What is the climate crisis? The climate crisis is the serious and growing harm caused by global warming. Burning coal, oil and gas adds greenhouse gases to the air, which strengthen the natural greenhouse effect and trap extra heat. Recent years have been about 1.4–1.6 °C warmer than before the Industrial Revolution, and 2024 was the warmest year on record.
Every number below comes from the World Meteorological Organization (WMO), the Intergovernmental Panel on Climate Change (IPCC), NASA, NOAA, Copernicus or the Met Office, checked in October 2026.
Climate crisis, global warming and climate change: what do the words mean?
People often use these terms as if they were the same thing. They are linked, but each one means something slightly different, and examiners like you to know the difference.
- Global warming is the long-term rise in Earth’s average surface temperature since the pre-industrial period (scientists use 1850–1900 as the baseline). It is one number for the whole planet.
- Climate change is wider. It covers every long-term shift that warming causes: changing rainfall, rising seas, shrinking ice, stronger heatwaves and new patterns of drought and flooding.
- The climate crisis (or climate emergency) describes how serious and urgent those changes have become for people and nature.
Weather is what happens over hours or days. Climate is the average pattern of weather over about 30 years. One cold week does not disprove global warming; what counts is the long-term trend.
Earth’s climate has changed naturally in the past, through ice ages and warm periods, over thousands of years. The difference now is speed and cause. The IPCC’s Sixth Assessment Report (AR6) concluded that human activities, mainly through emissions of greenhouse gases, have “unequivocally caused global warming”. Unequivocally means there is no reasonable doubt.
What is the greenhouse effect?
The greenhouse effect is the natural process that keeps Earth warm enough for life. Certain gases in the atmosphere let sunlight in but slow down the escape of heat back to space. Without them, our planet would be a frozen ball of ice.
Here is how it works, step by step:
- Sunlight arrives. Energy from the Sun reaches Earth mostly as visible light, plus some ultraviolet and infrared.
- Some is reflected. About 30% of incoming sunlight is reflected straight back to space by clouds, ice, snow and other bright surfaces.
- The rest is absorbed. Land, oceans and air absorb the remaining sunlight and warm up.
- The warm Earth gives off infrared. Like any warm object, the surface emits infrared radiation, which we feel as heat.
- Greenhouse gases absorb and re-emit it. Molecules such as carbon dioxide, methane and water vapour absorb some of this infrared and then re-emit it in all directions, including back down towards the surface. This keeps the lower atmosphere and the ground warmer than they would otherwise be.
The result is dramatic. NASA explains that without the natural greenhouse effect, Earth’s average surface temperature would be about −18 °C. With it, the average is about 15 °C. That difference of roughly 33 °C is what makes liquid water, rivers, forests and people possible.
Interestingly, the two gases that make up about 99% of dry air, nitrogen and oxygen, are not greenhouse gases. Their simple two-atom molecules hardly absorb infrared. The heat-trapping job is done by gases present in tiny amounts, which is why adding even a little more of them makes a measurable difference. You can read more about the layers and make-up of Earth’s atmosphere and how it compares with other worlds.
Is the greenhouse effect good or bad?
Both answers can be right, depending on which greenhouse effect you mean. This is one of the most common questions students get wrong, so it is worth being precise.
The natural greenhouse effect is good. It has kept Earth habitable for billions of years. The enhanced greenhouse effect is the problem. Since the Industrial Revolution, people have added extra greenhouse gases to the air, mainly by burning fossil fuels and clearing forests. More greenhouse gas means more infrared is absorbed and re-emitted, so more heat is held in the climate system. That extra heat is what drives global warming.
The clearest evidence comes from direct measurements. On 29 March 1958, the scientist Charles David Keeling recorded a carbon dioxide level of 313 parts per million (ppm) at Mauna Loa Observatory in Hawaii. Before the Industrial Revolution, the level was about 280 ppm. In May 2026, Mauna Loa recorded its highest reading ever: about 432 ppm.
