Greenhouse gas emissions (GHGE) are vaporous chemical compounds emitted into Earth’s atmosphere as a direct or indirect result of human activities. These gases exacerbate the greenhouse effect, causing climate change. The greatest quantity of these gases is carbon dioxide from burning fossil fuels: coal, oil, and natural gas. The largest sources of greenhouse gases (GHGs) include coal in China and large oil and gas corporations, including many state-owned companies in OPEC countries. Human-caused emissions have increased atmospheric carbon dioxide by about 50% since 1750.
Electricity generation and transport are also major emitters, the largest single source being coal-fired power stations (20% of GHG). Deforestation for industrial agriculture and other major changes in land use also emit carbon dioxide and methane. The largest source of anthropogenic methane emissions is agriculture, closely followed by gas venting and fugitive emissions from the fossil-fuel industry. The largest agricultural methane source is livestock. Agricultural soils emit nitrous oxide partly due to fertilizers. Similarly, fluorinated gases from refrigerants play a large role in total human emissions.
At current emission rates averaging six and a half tonnes per person per year, before 2030 temperatures may have increased by 1.5°C (2.7°F), which is the limit for the advanced economies of the G7 group of countries (Canada, France, Germany, Italy, Japan, the United Kingdom and the United States) and the aspirational limit of the Paris Agreement.
Global GHGE are about 50 gigatonnes (Gt) per year (6.6t per person) and for 2019 have been estimated at 57 Gt CO2 eq, including 5 Gt due to land use change.
Carbon dioxide (CO2), nitrous oxide (N2O), methane, three groups of fluorinated gases—sulfur hexafluoride (SF6), hydrofluorocarbons (HFCs) and perfluorocarbons—are the major anthropogenic GHGs These are regulated under the Paris Agreement.
Although chlorofluorocarbons (CFCs) are greenhouse gases, they are regulated by the Montreal Protocol (1987), which was motivated by CFCs` contribution to ozone depletion rather than by their contribution to global warming. Ozone depletion has only a minor role in greenhouse warming, though the two processes are sometimes confused. In 2016, negotiators from over 170 nations meeting at the summit of the United Nations Environment Programme reached a legally binding accord to phase out HFCs in the Kigali Amendment (2016) to the Montreal Protocol.
Measuring greenhouse gas emissions can be done several ways. Some variables that have been reported include:
- Definition of measurement boundaries: This process attributes emissions geographically, either to the area where they were emitted (the territory principle), or by the activity principle to the territory that produced the emissions. These two principles result in different totals when measuring, for example, electricity importation from one country to another, or emissions at an international airport.
- Time horizon of different gases: The contribution of given greenhouse gas is reported as a CO2 equivalent. The calculation to determine this takes into account how long that gas remains in the atmosphere.
The measurement protocol itself: This may be via direct measurement or estimation. The four main methods are: (1) the emission factor-based method, (2) mass balance method, (3) predictive emissions monitoring systems and (4) continuous emissions monitoring systems. These methods differ in accuracy, cost, and usability