Climate change

Weather, climate and the greenhouse effect

Weather is the condition of the atmosphere at one place at one moment: today's rain, today's temperature, the wind this afternoon. Climate is the average weather of a place over a long period, usually thirty years or more. That difference is the reason a cold week in January does not disprove climate change: one cold week is weather, while the slow rise in average global temperature over decades is climate.

The Earth is warmed by energy from the Sun. Some of that energy is reflected straight back into space by clouds, ice and light-coloured surfaces, and the rest is absorbed by the land and the oceans, which then radiate it back outwards as infrared (long-wave) radiation. In the atmosphere, certain gases - the greenhouse gases - absorb some of that outgoing infrared radiation and re-radiate it in all directions, including back down towards the surface. This is the natural greenhouse effect, and without it the average temperature of the Earth would be far colder than it is, and life as we know it would not exist. The greenhouse effect is not the problem; the problem is that we are strengthening it.

The main greenhouse gases are carbon dioxide, methane, nitrous oxide and water vapour. They are released naturally by volcanoes, by respiration, by the decay of plants and animals, and by the oceans, and they are removed naturally by plants, by the oceans and by the weathering of rocks. Human activity has added a large extra quantity of them - particularly carbon dioxide and methane - faster than the natural systems can remove them. With more greenhouse gases in the atmosphere, more outgoing infrared radiation is absorbed and re-radiated back to the surface, so the atmosphere warms. This extra warming caused by human emissions is the enhanced greenhouse effect.

The distinction the examiners want is precise. The natural greenhouse effect keeps the planet warm enough for life and has always existed. The enhanced greenhouse effect is the extra warming caused by the greenhouse gases that people have added to the atmosphere since the Industrial Revolution, mainly by burning fossil fuels, clearing forests and farming. Global warming is the rise in the Earth's average temperature that results; climate change is the wider set of changes that follow from it, including rainfall, storms, sea level and the timing of the seasons.

• Weather: the atmosphere at one place at one time. Climate: the average weather of a place over thirty years or more.
• Greenhouse gases absorb outgoing infrared radiation and re-radiate it, including back towards the surface: the main ones are carbon dioxide, methane, nitrous oxide and water vapour.
• The natural greenhouse effect keeps the Earth warm enough for life and is not a problem in itself.
• The enhanced greenhouse effect is the extra warming caused by the greenhouse gases people have added to the atmosphere.
• Global warming is the rise in average global temperature; climate change is the wider set of changes that follow.

The evidence for climate change

Climate change is not a prediction, it is a measurement, and the syllabus names four kinds of evidence you should be able to describe and evaluate.

Global temperature data. Weather stations around the world have been measuring temperature with thermometers for well over a century, and ships, buoys and satellites add measurements over the oceans. When these records are combined, they show a clear rise in the global average temperature, with the warmest years almost all falling in recent decades. The strengths of this evidence are that it is measured directly, it is global and it is continuous. Its limitations are that records are unevenly spread, with far more stations in Europe and North America than in Africa or over the oceans, that early instruments and methods changed over time, and that local effects such as the growth of a city around a weather station can distort a reading. Alongside the temperature records sit the measurements of the gases themselves, such as the continuous record of carbon dioxide in the atmosphere that has been taken at Mauna Loa in Hawaii since the late 1950s, which shows a steady rise.

Ice cores. In Antarctica and Greenland, snow falls year after year and is compressed into ice, trapping tiny bubbles of the atmosphere inside it. Drilling a core down through the ice and analysing those bubbles gives a direct sample of the air as it was when the snow fell, and the layers can be dated. Ice cores show that carbon dioxide and temperature have risen and fallen together over hundreds of thousands of years, and that the level of carbon dioxide in the atmosphere now is far higher than at any point in that record. The strength of the evidence is that it gives us a direct measurement of past atmospheres and takes us back long before thermometers existed. Its limitations are that cores come from a few polar and high mountain sites rather than from the whole planet, that the air in the bubbles is older than the ice around it, and that the dating becomes less precise further back.

Sea ice positions. Satellites have been photographing the polar regions since the late 1970s, measuring how far the sea ice extends each summer and winter. These records show the Arctic sea ice shrinking, with the summer minimum reaching further and further back from the coast, while the Antarctic has behaved differently. Ice sheets and glaciers on land can also be measured from space, and most of the world's glaciers have been retreating. The strength is that satellite data covers the whole polar region evenly and is repeatable. The limitation is the short record: it begins only in the satellite era, so it shows recent change but not what happened before.