The graph of these measurements, known as the Keeling curve, climbs steadily with a small zig-zag each year. The zig-zag happens because plants in the Northern Hemisphere absorb carbon dioxide in spring and summer, then release it in autumn and winter. The yearly average, though, has risen every single year since records began in 1958.
Explain the difference between the greenhouse effect and the enhanced greenhouse effect. (3 marks)
The greenhouse effect is natural: gases such as carbon dioxide and water vapour absorb infrared radiation from Earth’s surface and re-emit some back, keeping Earth about 33 °C warmer than it would be (1). The enhanced greenhouse effect is caused by humans adding extra greenhouse gases, mainly by burning fossil fuels and deforestation (1). This traps more heat and causes global warming (1).
What are the main greenhouse gases?
Greenhouse gases are gases in the atmosphere that absorb and re-emit infrared radiation. Several occur naturally, but human activity has raised the amounts of the most important ones far above their pre-industrial levels. The WMO’s Greenhouse Gas Bulletin, published in October 2025, gives the latest global averages for 2024.
| Gas | Formula | Main human sources | Global level in 2024 | Warming per kg compared with CO₂ (100 years) |
|---|---|---|---|---|
| Carbon dioxide | CO₂ | Burning coal, oil and gas; deforestation; cement making | 423.9 ppm (152% of pre-industrial) | 1 |
| Methane | CH₄ | Livestock, rice paddies, landfill, leaks from fossil-fuel production | 1,942 ppb (266% of pre-industrial) | About 27–30 |
| Nitrous oxide | N₂O | Fertilisers and other farming | 338.0 ppb (125% of pre-industrial) | 273 |
| Fluorinated gases (F-gases) | e.g. HFCs (historically also CFCs and HCFCs) | Refrigeration, air conditioning, some industrial processes | Tiny amounts | Many are hundreds to thousands of times stronger |
| Water vapour | H₂O | Not directly controlled by people; rises as air warms | Varies with temperature | Acts as a feedback |
ppm means parts per million and ppb parts per billion: 423.9 ppm means about 424 CO₂ molecules in every million molecules of air.
Carbon dioxide (CO₂)
Carbon dioxide is the most important greenhouse gas that humans add, because we release so much of it and it stays in the air for a very long time. Whenever a fuel containing carbon burns, the carbon combines with oxygen to make CO₂. For example, when methane in natural gas burns completely:
CH₄ + 2O₂ → CO₂ + 2H₂O
From 2023 to 2024, the global average level of CO₂ jumped by 3.5 ppm. The WMO reported that this was the largest one-year rise since modern measurements began in 1957, beating the previous record of 3.3 ppm set in 2015–16. Carbon moves naturally between the air, oceans, rocks and living things, which you can follow in this carbon cycle lesson. Burning fossil fuels moves carbon that was locked underground for millions of years into the air within a few human lifetimes.
Methane (CH₄)
Methane is the main ingredient of natural gas. It comes from cattle and sheep (microbes in their stomachs produce it), flooded rice fields, rotting waste in landfill and leaks from coal mines, oil wells and gas pipes. Its level in 2024 was 266% of the pre-industrial amount, the biggest relative rise of the three main gases.
Methane breaks down in the atmosphere in about a decade, much faster than CO₂. But while it lasts, it is very powerful. According to the IPCC, one kilogram of methane causes about 27 to 30 times as much warming as one kilogram of CO₂ over 100 years. That is why cutting methane leaks is seen as one of the quickest ways to slow warming.
Nitrous oxide (N₂O)
Nitrous oxide comes mostly from farming, especially when nitrogen fertilisers are spread on fields and soil microbes convert some of the nitrogen into N₂O. Kilogram for kilogram it is about 273 times more powerful than CO₂ over 100 years, and it also damages the ozone layer.
Fluorinated gases (F-gases)
F-gases such as HFCs are made by people for fridges, air conditioners and some industrial processes. Related older gases, the CFCs, also destroyed the ozone layer that shields us from ultraviolet light, and they were phased out under the Montreal Protocol of 1987. Some of their replacements, the HFCs, do not harm ozone but are still strong greenhouse gases.
