1The idea in one paragraph

Carbon dioxide and methane absorb the infrared radiation that the Earth's surface gives off, and human activity has raised the concentration of both, so the Earth is warming. Several positive feedback cycles then make the warming worse, and some ecosystems, such as boreal forests, may pass tipping points where they switch from storing carbon to releasing it. The impacts reach every level of life: polar animals lose the ice they breed and rest on, ocean food chains lose their nutrient supply, species move towards the poles and up mountains, and coral reefs bleach and dissolve. Carbon can be taken back out of the air by growing forests and restoring peat-forming wetlands. At HL, climate change also puts species that time their lives by different cues out of step with each other, lets insects fit more generations into a year, and changes which variants natural selection favours.

2The anthropogenic causes: carbon dioxide and methane

Anthropogenic means caused by humans. The guide limits the causes you need to two gases.

How greenhouse gases warm the Earth. Sunlight arrives mostly as visible and short-wave radiation, which passes through the atmosphere and is absorbed by the ground and the oceans. The warmed surface gives off energy as long-wave infrared radiation. Greenhouse gases, including carbon dioxide, methane and water vapour, absorb infrared radiation and re-emit it in all directions, some of it back towards the surface. So less energy escapes to space, and the lower atmosphere and the surface are warmer than they would otherwise be. Figure 1 shows the balance.

That natural greenhouse effect is what makes Earth habitable. The problem is its enhancement: more greenhouse gas means more infrared absorbed and a higher equilibrium temperature.

Carbon dioxide has risen from about 280 parts per million (ppm) before the industrial revolution to over 420 ppm in the 2020s. The human sources are:

• combustion of fossil fuels, coal, oil and natural gas, for electricity, transport, heating and industry, which releases carbon that was locked away for millions of years;
• deforestation, especially by burning, which releases the carbon stored in trees and soil and removes a carbon sink;
• cement production, which releases carbon dioxide from limestone.

Methane has more than doubled since pre-industrial times, from about 0.7 ppm to over 1.9 ppm. It is present in much smaller amounts than carbon dioxide, but each molecule absorbs far more infrared. The human sources are:

• cattle and other ruminants, whose gut microorganisms produce methane as they digest cellulose;
• flooded rice paddies, where anaerobic bacteria in waterlogged soil produce methane;
• landfill sites, where buried organic waste decomposes anaerobically;
• leaks from fossil fuel extraction, from gas wells, pipelines and coal mines.

Nature of science: correlation and causation. Air bubbles trapped in Antarctic ice preserve samples of the ancient atmosphere, and the ratio of oxygen or hydrogen isotopes in the ice records the temperature when it formed. Ice cores reaching back hundreds of thousands of years show that carbon dioxide concentration and temperature rose and fell together through the ice ages. Figure 2 sketches the kind of pattern they show.

• A positive correlation means that as one variable increases, the other tends to increase too. A negative correlation means that as one increases, the other tends to decrease. The ice-core data show a strong positive correlation.
• Correlation does not prove causation. Two variables can rise together because one causes the other, because the other causes the first, or because a third factor drives both. In the ice-core record, warming at the end of an ice age was often set off by changes in the Earth's orbit, and the warming then released carbon dioxide from the oceans, which amplified the warming. So the correlation alone cannot show that carbon dioxide drives temperature.
• Other evidence confirms the causal link. Laboratory measurements show exactly which wavelengths of infrared carbon dioxide absorbs. Satellites measure less infrared escaping to space at those wavelengths as carbon dioxide rises. The physics predicts the warming, and the observed warming matches it. A causal claim needs a mechanism and evidence beyond the correlation, and here both exist.

3Positive feedback cycles in global warming

A positive feedback cycle is one in which a change causes effects that increase that change (compare the negative feedback of homeostasis in D3.3). Warming triggers several, and each makes warming faster. Figure 3 sets out the five in the guide.