Historic writing and paintings. Written records, paintings, diaries and ships' logs describe the climate of the past in a way no instrument could. In Europe, documents record the Little Ice Age, a cooler period lasting several centuries in which winters were severe enough for the River Thames in London to freeze and for frost fairs to be held on the ice, and artists such as Pieter Bruegel the Elder painted snow-covered winter scenes in the sixteenth century. Records of harvest dates, of when vines could be picked, of when rivers froze and when mountain passes opened all give clues about how warm or cold each year was. The strength of this evidence is that it is detailed, human and local, and it reaches back before instruments. Its limitations are that it is written mostly by people in Europe and China, it describes what people noticed rather than measuring it, and it cannot be turned into an exact temperature.

Natural causes of climate change

The climate changed long before people could have caused it, so the natural causes have to be understood - partly because they explain the ice ages, and partly because they show why they cannot explain the warming of the last century.

Orbital changes (Milankovitch cycles). The Earth's orbit and axis are not fixed. The shape of the orbit changes from more circular to more elliptical and back, the tilt of the axis varies, and the axis itself wobbles like a spinning top. Each of these cycles changes the amount and the distribution of solar energy the Earth receives, and each works over a different timescale: tens of thousands to around a hundred thousand years. Together they pace the great swings between glacial (ice age) and interglacial periods. This is the main natural explanation for the ice ages, but it cannot explain the warming of the last hundred years, because these cycles are far too slow and because the Earth is not at a point in the cycle that would produce rapid warming now.

Sunspots. Dark patches on the surface of the Sun come and go in a cycle of roughly eleven years, and when there are more of them the Sun gives out slightly more energy. Periods of very low sunspot activity have coincided with cooler spells on Earth, so sunspots do have a real but small effect on climate. They cannot explain recent warming, because the amount of energy reaching the Earth from the Sun has not increased over the period in which the temperature has risen fastest.

Volcanic activity. Large eruptions blast ash and sulfur dioxide high into the atmosphere. The droplets of sulfuric acid that form from the sulfur dioxide reflect incoming sunlight back into space, so the surface cools, and the ash and droplets also spread around the world on the winds. The effect usually lasts only a year or two, until the particles settle out, but it can be significant: the eruption of Mount Tambora in Indonesia in 1815 was followed in 1816 by the 'year without a summer' in Europe and North America, with frosts and harvest failures, and the eruptions of Krakatoa in 1883 and Mount Pinatubo in the Philippines in 1991 were each followed by a temporary cooling of the global climate. Volcanoes can also release carbon dioxide, which warms, but the amount is tiny compared with human emissions. Volcanic activity cannot explain recent warming either, because the effect of an eruption is short-lived and because there has been no increase in major eruptions over the last century.

So the natural factors are real, they operate over very different timescales, and none of them fits the pattern of the last hundred years. That is the key argument: the recent warming is too fast, too steady and too closely matched to the rise in greenhouse gases to be explained by the Sun, by volcanoes or by the Earth's orbit.

Human causes: the enhanced greenhouse effect

The enhanced greenhouse effect is the extra warming caused by greenhouse gases that people have added to the atmosphere. Burning, clearing and farming all release them, and because the natural systems that remove carbon dioxide - plants, soils and the oceans - cannot keep up, the concentration in the atmosphere rises year after year.

The use of fossil fuels is the largest source. Coal, oil and natural gas are the remains of plants and animals that lived millions of years ago, so the carbon they contain had been locked out of the atmosphere until we dug it up. Burning them in power stations, factories, homes and vehicles combines that carbon with oxygen to make carbon dioxide, which is released into the atmosphere. The more energy a country uses, and the more of that energy comes from fossil fuels, the larger its emissions.

Deforestation adds to the problem in two ways. Trees absorb carbon dioxide from the air as they grow, so a standing forest is a carbon sink: a store of carbon that is keeping it out of the atmosphere. When forest is cleared for timber, for farmland, for grazing or for mining, that store is lost - the carbon held in the wood is released as the trees are burned or as they decay, and the carbon dioxide that the growing forest would have absorbed in future is not absorbed. Tropical rainforest is the most important example, because it holds so much carbon and because the clearance is often done by burning.

Agriculture releases greenhouse gases in several ways. Cattle, sheep, goats and other ruminant livestock produce methane as they digest their food, and so do flooded rice paddies, where bacteria break down plant material in waterlogged soil with no oxygen. Nitrous oxide is released from nitrogen fertilisers and from animal manure. Clearing land for fields and pasture releases carbon dioxide from the soil and the vegetation, and the machinery and the transport that support farming burn fossil fuels. Industry adds its own share, particularly cement manufacture, which releases carbon dioxide both from the fuel burned and from the chemical reaction in the kiln, and transport and a growing global population with rising energy demand push emissions up further.

