Greenhouse gases

Greenhouse gases trap heat radiating from the Earth to space.^[59] This heat, in the form of infrared radiation, gets absorbed and emitted by these gases in the atmosphere, thus warming the lower atmosphere and the surface. Before the Industrial Revolution, naturally occurring amounts of greenhouse gases caused the air near the surface to be warmer by about 33 °C (59 °F) than it would be in their absence.^[60] Without the Earth's atmosphere, the Earth's average temperature would be well below the freezing temperature of water.^[61] While water vapour (~50%) and clouds (~25%) are the biggest contributors to the greenhouse effect, they increase as a function of temperature and are therefore considered feedbacks. Increased concentrations of gases such as CO₂ (~20%), ozone and N

₂O are external forcing on the other hand.^[62]

Human activity since the Industrial Revolution has increased the amount of greenhouse gases in the atmosphere, leading to increased radiative forcing from CO₂, methane, tropospheric ozone, CFCs, and nitrous oxide. As of 2011, the concentrations of CO₂ and methane had increased by about 40% and 150%, respectively, since pre-industrial times.^[63] In 2013, CO₂ readings taken at the world's primary benchmark site in Mauna Loa surpassing 400 ppm for the first time.^[64] These levels are much higher than at any time during the last 800,000 years, the period for which reliable data have been collected from ice cores.^[65] Less direct geological evidence indicates that CO₂ values have not been this high for millions of years.^[66]

Global anthropogenic greenhouse gas emissions in 2010 were equivalent to 49 billion tonnes of carbon dioxide (using the most recent global warming potentials over 100 years from the AR5 report). Of these emissions, 65% was carbon dioxide from fossil fuel burning and industry, 11% was carbon dioxide from land use change, which is primarily due to deforestation, 16% was from methane, 6.2% was from nitrous oxide, and 2.0% was from fluorinated gases.^[67] Using life-cycle assessment to estimate emissions relating to final consumption, the dominant sources of 2010 emissions were: food (26–30% of emissions);^[68] washing, heating, and lighting (26%); personal transport and freight (20%); and building construction (15%).^[69]