Water vapour (H₂O)
Water vapour is the most abundant natural greenhouse gas, so why is it not top of the list of problems? Because people cannot control it directly. The amount of water vapour depends on temperature: warmer air can hold about 7% more water vapour for every 1 °C of warming. So when CO₂ warms the planet, the air holds more water vapour, which traps even more heat. Scientists call this a feedback, because it amplifies the original change rather than starting it.
Because the gases differ so much in strength, scientists convert them into a common unit called CO₂-equivalent (CO₂e) using each gas’s global warming potential (GWP). Try it for yourself:
What causes global warming?
Global warming is caused mainly by the extra greenhouse gases that people release. The biggest single cause is burning fossil fuels (coal, oil and natural gas) for electricity, heating, transport and industry. Other causes add to the total.
- Burning fossil fuels: power stations, cars, lorries, planes, boilers and factories release carbon dioxide.
- Deforestation: trees store carbon. When forests are cleared or burned, that carbon returns to the air as CO₂, and the land absorbs less CO₂ in future. Learn more about deforestation and reforestation.
- Farming: livestock and rice paddies release methane, and fertilisers release nitrous oxide.
- Industry: making cement releases CO₂ from limestone as well as from the fuel used to heat the kilns.
- Waste: food and other waste rotting in landfill releases methane.
Feedbacks that make warming worse
Once warming starts, some natural processes speed it up. These are called positive feedbacks:
- Ice–albedo feedback: bright ice and snow reflect sunlight. When they melt, darker ocean or land underneath absorbs more sunlight, causing more warming and more melting.
- Water vapour feedback: warmer air holds more water vapour, which traps more heat.
- Permafrost thaw: frozen ground in the Arctic holds old plant material. As it thaws, microbes break it down and release carbon dioxide and methane.
How much has the Earth warmed?
Scientists measure global warming against the average temperature of 1850–1900, which they call the pre-industrial baseline. Independent teams, including NASA, NOAA, the Met Office and Copernicus, calculate global temperature from millions of readings, and the WMO combines their results.
The IPCC’s AR6 report found that global surface temperature in 2011–2020 was already 1.1 °C above the pre-industrial level. Since then, the numbers have climbed further.
| Year | Rank | WMO (average of datasets) | Copernicus ERA5 |
|---|---|---|---|
| 2024 | Warmest on record | 1.55 °C (±0.13 °C) | 1.60 °C |
| 2025 | 2nd or 3rd warmest (depends on dataset) | 1.44 °C (±0.13 °C) | 1.47 °C (3rd) |
| 2023–2025 | The three warmest years in all eight datasets | 1.48 °C average | Above 1.5 °C average |
2024 was the first calendar year in which global temperature was more than 1.5 °C above pre-industrial levels. The WMO also reports that the eleven years from 2015 to 2025 are the eleven warmest on record.
What about 2026?
2026 is heading towards another very warm year. Copernicus reported that August 2026 was 1.65 °C above pre-industrial levels and tied with July 2023 as the warmest month ever recorded. Western Europe had its warmest summer on record, beating 2003, and drought hit countries including France and the UK. At the same time, a strong El Niño has been building in the Pacific Ocean. El Niño is a natural warming of the tropical Pacific that temporarily lifts global temperatures, and in September 2026 NOAA gave a more than 90% chance of a very strong event this winter. Whether 2026 or 2027 beats 2024 will only be known once the full data are in.
Has the world passed 1.5 °C?
Not yet, in the sense that the Paris Agreement means. In 2015, almost every country agreed to hold warming well below 2 °C and to try to limit it to 1.5 °C. That target refers to long-term average warming, usually judged over about 20 years, not a single hot year. However, the WMO’s latest five-year forecast gives a 91% chance that at least one year between 2026 and 2030 will exceed 1.5 °C, and a 75% chance that the 2026–2030 average will do so.
Global warming in the UK
The UK is feeling the change too. The Met Office confirmed that 2025 was the UK’s warmest year since records began in 1884, with a mean temperature of 10.09 °C, and also its sunniest. Summer 2025 was the UK’s warmest summer on record. A Met Office study found that climate change made the record-breaking 2025 temperature about 260 times more likely. On 19 July 2022, the UK recorded 40 °C for the first time, reaching 40.3 °C at Coningsby in Lincolnshire.