• Carbon dioxide from the deep ocean. The oceans hold vast amounts of dissolved carbon dioxide. Gases are less soluble in warmer water, and warming also changes ocean circulation, so some of the carbon dioxide stored in deep water is released to the atmosphere, causing more warming.
• Loss of reflective snow and ice. Snow and ice reflect most of the sunlight that falls on them; their reflectivity is called albedo. When they melt, the darker land or ocean beneath absorbs much more solar radiation, so the surface warms further and more ice melts.
• Faster decomposition of peat and permafrost organic matter. Permafrost is ground that stays frozen all year. In the Arctic it contains enormous amounts of dead plant material that has never decomposed, because decomposers cannot work in frozen soil. Peat builds up where waterlogging and cold slow decomposition. As they warm and dry, decomposers become more active and respire the organic matter, releasing carbon dioxide.
• Methane from melting permafrost. Where thawed permafrost is waterlogged, the decomposition is anaerobic and releases methane, a stronger greenhouse gas.
• Droughts and forest fires. Warming makes droughts more frequent and more severe in many regions. Drought-stressed forests grow less, so absorb less carbon dioxide, and they burn more easily. Fires release carbon dioxide directly.

4Boreal forests: from carbon sink to carbon source

The boreal forest, or taiga, is the belt of coniferous forest across northern Canada, Alaska, Scandinavia and Russia. For thousands of years it has taken in more carbon by photosynthesis than it released, storing it in trees and above all in thick layers of cold, slowly decomposing soil organic matter. It has been a net carbon accumulator, a sink. There is evidence that parts of it are passing a tipping point into becoming a net carbon source. Figure 4 shows the chain.

1. Warmer temperatures and decreased winter snowfall. Less snow means less meltwater in spring, when trees start growing, so soils are drier.
2. More frequent drought. Warmer summers increase evaporation and transpiration, so trees suffer water stress.
3. Reduced primary production. Stressed trees close their stomata, photosynthesise less and grow less. Satellites detect this as forest browning: a decrease in green, photosynthesising leaf cover, the opposite of the greening once expected from warming.
4. More frequent and intense forest fires. Dry trees and dry soil burn readily, and fires become larger and more frequent.
5. Legacy carbon combustion. Boreal soils contain deep layers of organic matter that built up over centuries and survived earlier, lighter fires. More intense fires burn down into these layers, releasing legacy carbon: carbon that had been stored for far longer than the interval between fires.

Once fires release more carbon than regrowth can absorb before the next fire, the forest as a whole loses carbon. That is the tipping point: the forest has switched from slowing climate change to adding to it, and each fire makes the next more likely.

5Polar habitat change: landfast ice and sea ice

Landfast ice is sea ice attached to the coast or to the sea floor in shallow water, so it does not drift. Sea ice more generally is frozen seawater, much of it drifting.

Emperor penguins (Aptenodytes forsteri) breed on landfast ice around Antarctica. They arrive in autumn, lay eggs in winter, and raise their chicks on the ice. The chicks cannot swim until they have replaced their downy feathers with waterproof adult feathers, in the southern summer. The ice must therefore stay attached for most of the year. If warming makes the landfast ice break out early, before the chicks have fledged, the chicks fall into the sea and drown or freeze. Colonies have already suffered complete breeding failure in years when the ice broke out early, and the potential loss of breeding grounds is a major threat to the species.

Walruses in the Arctic feed on clams and other animals on the shallow sea floor. They use floating sea ice as a platform to rest between feeding dives and to give birth and nurse calves, and the ice lets them reach feeding grounds far from shore. As summer sea ice retreats beyond the shallow continental shelf, walruses are forced to rest on land instead. They gather in huge crowds on beaches, where disturbance can cause stampedes that crush calves, and they must swim much further between their resting places and their feeding grounds, which costs energy.

6Warmer surface water and nutrient upwelling

In many parts of the ocean, winds and currents bring cold, deep water to the surface. This is upwelling. Deep water is rich in nutrients such as nitrate and phosphate, from organisms that died and sank and were decomposed. Where it reaches the sunlit surface, phytoplankton grow fast, and these areas support some of the richest fisheries on Earth.

When surface water warms, it becomes less dense and floats more strongly on the cold water below. The layers mix less, and the timing and extent of upwelling change: less nutrient-rich water reaches the surface, or it reaches it at the wrong time of year. Phytoplankton are starved of nutrients, primary production falls, and less energy flows up marine food chains to zooplankton, fish, seabirds and marine mammals. A well-known short-term example is El Niño: in El Niño years, warm surface water spreads along the coast of Peru, upwelling weakens, and the anchovy population and the seabirds that eat them crash. Climate change may make such conditions more frequent or more lasting.

