What causes climate change?
The main cause of recent climate change is the extra greenhouse gas that human activity adds to the atmosphere, mostly carbon dioxide from burning coal, oil and gas. Natural factors still affect the climate, but they do not explain the warming measured over the past century.
Greenhouse gases trap heat
Some gases in the atmosphere, including carbon dioxide and methane, let sunlight through but absorb heat radiating back from the surface. This is the greenhouse effect. It is natural and necessary: without it, Earth would be frozen. The issue is that adding more greenhouse gas strengthens the effect and warms the planet.
For how this works in more detail, see the greenhouse effect.
Burning fossil fuels adds CO2
Coal, oil and gas contain carbon that was locked away for millions of years. Burning them releases that carbon as CO2. Direct measurements show atmospheric CO2 has risen sharply since industrialisation, and it is now around 50% higher than before the industrial era.
Natural causes have been checked
Scientists have tested the Sun, volcanoes, orbital cycles and natural variability. Each affects the climate, but none explains the recent warming as well as human greenhouse gases. Solar output has been flat to slightly lower since the mid-twentieth century. Volcanoes emit far less CO2 than human activity. Orbital cycles act over tens of thousands of years.
The natural cycle question is covered in full on the natural cycle page, and the Sun specifically on is the Sun causing it?
Human emissions are the main cause
Put together, the chain is simple and measured at every step: fossil fuels add CO2, CO2 traps heat, and the planet warms in a pattern that fits greenhouse gases better than any natural cause. That is why human activity is judged to be the main cause of recent warming.
Frequently asked
The main cause is the extra carbon dioxide and other greenhouse gases humans add to the atmosphere, mostly by burning fossil fuels.
Yes, the Sun, volcanoes and natural variability still affect the climate. They just do not explain the timing, size and pattern of the warming over the past century as well as human emissions do.
The added CO2 carries the chemical fingerprint of fossil fuels, and the pattern of warming, including cooling high in the atmosphere, matches greenhouse gases rather than the Sun.