2024 was 1.55 °C above pre-industrial levels. Explain why this does not mean the Paris Agreement’s 1.5 °C limit has been broken. (2 marks)
The Paris limit refers to long-term average warming over about 20 years, not a single year (1). One year can be pushed higher by natural variation such as El Niño, so a single year above 1.5 °C is a warning sign but not a breach (1).
Effects of global warming
A rise of around 1.5 °C sounds small, but it is an average over the whole planet, including the oceans, which take a huge amount of energy to heat. Small changes in the global average bring big changes in extremes. Here are the main effects scientists have measured.
Hotter oceans
More than 90% of the extra heat trapped by greenhouse gases has gone into the ocean, according to the WMO. In 2025, Earth’s energy imbalance, the difference between the energy arriving and the energy leaving, reached a record high. Warmer seas fuel heavier rainfall and can bleach and kill coral reefs.
Rising sea levels
Sea level rises for two reasons: water from melting glaciers and ice sheets flows into the sea, and seawater expands as it warms (thermal expansion). The IPCC found that global sea level rose by about 0.20 m between 1901 and 2018. NASA satellites show a rise of about 10 cm since 1993, and the yearly rate has more than doubled, from about 0.20 cm a year in 1993 to about 0.44 cm a year by 2022 (NASA found it was higher still, about 0.59 cm, in 2024).
Melting ice
Mountain glaciers are shrinking in almost every region of the world. Arctic sea ice at its September minimum is shrinking by about 12% per decade, according to NASA and the US National Snow and Ice Data Center. In 2025 the minimum was 4.60 million square kilometres, tied for the tenth-lowest on record.
More extreme weather
The IPCC reports that heatwaves have become more frequent and more intense, heavy rainfall events have become heavier in many regions, and droughts and fire weather have increased in some areas. A warmer atmosphere holds more water vapour, so when it rains, it can pour.
More acidic oceans
The ocean absorbs some of the carbon dioxide we release. That CO₂ reacts with seawater to form carbonic acid. NOAA reports that the pH of surface seawater has fallen by about 0.1 since the industrial revolution, which means it is about 30% more acidic, because the pH scale is logarithmic. This makes it harder for corals, oysters and other shell-builders to grow.
CO₂ + H₂O ⇌ H₂CO₃
Wildlife, food and health
As temperatures shift, many plants and animals change when they breed or migrate, or move towards the poles. Species that cannot adapt fast enough are at risk, and the knock-on effects spread through the ecosystems and food chains they belong to. People are affected through heat stress, crop losses, water shortages and flooding. The IPCC estimates that about 3.3 to 3.6 billion people live in places that are highly vulnerable to climate change. Burning fossil fuels also causes air pollution, so cutting it helps health as well as the climate.
What can we do about the climate crisis?
The good news is that the main cause of the climate crisis is known, so the main solution is clear too: cut greenhouse gas emissions, especially from fossil fuels. Responses fall into two kinds.
- Mitigation means tackling the cause by reducing emissions or removing greenhouse gases from the air. Examples include renewable electricity, better insulation, electric transport, protecting forests and cutting methane leaks.
- Adaptation means preparing for the changes that can no longer be avoided. Examples include flood defences, heatwave plans for hospitals and care homes, drought-resistant crops and planting trees to cool city streets.
How fast do emissions need to fall?
The IPCC’s AR6 Synthesis Report found that to limit warming to 1.5 °C with no or limited overshoot, global greenhouse gas emissions would need to fall by about 43% by 2030 and 60% by 2035, compared with 2019 levels, with carbon dioxide emissions reaching net zero around the early 2050s. Net zero means any CO₂ still released is balanced by the same amount being removed from the air.
The world is not on that path yet. The UN Environment Programme’s Emissions Gap Report 2025 estimated that current policies would lead to about 2.8 °C of warming this century. If countries fully carry out their latest climate pledges, the estimate falls to about 2.3–2.5 °C. That is an improvement on earlier reports, but it is still well above the Paris goals.
