Climate Change

Greenhouse gases and the natural greenhouse effect

A greenhouse gas is a gas that absorbs radiation and emits the energy as thermal (heat) energy. The atmosphere is transparent to most of the short-wave radiation arriving from the Sun, so sunlight reaches the surface almost unhindered, but it is not transparent to the long-wave infrared radiation that the warmed surface gives off. When a greenhouse gas molecule absorbs that infrared radiation it gains energy and quickly re-emits it in all directions, including back down towards the surface. That returning energy is what makes the surface warmer than it would otherwise be.

The natural greenhouse effect follows a fixed sequence, and you should learn the steps in order. Solar radiation passes through the Earth's atmosphere. Some of it is absorbed by the land and the oceans, heating the Earth, and some is reflected back into space by clouds, ice, snow and light-coloured surfaces. The warmed surface then re-emits radiation as infrared, and this infrared radiation is absorbed and re-emitted back towards the Earth's surface by greenhouse gases and clouds. Without this natural process the average surface temperature would be about -18 °C instead of about 15 °C, water would be frozen at the surface almost everywhere and life as we know it could not exist. The greenhouse effect itself is therefore natural and essential; the difficulty is that human activity is strengthening it.

The main greenhouse gases are carbon dioxide (CO2), water vapour (H2O) and methane (CH4), with nitrous oxide and chlorofluorocarbons (CFCs) adding further warming. Water vapour is the most abundant of them and behaves as a feedback rather than as a control: warmer air holds more water vapour, which absorbs more infrared radiation and warms the air further. Methane absorbs far more heat per molecule than carbon dioxide does, but it breaks down in the atmosphere within about a decade, whereas carbon dioxide persists for centuries and nitrous oxide for about a century. CFCs, which were used in aerosols and refrigerants, are powerful greenhouse gases as well as the cause of ozone depletion.

The enhanced greenhouse effect: from global warming to climate change

The enhanced greenhouse effect is the extra warming caused by the increase in the concentration of greenhouse gases from human activity. Adding more greenhouse gas molecules to the atmosphere means that a larger fraction of the infrared radiation leaving the surface is absorbed and re-emitted back down again. The balance of energy shifts: less energy escapes to space than arrives from the Sun, so the extra energy is stored in the surface, the oceans and the lower atmosphere and the average temperature rises. This rise in the average temperature of the Earth's surface and lower atmosphere is global warming.

Global warming in turn produces climate change. Weather is the day-to-day state of the atmosphere - whether it is raining or sunny today - while climate is the average weather of a place worked out from many years of records, usually taken over 30 years. A single cold winter therefore does not disprove global warming: the warming is a change in the average of a great many readings, and as the average shifts, so does the frequency of hot spells, heavy rainfall and drought. Global warming can result in climate change, which is a long-term change in the patterns of temperature, rainfall and wind.

The warming is not the same everywhere. Land warms faster than the ocean, and the polar regions warm faster than the tropics because melting snow and ice expose darker sea and land that absorb more sunlight, so the ice-albedo feedback adds extra warming where the ice is being lost. Several other feedbacks make the system harder to predict. Warmer air holds more water vapour, which is itself a greenhouse gas; thawing permafrost releases methane that had been frozen into the ground; and a warmer ocean dissolves less carbon dioxide, so that part of the ocean which has been absorbing carbon dioxide weakens as it warms.

Two openings are worth memorising because they earn marks in longer answers. First, the sequence: more greenhouse gas -> more infrared radiation absorbed and re-emitted -> surface and ocean temperature rise -> ice melts, seawater expands, rainfall patterns and weather extremes change. Second, the difference between them: the enhanced greenhouse effect is the process, global warming is the rise in average temperature, and climate change is the wider change in the pattern of climate.

• Enhanced greenhouse effect: extra greenhouse gases absorb and re-emit more of the infrared radiation leaving the surface.
• Global warming: a rise in the average temperature of the Earth's surface and lower atmosphere.
• Climate change: a long-term change in the average weather and in the pattern of weather at a place.
• Feedbacks to quote: the ice-albedo feedback, more water vapour in warmer air, methane released from thawing permafrost, and a warmer ocean absorbing less carbon dioxide.

