D4.3 Climate change

The Earth’s surface is warmed by sunlight, and it would be far colder without the greenhouse effect. Climate change today is driven by anthropogenic (human-caused) increases in two greenhouse gases: carbon dioxide and methane.

Rising carbon dioxide and methane

The CO₂ rise is the long-term trend in the Keeling curve (C4.2 Transfers of energy and matter). Natural processes balance in the long run: photosynthesis removes roughly as much CO₂ as respiration releases, and methane made by methanogens in wetlands is gradually oxidized in the atmosphere. Human activities add carbon from fossil stores faster than oceans and plants can absorb it.

Each molecule of methane absorbs far more infrared than CO₂: per kilogram it causes about 80 times as much warming over 20 years. It stays in the air for only about a decade, though, whereas much of the extra CO₂ persists for centuries, so CO₂ causes the larger share of warming.

Correlation, causation and the ice-core record

The two curves show a strong positive correlation, but correlation alone cannot show which variable causes which, or whether a third factor drives both. In fact, at the end of ice ages slight changes in Earth’s orbit began the warming, and CO₂ released from the warming ocean then amplified it: CO₂ was both effect and cause. The evidence that today’s CO₂ rise causes warming comes from several independent lines:

• Mechanism: laboratory measurements since the 1850s show CO₂ and CH₄ absorb infrared.
• Fingerprint: the extra CO₂ has the isotope signature of fossil carbon (no ¹⁴C and relatively little ¹³C).
• Pattern: the lower atmosphere is warming while the upper atmosphere cools, which fits more greenhouse gas, not a brighter Sun.

Global surface temperature in 2011–2020 averaged about 1.1 °C above 1850–1900, and 2024 was the warmest year on record, about 1.5 °C above.

In positive feedback, a change sets off effects that increase that same change. Several processes in the climate system respond to warming by causing more warming, so the warming from our emissions is amplified.

Permafrost soils hold roughly 1,400–1,600 billion tonnes of carbon, about twice as much as the atmosphere, which is why their thaw is a major concern.

The feedbacks do not usually cause runaway warming: each round of feedback adds less than the round before, so warming settles at a higher level. But where a feedback becomes self-sustaining in one part of the system, a tipping point can be passed.

Boreal forest (taiga) stretches across Canada, Alaska, Scandinavia and Russia. It is dominated by conifers such as spruce, pine, fir and larch, and holds about a third of the carbon in the world’s forests, much of it in cold, slowly decomposing soils and peat. For thousands of years it has been a net carbon sink: photosynthesis exceeded respiration and combustion, so carbon accumulated.

How warming can flip it to a source

The high latitudes are warming roughly two to four times faster than the global average. In the taiga this brings:

1. Warmer temperatures and less winter snow. Snow melts earlier, so less meltwater is stored in the soil for summer.
2. Drought. Trees suffer water stress in hot, dry summers, and drought-weakened trees are more easily killed by insect pests.
3. Lower primary production and browning. Satellite images show “browning”, a fall in green leaf cover, over large areas, as photosynthesis declines.
4. More frequent and more intense fires. Drier fuel burns more often and more deeply.
5. Combustion of legacy carbon. Deep organic soil layers that survived previous fires, storing carbon for decades or centuries, now burn too.

When losses from fire, respiration and decomposition exceed the carbon taken up by photosynthesis, the forest changes from net carbon accumulation to net carbon loss. Because that release causes more warming, which causes more fire and drought, the change can become self-reinforcing. Burned stands may regrow as broadleaf woodland or grassland that stores less carbon, so the system does not simply return to its old state.

This is an example of a tipping point: a threshold beyond which a small extra change shifts a system into a different state that is hard to reverse (tipping points are also discussed in D4.2 Stability and change). Canada’s record 2023 fire season burned more than 15 million hectares, and in some recent years boreal fires have released as much carbon as a large industrial country emits from fossil fuels.

Polar ice is a habitat: a solid platform on which animals rest, breed and reach their food. Two types matter here:

• Landfast ice (fast ice) is sea ice frozen to the coast or to grounded icebergs, so it stays in place.
• Sea ice (pack ice) floats freely and drifts with winds and currents.

Both are forming later, shrinking and breaking up earlier. The Arctic’s September sea-ice extent has fallen by roughly 13% per decade since satellite records began in 1979.

Emperor penguins (Aptenodytes forsteri): losing breeding grounds

Emperor penguins breed on landfast ice around Antarctica in the southern winter. Eggs are laid around May–June, incubated by the males on their feet, and the chicks need the ice to stay stable until about December–January, when they have grown waterproof adult feathers and can go to sea. If the fast ice breaks out early, chicks still covered in down fall into the sea and drown or freeze.

In 2022, record-low sea ice in the Bellingshausen Sea region led to total breeding failure at four of five colonies there, because the ice broke up before the chicks fledged. Models predict that most colonies could be lost by 2100 if warming continues at a high rate.

Walruses: losing sea-ice platforms

Walruses dive to feed on clams and other animals on the sea floor of shallow continental shelves. Females and calves rest on sea ice between dives, which lets them feed over wide areas far from land and keeps calves away from predators and crowds. In the Chukchi Sea, summer ice now often retreats beyond the shelf over water too deep to feed in, so walruses must haul out on beaches instead:

• tens of thousands crowd together, and calves can be crushed when the herd stampedes;
• they must make longer swims from shore to feeding areas, using more energy;
• feeding grounds near the haul-out sites are depleted.