The mechanism is the same in every case: more greenhouse gas in the atmosphere -> more outgoing infrared radiation absorbed and re-radiated back towards the surface -> more heat trapped -> a rise in the average global temperature, which in turn drives the wider changes in rainfall, storms and sea level.

The impacts of climate change: present day and predicted future

The impacts of climate change are already being felt, and they will grow. Because they are unevenly spread, sort them by scale - local, regional and global - and by type - social, economic and environmental - then say who is worst affected and why.

Rising sea levels. Sea level rises for two reasons: the water in the oceans expands as it warms (thermal expansion), and melting glaciers and ice sheets on land add more water to the ocean. Low-lying coasts face flooding and erosion, salt water is pushed into rivers and into the freshwater supplies under the ground, and beaches, wetlands and mangroves are lost. For a coastal city or a low-lying island nation, this threatens land, homes, farmland, roads, airports and freshwater, and in the most extreme cases the loss of a large part of the country itself.

Changes to global temperature. The average global temperature is rising, and with it the number and intensity of heatwaves - periods of unusually hot weather that cause deaths, especially among the old and the ill, and that damage crops and livestock. Glaciers and ice sheets are retreating, sea ice is shrinking, and in the polar regions the ground that has been frozen for thousands of years is thawing, which damages buildings, roads and pipelines and releases more greenhouse gases as the frozen plant material decays.

Changes to weather patterns. A warmer atmosphere holds more water vapour and more energy, so the pattern of rainfall and storms changes rather than simply getting drier or wetter everywhere. Some regions get heavier and more intense rainfall and more serious flooding, while others get longer and more severe droughts, and the timing of the seasons shifts, so the monsoon or the spring rains arrive at the wrong time for farmers. Tropical cyclones can carry more energy and more water, and in some regions they are becoming more intense, with heavier rain and stronger storm surges.

Changes to food production. Crops respond to temperature, to rainfall, to carbon dioxide and to pests, so food production is affected unevenly. In many low-latitude regions, where it is already hot and where many people depend on rain-fed farming, rising temperatures and unreliable rainfall reduce yields of staple crops such as wheat, rice and maize, and there is less water available for irrigation. Pests and diseases spread into regions that were previously too cool for them. In some higher-latitude regions, such as parts of northern Europe and Canada, a longer growing season and less frost may actually increase yields, which is why the impact is described as uneven rather than simply bad. Rising sea level and salt water also remove farmland from production in deltas and on low-lying coasts.

Water supply, health and ecosystems. Less reliable rainfall and shrinking glaciers mean less water for drinking, irrigation, industry and hydroelectric power in the regions that depend on them, and competition for water can increase tension between neighbours. Human health is affected by heat stress, by a wider spread of diseases such as malaria into previously cool regions, by air pollution from fires and dust, and by the disruption and malnutrition that follow floods and droughts. Ecosystems are already changing: coral reefs bleach when the water gets too warm, and the fish, tourism and coastal protection that depend on them suffer; species of plants and animals shift towards the poles or up mountains to stay in the conditions they can tolerate, and those that cannot move fast enough are at risk of extinction.

Why the impacts differ. A wealthy country can afford sea defences, irrigation, air conditioning, early warning systems, insurance and new crop varieties. A poorer country usually depends more on farming, has less money to spend on defences, and is more exposed to floods and droughts - so the same amount of warming causes far more suffering. Small island states and low-lying deltas, such as those of Bangladesh and Egypt, are the most exposed of all, because their land, their farmland and their freshwater are all close to sea level.

Mitigation: reducing the causes, including international agreements

Mitigation means reducing the causes of climate change: cutting the greenhouse gases released into the atmosphere, and increasing the amount that is taken out of it. It works globally and slowly - the benefit is shared by everyone, but it takes decades for the effect on temperature to show.

Cutting emissions from energy. The single most effective mitigation technique is to produce electricity without burning fossil fuels. Renewable energy - wind, solar, hydroelectric, geothermal, tidal and wave power - produces electricity with very low emissions, and once the turbines or panels are built the fuel costs nothing. Nuclear power also produces very little carbon dioxide, though it creates radioactive waste and is expensive and unpopular. Alongside cleaner supply comes lower demand: energy efficiency measures such as insulating buildings, using efficient appliances and lighting and heating homes with heat pumps reduce the energy needed in the first place.

