The greenhouse gases in the atmosphere trap some of the heat from incoming sunlight, keeping it close to Earth and stopping it from radiating away into space. As the levels of these gases increase, global temperatures rise.
Some of these gases, like carbon dioxide (CO2), have been naturally present in the atmosphere for a long time. However, human activities are increasing the concentration of these gases and the warming effect they have.
CO2 emissions come from burning fossil fuels for energy (coal, oil and gas) as well as solid waste and the biological cycle of plants. It can remain in the atmosphere for years or even centuries. Other greenhouse gases like methane, nitrous oxide and fluorinated gases (such as sulphur hexafluoride, trichlorofluoride and perfluorocarbons) have shorter atmospheric lifetimes, but still contribute significantly to warming.
In the last century, humans have been emitting more of these gases than at any other time in history. This is because the industrial revolution and rapid growth of developing countries have led to a large increase in the demand for energy, food and other goods. The increase in demand has been accompanied by more efficient production methods and the use of fossil fuels which produce large amounts of greenhouse gases.
NOAA scientists measure the concentrations of the major greenhouse gases in the air at 40 remote sites around the world every week. The measurements are combined to make a smoothed global average and then compared to a baseline year (indexed to 1990). The charts below show the total concentrations of these gases in the atmosphere over time.