Why concentrations are rising: fossil fuels, cement, land use and farming

Combustion of fossil fuels is the largest single source of extra carbon dioxide. Coal, oil and natural gas are burned in power stations, factories, homes and vehicles, and the carbon that was locked into the rock over hundreds of millions of years is returned to the atmosphere as carbon dioxide within a few hundred years. Coal releases the most carbon dioxide for each unit of energy, and natural gas the least, but all three add to the total. Increasing human population and increased energy usage multiply this: more people need more homes, food, goods and journeys, and rising wealth means each person can afford more electricity, heating, air conditioning and travel, so total demand keeps climbing.

Cement manufacture releases carbon dioxide in two ways. Limestone, which is calcium carbonate, is heated with clay in a kiln to produce calcium oxide (lime) and carbon dioxide, and that carbon dioxide comes from the chemical change itself, not only from the fuel burned to heat the kiln. Roughly half of the emissions from cement come from this reaction, so even a kiln powered by clean electricity would still release carbon dioxide, and cement is used in such quantities that it is a major global source.

Agriculture adds methane and nitrous oxide. Cattle, buffalo, sheep and goats are ruminants: the microbes in the first stomach ferment the grass they eat and produce methane, which the animal belches out. The same happens as heaps of manure and slurry decompose. Flooded rice fields are also a source, because the water keeps the soil oxygen-free, and microbes in waterlogged mud produce methane instead of carbon dioxide. Nitrogen fertiliser and manure add nitrous oxide. Livestock also need land, so forests are often cleared for pasture and for feed crops, which doubles the effect on the atmosphere.

Changes in land use and deforestation alter the balance in two ways at once. Draining a wetland or a peat bog lets air into the peat, so the stored carbon decomposes and escapes as carbon dioxide and methane, and drained peat also dries out and burns readily in fires. Cutting down forest removes trees that were removing carbon dioxide from the air by photosynthesis, and the felled timber is usually burned or left to rot, which releases carbon dioxide as well. Forests are therefore both a store of carbon and a working sink, and destroying them does the damage twice over.

The evidence that the climate is changing, and how the trend is measured

The most direct evidence comes from thermometers. Weather stations on land record the air temperature in a standard way, ships and drifting buoys record the temperature of the sea surface, and satellites measure both from orbit. Thousands of station records are combined into a global average, and the instrumental record goes back to about 1850. The records are not perfect: the network is thin over the oceans, over Africa and in polar regions, some stations have moved, some have been swallowed by a growing city whose buildings and tarmac create an urban heat island, and instruments have changed. Each of these problems is corrected for, and several different groups produce their own version of the global curve, which is why the trend is not the work of a single thermometer.

The second kind of evidence is proxy data, which gives the temperature and the composition of the air long before thermometers existed. Ice cores drilled in Antarctica and Greenland are the most valuable: each year's snowfall traps a little air, and drilling down through the ice recovers bubbles of ancient atmosphere, so the carbon dioxide and methane concentrations of the past can be measured directly, while the isotopes in the ice itself record the temperature at the time. The record extends back hundreds of thousands of years and shows that carbon dioxide and temperature rose and fell together. Tree rings, corals, stalactites and lake and ocean sediments are other proxies: proxies are indirect, so they show the pattern and the direction of change rather than an exact temperature for one year.

A third line of evidence is the physical changes that a warming world should produce, and all of them agree with the thermometer record. Glaciers are retreating in the Alps, the Himalayas and the Andes; the Greenland and West Antarctic ice sheets are losing mass; Arctic sea ice is shrinking in summer; permafrost is thawing; the ocean is storing more heat; and sea level is rising, as measured by tide gauges and by satellite altimetry. Living things show the same signal: spring arrives earlier, leaves open sooner, birds and insects appear before they used to, warm-water species are moving towards the poles and up mountains, and heat-sensitive corals bleach in marine heatwaves.

The trend is now clear enough to state simply. The average global surface temperature has risen by more than 1 °C since the late nineteenth century, most of it since the 1970s, and the warmest years on record have all occurred in the last decade. Global mean sea level rose by about 0.20 m between 1901 and 2018, and the rate of rise has increased in recent decades as the ice sheets have begun to contribute more. Notice that the trend is measured in averages over decades, not in single years: one hot summer or one cold winter tells you about the weather, not about the climate.

