Scientists have measured the concentration of carbon dioxide in the atmosphere, the amount of fossil fuels burned and the average global temperature over many years. All three have risen together since the Industrial Revolution, when people began burning large amounts of coal, oil and gas. When two variables rise together like this there is a correlation. The link makes sense because burning fossil fuels releases carbon dioxide, and carbon dioxide is a greenhouse gas that traps heat in the atmosphere.
A correlation alone does not prove that one change causes the other, so scientists weigh up the evidence. Climate models, the known physics of the greenhouse effect and the match between rising fuel use and rising carbon dioxide all support the idea that human activity is a major cause of the warming. Many independent research groups have reached the same conclusion, which makes it more reliable.
There are also uncertainties. Measurements are taken at different locations, such as cities, oceans and remote mountains, and local conditions can change a reading. A city is often warmer than the countryside around it, for example. Records from the distant past are less accurate because thermometers were less precise and fewer places were measured, and older data often has to be worked out indirectly from sources such as ice cores and tree rings. A fair evaluation says the evidence is strong, but the data are not perfect.
