Scientific certainties and uncertainties
The Earth's climate has varied in the past over many different timescales. Right now we are in an interglacial, a relatively mild period between ice ages. But humans are changing the climate system.
Some gases in our atmosphere, for example carbon dioxide, methane, nitrous oxide, and especially water vapour, trap heat emitted from the Earth's surface, keeping the planet about 30°C warmer than it would otherwise be. This is the 'natural greenhouse effect' and is well understood scientifically.
Human activities, especially burning fossil fuels like coal and oil, have increased the level of these greenhouse gases in our atmosphere. This is throwing the climate system out of balance.
Climate change is the most important environmental issue we face this century. Every one of the hottest 15 years on record has occurred since 1980 - the five hottest since 1997.