The global distributions of ecosystems

The distribution of large-scale ecosystems (biomes) is determined by climate. Latitude, air pressure and winds are important factors that determine the climate of a place.

Latitude

Latitude is one of the most important factors in determining global climate patterns.

In the lower latitudes, such as the tropics, temperatures are the highest. This is because the sun's rays travel a shorter distance to the Equator and are therefore more concentrated.

However, in the higher latitudes, such as the polar regions of the world, temperatures are lowest. This is due to the sun's rays travelling a longer distance and being spread over a wider area of the Earth's surface. When these rays approach the Earth, they do so at a sloping angle, resulting in lower surface temperatures.

High and low air pressure

Differences in temperature lead to variations in air pressure around the world. Low-pressure areas are created when air rises. This is called low pressure because the weight of the air above the Earth's surface is lower than average. High-pressure areas are created when air sinks. This is called high pressure because the weight of the air is above average when it sinks to the Earth's surface.

Low-pressure areas are associated with cloud and precipitation (rainfall) because:

  1. as the air rises it cools, condenses and forms clouds
  2. the water droplets in the clouds increase in size
  3. they eventually become too heavy to be held and fall as precipitation

The air above the Equator is very hot and rises, creating an area of low pressure. The Equator experiences high amounts of rainfall due to this rising air resulting in a warm and wet equatorial climate (eg Amazon and Congo tropical rainforests).

High-pressure areas are associated with dry, warm and settled weather conditions. This is because sinking air does not result in precipitation.

The Sun's rays are more intense at the Equator. Warms air here rises, moves towards the poles, cools, then sinks. This causes pressure cells.

Winds

The air travels in the upper atmosphere and sinks at approximately 30° north and 30° south of the Equator. When the air sinks it creates an area of high pressure.

These high-pressure areas experience very dry and warm conditions resulting in a hot desert climate (eg the Sahara and Kalahari deserts). Winds blow from areas of high to low pressure, which transfers the air from where it is sinking to where it is rising. This continual transfer of wind maintains the pressure belts of high and low pressure which creates different global climatic zones.