Climate Change and Ecosystems

Ecosystems do not experience “climate change”. They experience a specific thing: water that is two degrees warmer, a dry season that lasts three weeks longer, a spring that arrives early. This page is about turning those specific changes into the chains of cause and effect that exam questions are really asking for.

Local impact: coral bleaching

Corals are animals, but they live in partnership with tiny algae called zooxanthellae inside their tissues. The algae photosynthesise and hand over a large share of the sugars they make; in return they get shelter and nutrients. That partnership is why reefs can thrive in tropical water that is otherwise nutrient-poor, and it is also the coral’s weak point.

When sea temperature rises a degree or two above the normal summer maximum and stays there, the algae start producing substances that are toxic to the coral. The coral expels them. Now the coral is transparent, so you see straight through to the white calcium carbonate skeleton beneath — that is bleaching. Bleaching is not death. If the water cools quickly enough, algae can recolonise. If it does not, the coral starves.

Local impact: desertification

Desertification is land in dry regions losing its productivity until it behaves like desert. Climate change drives it through longer droughts and higher temperatures that increase evaporation, so soil dries out and plants die. Once the vegetation is gone the soil has nothing holding it, so wind blows it away and the rare heavy downpours wash it off instead of soaking in.

Human land use turns a bad situation into a very bad one. Overgrazing removes the plant cover, and deforestation removes the roots and the shade. The result is soil erosion, falling agricultural output and communities forced to move. The Sahel in Africa is the standard case study: a semi-arid belt where livelihoods depend almost entirely on rain-fed farming and grazing.

Global impact: ocean circulation and sea level

The Atlantic Meridional Overturning Circulation (AMOC) works because cold, salty water in the far North Atlantic is dense enough to sink, pulling warm surface water — including the Gulf Stream — northwards behind it. Melting Greenland ice pours fresh water into exactly that region. Fresh water is less dense than salt water, so the surface water no longer sinks as readily and the whole circulation slows. Since that current carries heat towards Europe, a slowdown could mean colder European winters even in a warming world, along with disrupted fisheries as fish follow shifting water temperatures.

Sea level rises for two separate reasons, and you need both for full marks. First, land ice on Greenland, Antarctica and mountain glaciers melts and adds water to the ocean. Second, water expands as it warms — thermal expansion — so the same amount of water takes up more space. The consequences are coastal flooding and erosion, loss of mangroves and salt marshes, and low-lying nations such as the Maldives facing the prospect of becoming uninhabitable.

Biome shifts: ecosystems on the move

A biome sits where it does because the temperature and rainfall there suit it. Change those conditions and the suitable zone moves. Species track the conditions they need, so whole communities creep polewards and uphill. In North America temperate forest is pushing northwards into what was boreal forest. In the Arctic, tundra is turning into shrubland as warmth allows woody plants to survive.

Resilience: why the second shock is the dangerous one

Resilience is an ecosystem’s ability to absorb a disturbance and still recover. Two things determine it, and climate change damages both.

Biodiversity

A diverse ecosystem carries spare capacity. If one species is hit, others with a similar role can take over, so the system keeps working. Tropical rainforest recovers well from disturbance for exactly this reason, while a monoculture farm collapses when one pest arrives. Climate change reduces biodiversity through habitat loss, disrupted food webs and extreme weather — and each species lost removes another piece of the spare capacity.

Habitat fragmentation

Connected habitat lets species migrate as conditions shift. Fragmented habitat traps them in islands with no route out. Climate change fragments habitat by spreading deserts between ecosystems, by burning gaps through forests with more frequent fires, and by breaking up sea ice that polar bears use to hunt.

Put the two together and the danger becomes clear. A bleached reef with reduced biodiversity is less able to survive the next stressor — ocean acidification, a cyclone, a disease outbreak. The first shock does not have to be the fatal one.