How does the greenhouse effect work?

Sunlight passes through the air and warms the land and oceans. The warm Earth then gives off heat as invisible infrared radiation. Some gases in the air absorb part of this heat and send it back down, like a blanket. Without any of these gases the planet would be far too cold for life. The problem is that human activity has added much more of them since about 1750, so the blanket has become thicker.

The IPCC (AR6 Working Group I, Summary for Policymakers, 2021) reports that in 2019 average concentrations reached 410 parts per million (ppm) for carbon dioxide, 1866 parts per billion (ppb) for methane and 332 ppb for nitrous oxide. It also found that carbon dioxide levels in 2019 were higher than at any time in at least 2 million years, and that global surface temperature in 2011-2020 was 1.09°C higher than in 1850-1900.