Impacts of climate change on the physical world and on people

Temperature and sea level. The surface of the Earth and the surface of the ocean both warm. Sea level rises for two reasons: meltwater from glaciers and the Greenland and West Antarctic ice sheets adds water to the ocean, and seawater expands as it warms, which is thermal expansion. Thawing permafrost damages buildings, roads and pipelines built on frozen ground because the ground slumps, and it releases methane that had been locked in the soil. Sea level rise matters most on low coral islands, such as the Maldives, where the highest natural ground is only about 2.4 m above the sea, and on low-lying river deltas such as the Ganges-Brahmaputra in Bangladesh and the Nile in Egypt, where salt water pushes into the groundwater and turns farmland saline.

Ocean acidification is a separate problem caused by the gas itself rather than by the warming. Carbon dioxide dissolves in seawater and forms a weak acid, so the pH of the surface ocean falls - by about 0.1 units since pre-industrial times. Because the carbonate ions in the water are used up by that reaction, corals, molluscs and some plankton find it harder to build their skeletons and shells of calcium carbonate. Coral bleaching is driven by heat, and acidification slows the recovery of the reef afterwards, so the two effects together reduce the reefs that shelter juvenile fish and support fisheries and tourism.

Ecosystems. A warming climate moves the conditions that species need, usually towards the poles and up hillsides, but habitats cannot move with them: a coral reef, a mangrove forest, a cloud forest or an alpine meadow has nowhere to go, so it shrinks and is lost. Where cool-water species retreat and warm-water species arrive, biodiversity changes and food chains are disrupted, partly because the timing of events slips apart - insects may hatch before the chicks that feed on them do. Warmer winters also let pests and invasive species survive and spread into places that used to be too cold for them.

People. Extreme weather events become more frequent and more severe: heatwaves, heavy rainfall and flooding, drought and wildfire. Crops change: yields usually fall in tropical and subtropical regions where heat and drought already limit growth, while some cooler regions may gain, and the effect of fertilisation by extra carbon dioxide is held back by shortages of water and nutrients. Falling yields and lost land raise food prices, and where harvests fail there are food shortages. Water supply becomes less reliable where the glaciers that feed rivers shrink and where rainfall becomes more variable, so irrigation competes with drinking water and hydropower. Health is affected by heat stress, by smoke from wildfires, by diseases and accidents after floods, and by mosquito-borne diseases such as malaria and dengue spreading into new, previously cooler regions. Finally, people are forced to move when their land disappears under the sea, when their farmland turns saline or when drought makes grazing impossible, which is why the Pacific atoll states and the drylands of the Sahel are both areas of climate migration.

Mitigation: strategies for reducing the carbon footprint

Your carbon footprint is the total mass of greenhouse gases released by a person, an organisation or a product, usually measured in tonnes of carbon dioxide equivalent. Mitigation means cutting the emissions that cause climate change, and it is the only way to stop the problem from growing. The largest single lever is a reduced use of fossil fuels: generating electricity from wind, solar, hydro, geothermal and nuclear power, heating buildings with electric heat pumps instead of gas, and moving vehicles onto electricity or hydrogen. The limitations are real. Renewables need large amounts of capital, new transmission lines and storage because their output varies with the weather, electric vehicles and batteries need mined lithium, cobalt and nickel, and existing coal stations are expensive to shut down early while many governments still subsidise fossil fuels. Increased energy efficiency - insulated buildings, LED lighting, efficient motors and appliances - delivers the cheapest cuts because it saves fuel as well as emissions, but older buildings are costly to retrofit and the money saved may simply be spent on other energy use.

