section_path: Causes of recent global temperature rise/Greenhouse gases
section_name: Greenhouse gases
paragraph_text: Greenhouse gases
  
  Greenhouse gases are transparent to sunlight, and thus allow it to pass through the atmosphere to heat the Earth's surface. The Earth radiates it as heat, and greenhouse gases absorb a portion of it. This absorption slows the rate at which heat escapes into space, trapping heat near the Earth's surface and warming it over time.
  
  While water vapour (≈50%) and clouds (≈25%) are the biggest contributors to the greenhouse effect, they primarily change as a function of temperature and are therefore mostly considered to be feedbacks that change climate sensitivity. On the other hand, concentrations of gases such as CO (≈20%), tropospheric ozone, CFCs and nitrous oxide are added or removed independently from temperature, and are therefore considered to be external forcings that change global temperatures.
  
  Before the Industrial Revolution, naturally-occurring amounts of greenhouse gases caused the air near the surface to be about 33 °C warmer than it would have been in their absence. Human activity since the Industrial Revolution, mainly extracting and burning fossil fuels (coal, oil, and natural gas), has increased the amount of greenhouse gases in the atmosphere, resulting in a radiative imbalance. In 2019, the concentrations of CO and methane had increased by about 48% and 160%, respectively, since 1750. These CO levels are higher than they have been at any time during the last 2 million years. Concentrations of methane are far higher than they were over the last 800,000 years.
  
  Global anthropogenic greenhouse gas emissions in 2019 were equivalent to 59 billion tonnes of CO. Of these emissions, 75% was CO, 18% was methane, 4% was nitrous oxide, and 2% was fluorinated gases. CO emissions primarily come from burning fossil fuels to provide energy for transport, manufacturing, heating, and electricity. Additional CO emissions come from deforestation and industrial processes, which include the CO released by the chemical reactions for making cement, steel, aluminum, and fertilizer. Methane emissions come from livestock, manure, rice cultivation, landfills, wastewater, and coal mining, as well as oil and gas extraction. Nitrous oxide emissions largely come from the microbial decomposition of fertilizer.
  
  While methane only lasts in the atmosphere for an average of 12 years, CO lasts much longer. The Earth's surface absorbs CO2 as part of the carbon cycle. While plants on land and in the ocean absorb most excess emissions of CO every year, that CO is returned to the atmosphere when biological matter is digested, burns, or decays. Land-surface carbon sink processes, such as carbon fixation in the soil and photosynthesis, remove about 29% of annual global CO emissions. The ocean has absorbed 20 to 30% of emitted CO over the last 2 decades. CO is only removed from the atmosphere for the long term when it is stored in the Earth's crust, which is a process that can take millions of years to complete.