7Range shifts: poleward and upslope

Every species survives within a range of temperatures. As the climate warms, the zone of suitable temperatures moves towards the poles and up mountains. Species that can move follow it: they colonise new areas at the cool edge of their range and die out at the warm edge. Figure 5 shows the upslope version.

Tropical montane birds in New Guinea. On mountains in New Guinea, researchers resurveyed bird species at sites that had been surveyed about half a century earlier, recording the elevations at which each species occurred. Many species had shifted their ranges upslope, consistent with the warming over that period. For species already living near mountaintops, there is nowhere further up to go, so their ranges shrink: the so-called "escalator to extinction".

North American trees. Trees cannot walk, but their ranges shift through where seedlings establish and where adults die. Surveys across the eastern United States comparing where seedlings grow with where mature trees grow have found that many species are regenerating further north than their adults, and failing to regenerate at the southern edge of their range. The result is range contraction in the south and northward spread in the north. Trees spread slowly, over generations, so many species are expected to lag behind the shifting climate.

8Coral reefs: a threat of ecosystem collapse

Coral reefs are built by corals: small animals living in colonies, each laying down a skeleton of calcium carbonate. Reef-building corals contain photosynthetic algae called zooxanthellae in their tissues, which supply most of their food. Reefs cover a tiny fraction of the sea floor, yet they are commonly estimated to support about a quarter of all marine species. Climate change threatens them in two ways, shown in Figure 6.

Ocean acidification, caused by increased carbon dioxide. Much of the extra carbon dioxide dissolves in the ocean, forming carbonic acid, which lowers the pH:

Corals need carbonate ions to build calcium carbonate. With fewer carbonate ions available, calcification is suppressed: skeletons grow more slowly and are weaker, and existing reef can start to dissolve.

Coral bleaching, caused by increased water temperature. When the water stays a degree or two above the normal summer maximum for weeks, the corals expel their zooxanthellae. Without them the coral turns white, which is bleaching, and loses its main food supply. If the heat lasts too long, the corals starve and die. The Great Barrier Reef has suffered repeated mass bleaching in the past decade.

Collapse. Corals are the foundation of the reef ecosystem. When they die, the reef structure erodes, the fish and invertebrates that shelter and feed there disappear, and algae take over. Loss of corals causes collapse of the reef ecosystem, with the loss of most of its species and of the coastal protection and fisheries it provides.

9Taking carbon back: afforestation, regeneration and peatland restoration

Carbon sequestration is the capture and long-term storage of carbon dioxide from the atmosphere. Living systems can do it by photosynthesis, storing the carbon in biomass and soil. The guide names three approaches.

• Afforestation: planting trees on land that has not been forested in recent times, such as grassland or former farmland.
• Forest regeneration: allowing or helping forest to regrow on land where it has been cleared, by natural seeding from nearby trees or by planting.
• Restoration of peat-forming wetlands: rewetting peatlands that were drained for farming or forestry. Peat is partly decomposed plant matter that builds up in waterlogged soils, where lack of oxygen and often acidic conditions slow decomposition, so more carbon is fixed than released. Drained peat dries and decomposes, releasing carbon dioxide; rewetting stops the loss and restarts accumulation. Peat forms naturally in temperate and boreal zones, and very rapidly in some tropical ecosystems, such as the peat swamp forests of South-East Asia and the Congo Basin.

Nature of science: an active scientific debate. Scientists disagree over whether plantations of non-native trees or rewilding with native species offer the better approach to sequestration. The arguments run like this.

A strong answer does not declare a winner. It says that the best choice depends on the land, the timescale and the goals: the fastest carbon uptake, the most secure long-term store, or biodiversity as well as carbon.

14Linking questions

What are the impacts of climate change at each level of biological organisation? Molecules: enzymes and proteins work less well outside their temperature range, and carbonate chemistry changes with pH. Cells: coral cells expel their zooxanthellae. Organisms: penguin chicks drown, walruses crowd on land, insects develop faster. Ecosystems: ranges shift, reefs collapse, forests switch from sinks to sources, and synchrony between species breaks down.

What processes determine the distribution of organisms on Earth? Each species' tolerance of abiotic factors, above all temperature and water; its interactions with other species; its ability to disperse; and history, including where it evolved. Climate change moves the first and so moves distributions, as the range shifts in section 7 show.