Food and farming offer the next largest savings. Sustainable food choices mean eating more plants and less meat, eating food in season and buying locally produced food, because growing feed and grazing livestock takes far more land and energy per calorie than growing crops for people. The limitation is that local food is not automatically low-carbon: tomatoes grown locally in a heated greenhouse in winter can release more carbon dioxide than tomatoes trucked in from a warm country, and transport is usually a smaller part of a food's footprint than the way it was produced, so what you eat matters more than how far it travelled. Reducing livestock farming attacks methane directly, since cattle are the largest agricultural source, and it can be done by eating less beef and dairy or by breeding and feeding animals more efficiently. Against this, livestock supports the livelihoods of hundreds of millions of smallholders, meat and milk are culturally important, and demand for meat is rising in middle-income countries as incomes grow.

A reduction in the number of children per woman lowers the long-run size of the population and therefore the total demand for energy, food and land. It works through better family planning, girls staying in school and lower child mortality, so families choose to be smaller; the limitation is that it takes a generation to show any effect, emissions depend even more on how much each person consumes than on how many people there are, and any attempt to force smaller families has serious consequences, as China discovered with its one-child policy, which left an imbalanced sex ratio and a rapidly ageing population.

Land and technology provide the fourth group. Reforestation and afforestation remove carbon dioxide from the air by photosynthesis and store it in wood and soil, while also providing habitat, shade and protection from soil erosion; but a new forest takes decades to build up its carbon store, that store can be lost to fire, pests or later felling, an even-aged plantation of a single species stores far less carbon and supports far less wildlife than natural forest, and planting trees on drained peat or on naturally treeless grassland can release more carbon than it saves. Carbon sequestration means capturing carbon dioxide, removing it from the atmosphere and storing it: it can be captured at a power or cement plant and pumped into deep saline rock or a depleted oil or gas field, as happens at the Sleipner field in Norway, or by burning biomass in a power station and burying the carbon dioxide, or by drawing carbon dioxide out of the air directly. Capture is expensive, it uses a large amount of the plant's own energy, it can only be fitted to large fixed sources such as power stations and cement works, so it does nothing for cars or home heating, and the stored gas has to be monitored for leaks for a very long time. Soil carbon, biochar and protecting existing peatland store carbon more cheaply but on a limited scale.

• Reduced use of fossil fuels: renewables, nuclear and electrified transport cut emissions, but cost, intermittent output, storage needs and mined battery metals limit how fast it can be done.
• Sustainable food choices: a plant-based diet, seasonal food and locally produced food use less land and energy - but local food grown out of season can emit more, and what you eat matters more than how far it travelled.
• Reduction in livestock farming: fewer cattle cut methane directly, but livestock supports many livelihoods and demand for meat is rising.
• Reduction in the number of children per woman: lowers long-run demand through family planning and girls' education, but it is slow and coerced family limitation causes other harm.
• Increased energy efficiency: the cheapest cuts, because less fuel is used; older buildings are expensive to retrofit and savings may be spent on other energy use.
• Reforestation and afforestation: they remove carbon dioxide and provide habitat, but take decades to work and can be lost to fire, pests or later felling.
• Carbon sequestration: capture, remove and store carbon dioxide, but it is costly, energy-hungry and only practical for large fixed sources.
• Transport policies: congestion charges and low-emission zones, better public transport, cycle routes and rail freight cut fuel use, but they are unpopular, they hit poorer drivers hardest and they need a viable alternative to exist.

Policy, investment and adaptation: living with a changing climate

Governments act on emissions through international agreements and policies, taxation, research and climate models, and increasingly through artificial intelligence. The Paris Agreement, agreed in 2015, set out to hold the rise in global average temperature well below 2 °C above pre-industrial levels and to pursue efforts to limit it to 1.5 °C; each country submits a nationally determined contribution and these are reviewed and raised every five years, with finance promised to poorer countries. Its strength is that almost every country signed it; its weakness is that the pledges are voluntary and not legally binding, so countries can miss their targets, and the United States even withdrew for a period. Taxation puts a price on emitting: Sweden has had a carbon tax since 1991, and the European Union's emissions trading scheme sets a cap on emissions and lets companies trade allowances. A carbon price encourages efficiency and low-carbon investment, but it raises costs for households and industry, it can be regressive, hitting poorer families hardest unless the revenue is recycled, and it can cause carbon leakage, where heavy industry simply moves to a country with no carbon price so that the emissions still happen, just somewhere else - the EU scheme's early years were also weakened by too many free allowances, which pushed the price down.

