The climate is on the move: Global warming is raising the Earth's average temperature and changing where and when it rains. Because each biome matches a climate, a moving climate means moving biomes.
The points to remember
- The Earth's average temperature is about 1.1-1.2 °C higher than in 1850-1900.
- Climate is the long-term average, so a changing climate means the averages are moving, not one hot day.
- Places are getting warmer, with longer growing seasons and fewer frosts.
- Rainfall patterns are changing: some regions get wetter, others drier.
- The poles and high mountains are warming fastest.
- So the climate zones themselves are moving towards the poles and uphill.
Real example: on 19 July 2022 the UK passed 40 °C for the first time, at Coningsby in Lincolnshire. One day is weather, but such days are becoming more common because the climate itself has warmed.
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Why do biomes move? Every species has a tolerance range. Change the climate, and the places that suit it change too.
The points to remember
- Each species lives within a tolerance range of temperature and water.
- A biome exists where its climate is: set by temperature and rainfall.
- When the climate warms, a place moves outside the range of some of its species.
- They die out at the warm edge and spread into new areas at the cool edge.
- So the whole biome's boundary moves: towards the poles and to higher altitude.
Remember it as: Up and out: biomes climb the mountains and head for the poles.
Warmer
- Winters in the north-eastern USA are milder than a century ago.
Out of range
- Sugar maples need cold winters; at the warm southern edge their seedlings do worse.
New space
- Further north, in Quebec, the climate now suits them better.
Shift
- Scientists expect the maple forest to move north into Canada.
Link the climate to the species: Do not just say 'the biome moves'. Say what changed (temperature or rainfall), that species need conditions within their tolerance range, and which way the boundary moves.
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Moving away from the equator, the climate gets colder. So as the world warms, each biome's conditions are found further towards the poles.
The points to remember
- Warmer conditions let biomes move towards the poles (north in the northern hemisphere).
- Boreal forest and shrubs spread north into the tundra.
- Warm-climate biomes spread into cooler areas: tropical and sub-tropical conditions cover more land.
- The coldest biomes shrink: Arctic tundra has the Arctic Ocean behind it.
Real example: in northern Alaska, photographs from about 1950 and 2000 show tall shrubs such as alder and willow spreading across what was open tundra. The boreal forest is following.
Nowhere left to go: Poleward shifts squeeze the coldest biomes. Arctic tundra cannot move further north than the Arctic Ocean, so it shrinks.
Climbing a mountain is like travelling towards the poles: it gets colder, so the biomes sit in bands. Warming lifts every band.
The points to remember
- Air cools about 0.6 °C per 100 m of height, so a mountain has zones like bands.
- Warming moves every zone uphill: each 1 °C is roughly 150 m.
- The treeline rises and forest grows into alpine meadow.
- Alpine biomes near the top shrink: a mountain gets narrower towards the summit.
- At the top there is nowhere higher to go, so alpine species can be lost.
Real example: a study of 302 mountain summits across Europe, many in the Alps, found more plant species on them than a century ago, and they were arriving five times faster in 2007-2016 than in 1957-1966, as lowland plants climb.
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Global warming changes rain as well as heat. Since biomes depend on both, a change in rainfall moves them too.
The points to remember
- Biomes depend on rainfall as well as temperature.
- Where it gets wetter, wet biomes (such as wet sub-tropical forest) grow, and dry ones shrink.
- Where it gets drier, forest can turn to savanna and grassland to desert.
- Longer droughts bring more fires, which speed the change.
- So biomes shift sideways as well as poleward and uphill.
Wetter
- Wet forest spreads
- Dry biome area shrinks
- Wetlands grow
Drier
- Forest turns to savanna
- Grassland turns to desert
- More fires
Real example: in the south-east Amazon, the dry season has become longer and hotter, and fires are more common. Parts of the forest there now release more carbon than they take in, and scientists warn they could turn into savanna.
The trend is poleward and uphill, but real biomes do not slide neatly across a map. Many move slowly, in pieces, or are blocked.
The points to remember
- Barriers block movement: seas, mountains, cities and farmland; habitats are fragmented.
- Soils take decades to centuries to form, so trees cannot root in new places quickly.
- Trees live long and spread slowly by seed, so biomes lag behind the climate.
- Species move at different speeds, so biomes break up into new mixes rather than move whole.
- Growing seasons and migration patterns change, so ecosystems become less stable.
- A biome shift is caused by climate change, not by people clearing land.
Speeds up shifts
- Fast warming at poles and peaks
- Droughts and fires
- Pests surviving milder winters
Slows or blocks shifts
- Seas, mountains, cities
- Slow soils and slow trees
- Farmland in the way
Real example: Joshua trees in California's Mojave Desert spread their seeds only a short way, so as the lower desert gets hotter and drier, young trees now survive mainly in the higher, cooler parts of Joshua Tree National Park; the old trees below cannot move.
Not the same as clearing land: A biome shift is caused by the changing climate. Cutting down a forest for farms is land-use change, not a biome shift, although it can block one.
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How this comes up: Paper 2, Section B (a): outline how global warming is shifting biomes, with an example [4].
Scientists who survey plants on mountains in the Alps find species growing higher up the slopes each decade.
Outline how global warming is leading to shifts in biomes.
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