The short version
Earth’s climate is warming because human activity has added large amounts of heat-trapping gases to the atmosphere, mainly by burning coal, oil and gas and by clearing land. These gases slow the rate at which heat escapes to space, so the surface and lower atmosphere warm. That one idea sits behind most of what you read about climate change. The rest is detail about how much, how fast and with what effects.
The greenhouse effect
The greenhouse effect is a natural process, and life depends on it. It works in three steps.
- Sunlight passes through the atmosphere and is absorbed by the land, oceans and air, which warms them.
- The warmed surface gives off heat as infrared radiation, a form of energy we feel as warmth but cannot see.
- Some gases in the air absorb that infrared radiation and send it out again in all directions, including back towards the ground. The gases that do this are the greenhouse gases.
Without this effect, the planet would be far colder, well below freezing on average. The problem is not the greenhouse effect itself but that we are strengthening it. The name comes from a loose comparison with a garden greenhouse, though a real greenhouse mostly works by blocking air movement, so the analogy is imperfect.
The gases doing the work are mostly present in tiny amounts. Nitrogen and oxygen, which make up most of the air, barely interact with infrared radiation. Water vapour, carbon dioxide, methane and a few others do, which is why small changes in their amounts can matter. The main ones, and how they differ, are covered in the article on greenhouse gases.
From emissions to a warmer atmosphere
Carbon dioxide is released when fossil fuels are burned for electricity, heating, transport and industry. It is also released when forests are cleared or burned and when some industrial processes, such as cement making, take place. Methane comes largely from livestock, rice growing, landfill and leaks from fossil fuel systems, and nitrous oxide is linked mainly to fertiliser use.
Not all of this stays in the air. Plants, soils and the oceans absorb a considerable share of the carbon dioxide we emit each year, which is why the amount in the atmosphere rises more slowly than the amount we release. These natural stores are discussed in more detail in the piece on carbon sinks, including why they cannot absorb unlimited amounts.
Carbon dioxide is also long-lived. A portion of what is emitted today can remain in the atmosphere for centuries, so the warming effect builds up over time rather than disappearing when emissions pause. This is one reason scientists talk about total accumulated emissions, not only yearly ones.
From extra heat to warming
More trapped heat does not simply mean a hotter air temperature everywhere. Most of the extra energy goes into the oceans, with smaller amounts warming the air, land and melting ice. Global average surface temperature is now a little over 1°C above the late 1800s, according to the main temperature records. That sounds small, but a global average hides a lot. Land warms faster than oceans, and some regions, such as the Arctic, have warmed faster than the global figure.
A small shift in the average also changes the extremes. When the whole range of temperatures moves upward, very hot days become more common and very cold days less common, though cold spells still occur. This is also why one chilly week says little about the long-term trend, a point covered in the article on climate versus weather.
Feedbacks: what speeds up or slows down warming
The climate system contains feedbacks, which are responses that amplify or dampen the original change.
- Water vapour. A warmer atmosphere can hold more water vapour, itself a greenhouse gas, which adds to the warming.
- Ice and snow. Bright ice reflects sunlight. As it melts, darker land or water absorbs more heat, which encourages further melting.
- Carbon stores. Warming can affect how much carbon forests, soils and oceans take up or release, though the size of this effect is uncertain.
- Clouds. Clouds can both cool and warm, and how they will respond is one of the harder questions in climate science.
Feedbacks are a main reason projections come with ranges rather than single numbers.
What changes on the ground
The consequences vary by place, but the broad patterns scientists expect and observe include:
- more frequent and intense heat waves;
- rising sea levels, because ocean water expands as it warms and ice on land melts;
- shifts in rainfall, with heavier downpours in some regions and longer dry spells in others;
- pressure on ecosystems, crops and water supplies;
- ocean warming and increasing acidity as the sea absorbs carbon dioxide.
Linking an individual storm or heat wave to climate change is a specialised task, and researchers usually describe how much more likely or intense climate change made an event, not whether it caused the event outright.
What is well established and what is less certain
The basic physics of the greenhouse effect has been understood for well over a century, and the rise in atmospheric carbon dioxide from human activity is measured directly. That the planet is warming is supported by many independent records, including land and sea temperatures, shrinking glaciers and rising sea levels.
The uncertainty lies mostly in the details: exactly how sensitive the climate is to a given amount of extra carbon dioxide, how quickly ice sheets will respond, how regional rainfall will change, and what choices societies will make about emissions. Good reporting separates those open questions from the settled basics, and our piece on environmental news offers some ways to do that.
What this means for everyday choices
The chain above explains why the main levers are those that cut carbon dioxide and other gases at source: how electricity is generated, how homes are heated, how people travel and what is grown and eaten. For most households, home energy use, transport and food tend to be the larger parts of their personal share, although the balance varies a lot between people and countries.
No single habit solves the problem, and individual action works alongside policy, business and infrastructure changes. Understanding the mechanism helps you judge claims sensibly: whether an action actually lowers emissions, by roughly how much compared with alternatives, and whether it is a real change or just a label. Tools such as footprint calculators can help with the first step of seeing where your own emissions come from, as long as their limits are understood.