In the open ocean, mineral nutrients such as nitrate, phosphate and iron are quickly used up by phytoplankton in the sunlit surface layer. Dead organisms and faeces sink, so the nutrients accumulate in deep water, where there is too little light for photosynthesis.

Upwelling

In some places, winds and the Earth’s rotation push surface water away from a coast, and cold, nutrient-rich deep water rises to replace it. This upwelling supplies nutrients to the surface, so phytoplankton grow fast. High primary production supports large populations of zooplankton, fish (such as the Peruvian anchoveta), seabirds and marine mammals. Upwelling zones off Peru, California and south-west Africa cover only a small fraction of the ocean surface but yield a large share of the world’s fish catch.

How warming disrupts it

Warm water is less dense than cold water, so as the surface warms it floats more stably on top of the cold deep water. This stratification reduces mixing, and changes in winds and currents can alter the timing and strength of upwelling. The result:

warmer surface water → less upwelling of nutrients → less phytoplankton growth → lower primary production → less energy passed along marine food chains

A natural example shows the effect. During El Niño events, trade winds weaken and warm water spreads east across the Pacific, suppressing upwelling off Peru. In 1972–73 this contributed to the collapse of the anchoveta fishery, and seabird colonies that depend on the fish declined sharply.

Every species can survive only within its range of tolerance for temperature and other factors (B4.1 Adaptation to environment). As the climate warms, the zones with suitable conditions move towards the poles and up mountains (temperature falls with altitude). Populations at the warm edge of a range decline, while new populations can establish at the cool edge, so the range shifts. Animals do this by moving; plants do it through seed dispersal and differences in seedling survival.

Upslope: tropical montane birds in New Guinea

Researchers repeated bird surveys first carried out by Jared Diamond on mountains in Papua New Guinea. On Karkar Island (surveyed 1969, resurveyed 2013), the upper elevation limits of the montane species had moved upslope, by about 152 m on average. On Mount Karimui (1965 and 2012) upper limits rose by about 113 m. Tropical montane species often have narrow temperature tolerances, so they respond strongly. Species already near a summit have nowhere higher to go and may become extinct (A4.2 Conservation of biodiversity).

Poleward: North American trees

Trees cannot move, and they live for decades or centuries, so their ranges change slowly. A study of 92 tree species in the eastern USA compared where seedlings grow with where adult trees grow:

• about 21% showed a northward spread (seedlings beyond adults in the north, absent in the south);
• about 59% showed range contraction at both edges (seedlings failing at the southern edge but not yet spreading north);
• about 16% shifted south.

Other studies show conifers tending to move north, while many broadleaf species have shifted west, perhaps following rainfall. Trees are often too slow to keep up with the speed of warming.

Tropical coral reefs are built by reef-building (hard) corals: colonies of small polyps that secrete skeletons of calcium carbonate (CaCO₃). Inside the polyps’ cells live zooxanthellae, photosynthetic algae that supply the coral with sugars, in a mutualism (C4.1 Populations and communities). Reefs cover far less than 1% of the sea floor but support roughly a quarter of all marine species. Rising CO₂ threatens them in two separate ways.

Ocean acidification: caused by increased CO₂

The oceans have absorbed roughly a quarter of the CO₂ emitted by humans. Dissolved CO₂ reacts with water:

CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻

The extra hydrogen ions lower the pH, from about 8.2 before industrialization to about 8.1 now (about 30% more H⁺, because pH is a logarithmic scale). They also react with carbonate ions:

H⁺ + CO₃²⁻ → HCO₃⁻

Corals need carbonate ions to combine with calcium (Ca²⁺ + CO₃²⁻ → CaCO₃), so fewer carbonate ions means suppressed calcification: skeletons grow more slowly and are weaker, and if carbonate falls far enough, existing CaCO₃ starts to dissolve.

Bleaching: caused by high temperature

When water stays about 1 °C or more above the usual summer maximum for several weeks, the zooxanthellae are damaged and the corals expel them. The corals turn white (bleaching) and lose most of their food supply. If the heat passes quickly the algae can return, but long or repeated bleaching kills the corals. Mass bleaching events have become more frequent; the fourth global event began in 2023.

From coral loss to reef collapse

Corals are the foundation species of the reef: their skeletons create the three-dimensional structure that fish and invertebrates use for food, shelter and breeding. When corals die, the reef is overgrown by seaweed, erodes, and the community that depends on it collapses: a whole ecosystem is lost, not just one species.

Carbon sequestration means removing CO₂ from the atmosphere and storing it for a long time. Biological approaches use photosynthesis to build long-lasting carbon stores: wood in trees and peat in wetlands.

The plantation debate

Scientists actively debate the best approach:

• Non-native plantations (e.g. Sitka spruce in Britain, eucalyptus or pine elsewhere) grow fast and sequester carbon quickly, and produce timber. But they are usually monocultures with low biodiversity, are vulnerable to pests and fire, and much of their carbon returns to the air if the wood is harvested and burned.
• Rewilding with native species creates diverse, more resilient forests that support native wildlife and keep carbon stored for longer, though growth can be slower. Some argue that non-native species may be better suited to a warmer future climate.

Where peat forms

Peat forms where dead plant matter accumulates faster than it decomposes: in waterlogged soils, which are anaerobic and often acidic, so saprotrophs work very slowly. This happens in bogs and fens of the temperate and boreal zones, where peat builds up about a millimetre a year, and very rapidly in some tropical peat swamp forests, such as in South-East Asia and the Congo Basin. Peatlands cover only about 3% of the land but store roughly 600 billion tonnes of carbon, more than all the world’s forest vegetation.