Research and climate models are the basis of planning. Powerful computer models of the atmosphere, ocean, ice and land are run for different pathways of future emissions to project temperatures, rainfall patterns, sea level and the frequency of extremes, and governments, insurers and engineers use those projections to decide where to build and how high a defence must be. The limitations are that models simplify an extremely complex system, they agree far more closely on global averages than on the rainfall and storminess of one small region, and projections for decades ahead depend on which emissions pathway the world actually follows - which is a political choice, not a scientific one. Uncertainty is sometimes used as an excuse for delay, although the direction of the change is not seriously disputed. Artificial intelligence now complements the models: machine learning speeds up and narrows down their output to the local scale, improves forecasts of extreme events, helps analyse satellite images to detect deforestation, methane leaks and illegal fishing, and can optimise the operation of an electricity grid so that wind and solar output is used more efficiently. Its limitations are that training large AI models consumes very large amounts of electricity and water; the predictions are only as good as the data used; and a better forecast does not by itself reduce a single tonne of emissions.

Climate change adaptation is defined as altering behaviour, practices and infrastructure to adapt to the impacts of climate change. It is not the same as mitigation: mitigation reduces the cause, while adaptation reduces the damage that is already happening, and both are needed because the greenhouse gases already released will keep warming the planet for decades. The first adaptation is in farming. Crops that resist warmer, drier or wetter conditions - drought-tolerant millet and sorghum, salt-tolerant rice, and flood-tolerant rice such as the Sub1 varieties bred at the International Rice Research Institute in the Philippines, which survive two weeks under water - keep harvests coming as the climate shifts, along with changing planting dates, more efficient irrigation and farm water storage. The limits are that improved seed costs money and must be bought again, subsidies may not reach the poorest farmers, a tolerant variety often yields less than the best variety would in a good year, irrigation needs water that may not be there, and knowledge and credit have to reach the farm for any of it to happen.

The second adaptation is in defence and construction. Improved flood defences include sea walls and dykes, storm surge barriers such as the Thames Barrier in London and the Maeslantkering in the Netherlands, river embankments, beach recharge and, more cheaply, restoring mangroves, salt marshes and dunes, which absorb wave energy and grow upwards as the sea rises. They also include urban drainage, permeable paving and flood storage reservoirs upstream. The limitations are that hard defences are extremely expensive, they only protect up to their design standard, so a higher sea level or a bigger storm will overtop them and the standard has to be raised again, they can increase erosion further along the coast by starving it of sediment, and people feel safer behind a wall, so more building takes place behind it. Legislation about building near coasts and on floodplains - planning restrictions and set-back lines, flood risk maps, building codes and insurance rules, and managed retreat where homes are bought out and the land returned to the sea - keeps new development out of danger. Such rules are difficult and unpopular to enforce, land is scarce in growing cities, people are reluctant to abandon homes their families have lived in for generations, and the poorest households often end up on the most dangerous land anyway. Building design and materials adapt the buildings themselves: houses raised on stilts or on floating foundations as in the Netherlands and Bangladesh, reflective cool roofs and better insulation for heat, natural ventilation and shading instead of air conditioning, green roofs, rainwater harvesting and water tanks, and roofs strapped down to resist cyclones. These measures cost more at the outset, and the poorest households cannot afford them without help.

Judging the two approaches is the heart of the topic. Mitigation is the only strategy that can stop the problem growing, and it benefits everyone in the world at once, but that is exactly its weakness: the benefit is slow, it is shared by every country whether or not they pay for it, and so each country has an incentive to leave the cost to others, which is why international agreements have no teeth and emissions have kept rising. Adaptation brings visible local benefits quickly, protects lives and farmland now and can be fitted to the place, but it is expensive, it has to be repeated and raised as the impacts worsen, and it cannot protect everything: a glacier, an ice sheet or a low-lying atoll cannot be defended in the long run. Adaptation also shifts costs onto local taxpayers, which is why poorer countries, who are usually the most exposed and the least responsible for the emissions, need international finance and technology transfer. The strongest answers combine both and make a judgement: mitigate urgently because the cost of adaptation rises without limit if the warming continues, and adapt now because the impacts are already here.