Transport, industry and land use. Transport can be decarbonised by shifting journeys to public transport, walking and cycling, and by replacing petrol and diesel vehicles with electric ones charged from low-carbon electricity. Industry can use cleaner fuels, recycle materials and capture the carbon dioxide released by processes such as cement and steel making. On land, afforestation and reforestation - planting new forest and restoring cleared forest - increase the amount of carbon dioxide taken out of the atmosphere, and protecting existing forest, especially tropical rainforest, keeps a very large store of carbon where it is. Farming can be changed too, by using less fertiliser, by managing manure and rice paddies to release less methane, and by reducing the amount of land cleared for pasture. Carbon capture and storage takes carbon dioxide from a power station or factory and pumps it deep underground, but the technology is expensive, it does not remove the need to cut emissions, and it is not yet used at a large scale.

International agreements. Climate change is a global problem, so national action alone cannot solve it. The Kyoto Protocol (1997) was the first major international agreement to set binding targets for reducing greenhouse gas emissions, but only for developed countries, and some of the largest emitters did not take part, so its effect on global emissions was limited. The Paris Agreement (2015) is different: almost every country in the world agreed to keep the rise in global average temperature to well below 2°C above pre-industrial levels and to pursue efforts to limit it to 1.5°C. Instead of targets imposed from outside, each country sets its own nationally determined contribution and reports on it, and the agreement commits richer countries to help poorer ones with finance and technology. Its weakness is that it has no power to force a country to meet its own promise. Around these global agreements sit national agreements and policies: carbon taxes and emissions trading schemes that put a price on releasing carbon dioxide, subsidies and grants for renewable energy and electric vehicles, targets to phase out coal-fired power stations, building regulations that require insulation, and bans on the most polluting practices. Individuals add to this through their own choices - using less energy and water, eating less meat, flying less, recycling and voting for parties with serious climate policies - but individual action is small compared with what governments and industry can do, which is why international and national agreements matter most.

Adaptation, and evaluating mitigation against adaptation

Adaptation means accepting that the climate is changing and reducing the damage it does. It acts on the impacts, it works locally and it produces a benefit immediately, but it does nothing to slow the warming itself.

Coastal and river defences. Sea walls, revetments, breakwaters and groynes protect the coast from erosion and flooding, and flood barriers such as the Thames Barrier close to protect a city from a tidal surge. Coastal management can also work with nature: restoring mangroves, salt marshes and coral reefs absorbs wave energy and holds sediment in place, and in some places the cheapest option is managed retreat, allowing the sea to flood low-value land and rehousing the people who lived there. On rivers, embankments, flood storage areas, wetland restoration and better drainage reduce the risk of flooding.

Water and food. Where rainfall is becoming less reliable, countries store water in reservoirs and tanks, harvest rainwater, reduce waste through metering and leak repair, recycle water, and in coastal or very dry regions use desalination to make fresh water from the sea. Irrigation becomes more efficient with drip systems that deliver water straight to the roots. Farmers adapt by planting drought-resistant and salt-tolerant crop varieties, by changing the dates they sow and harvest to match a shifted season, by diversifying what they grow, by terracing and contour ploughing to hold water and soil on the slopes, and by keeping more livestock feed in reserve for bad years.

Planning, technology and health. Land-use planning keeps new housing, hospitals and industry off land that will flood, and building codes require raised floors, flood-proof materials and drainage that can cope with heavier rain. Early warning systems for cyclones, floods and heatwaves, and insurance and disaster funds, spread the cost and save lives. In hot regions, cooling centres, changes to working hours and public health measures against heat stress and spreading disease reduce the human cost. Technology helps throughout: drought-tolerant seed, irrigation sensors, satellite crop monitoring, and desalination plants.

Evaluating mitigation and adaptation. Mitigation and adaptation are not alternatives - they do different jobs, and a country needs both. Mitigation tackles the cause, so it is the only way to stop climate change getting worse, and its benefit is shared by the whole world. That is also its weakness: because everyone benefits whether or not they pay, each country has an incentive to let others make the cuts - the free-rider problem - and because the atmosphere holds the gases for a long time, the effect of cuts today is slow. Mitigation is also expensive, and the countries that have emitted least have the least money to spend on it. Adaptation tackles the impacts, so it delivers a local and immediate benefit and can be cheaper for a single community, but it cannot stop the problem, it can only protect those who can afford it, and there are limits: no sea wall can save an island that is submerged, and no irrigation can water a region with no water. The defensible judgement is therefore that mitigation matters most in the long run, because without it adaptation becomes a losing race against an ever-rising sea and an ever-hotter atmosphere, while adaptation matters most in the short run and is the only option for the poorest and most exposed countries, which have contributed least to the problem. The balance a country chooses depends on its wealth and its geography: a rich, high-latitude country can afford both, while a poor, low-lying country has to adapt to survive and must rely on international finance to do it.