels the storm. They are called hurricanes in the Atlantic and eastern Pacific and typhoons in the western Pacific.

(HL) Rises in ocean temperatures resulting from global warming are increasing the intensity and frequency of hurricanes and typhoons because warmer water and air have more energy. Example: Typhoon Haiyan struck the Philippines in 2013.