Cleaner energy and transport
Electricity from wind and sunlight produces no carbon dioxide while it is being generated. Each source has strengths and limits, which you can compare in the advantages and disadvantages of wind energy and in this guide to how solar panels work. Transport is another big source of emissions; switching to buses, trains, cycling and electric cars charged with clean electricity reduces it.
What can I do?
Big changes depend on governments and businesses, but individual choices add up and influence others. Here is a checklist you can tick off; your ticks are saved in your browser.
- Walk, cycle or take the bus or train for short journeys.
- Switch off lights and devices you are not using, and turn the heating down by a degree.
- Plan meals and use leftovers to cut food waste.
- Eat more plant-based meals each week.
- Find out how your local council plans to reduce emissions.
Common doubts about global warming
Some people still question whether global warming is real or caused by humans. You can read about why in our article on doubts on global warming. The evidence, however, comes from many independent sources: thermometers, satellites, ocean floats, ice cores and direct CO₂ measurements.
Frequently asked questions about the climate crisis
What is the climate crisis?
The climate crisis is the serious and urgent harm caused by human-driven global warming. Extra greenhouse gases from burning fossil fuels and clearing forests trap more heat, raising temperatures and sea levels, melting ice and making heatwaves, droughts and heavy rainfall more severe. It affects people, wildlife, food and water supplies around the world.
What is the greenhouse effect in simple terms?
The greenhouse effect is the way certain gases in the air keep Earth warm. Sunlight warms the surface, which gives off heat as infrared radiation. Greenhouse gases absorb some of that heat and send part of it back down. Without this natural effect, Earth would average about −18 °C instead of about 15 °C.
What is the difference between global warming and climate change?
Global warming is the long-term rise in Earth’s average surface temperature since the pre-industrial period. Climate change is broader: it includes all the long-term changes that warming causes, such as shifting rainfall patterns, rising seas, shrinking ice and more extreme weather. Global warming is one cause; climate change is the full set of effects.
What are the main greenhouse gases?
The main greenhouse gases are carbon dioxide, methane, nitrous oxide, fluorinated gases and water vapour. Carbon dioxide from burning fossil fuels is the largest human contribution. Methane comes from livestock, landfill and fossil-fuel leaks, and nitrous oxide mainly from fertilisers. Water vapour is abundant but acts as a feedback that amplifies warming.
How much has the Earth warmed?
The IPCC found Earth was about 1.1 °C warmer in 2011–2020 than in 1850–1900. Recent years have been warmer still: the WMO puts 2024 at about 1.55 °C and 2025 at about 1.44 °C above pre-industrial levels, and the three-year average for 2023–2025 at about 1.48 °C.
What was the warmest year on record?
2024 is the warmest year on record, at about 1.55 °C above the 1850–1900 average according to the WMO, and 1.60 °C according to Copernicus. It was the first calendar year above 1.5 °C. 2025 ranked second or third depending on the dataset, and 2015–2025 were the eleven warmest years measured.
Is the greenhouse effect good or bad?
The natural greenhouse effect is good, because it keeps Earth warm enough for liquid water and life. The problem is the enhanced greenhouse effect, caused by people adding extra greenhouse gases. That traps more heat than the climate system has had for thousands of years and drives global warming and climate change.
What is the 1.5 °C limit?
Under the 2015 Paris Agreement, countries agreed to keep warming well below 2 °C and try to limit it to 1.5 °C above pre-industrial levels. The limit refers to long-term average warming over about 20 years, so a single year above 1.5 °C, such as 2024, does not by itself mean the target has been missed.
How can we stop global warming?
The IPCC says human-caused warming will roughly stop rising once net carbon dioxide emissions reach zero. That means replacing fossil fuels with clean energy, using energy more efficiently, switching to electric transport, protecting and restoring forests and cutting methane. The IPCC says emissions must fall about 43% by 2030, compared with 2019, to keep 1.5 °C within reach.
