Black carbon results from incomplete combustion of biomass and fossil fuels. Its major anthropogenic sources are biomass and fossil fuel burning for heat and cooking, transportation (diesel and gasoline vehicles without new filtration systems), and agricultural open burning. Wildfires also produce large amounts of black carbon.
Black carbon has a short atmospheric lifetime, on the order of a few days to weeks, so effects are strongly regional. Black carbon particles give soot its black color and, like any black surface, strongly absorb sunlight. In snow-covered areas, the deposition of black carbon darkens snow and ice, increasing their absorption of sunlight and making them melt more rapidly.
Black carbon may contribute to the acceleration of sea ice loss, and melting of glaciers, which are a major source of fresh water for millions and contribute to sea level rise. Additionally, black carbon, as small particulate matter (PM2.5), contributes to respiratory and cardiovascular illnesses, and other air pollutants are often emitted from the same sources. Reductions in this pollutant would slow climate warming and have significant co-benefits for human health, particularly in developing countries where a large proportion of black carbon comes from household biomass burning.
Black carbon emissions are expected to fall in many parts of the world, but concerted efforts globally could result in significant emissions reductions by 2030. In the case of cooking by burning wood or biomass, the move to cleaner burning fuels would immediately reduce black carbon and would not be too expensive (at least globally). Transitioning from coal plants to renewables or other cleaner energy sources would also reduce black carbon emissions.
Methane emissions are generated primarily by agriculture, including from farming rice and ruminant livestock (methane is produced by bacteria in their stomachs), natural gas and petroleum systems, and waste disposal in landfills. It also has natural sources like wetlands. Methane has an atmospheric lifetime of about 12 years and a global warming potential of about 25 over a 100-year period, meaning a pound of methane has 25 times the warming power of a pound of carbon dioxide over this timespan. It makes up about 10 percent of greenhouse gas emissions in the United States and roughly 16 percent worldwide.
Reductions in methane emissions can also improve local air quality by reducing tropospheric ozone, which methane helps to produce. Emissions of methane are expected to continue increasing through 2030, unless immediate action is taken. At oil and gas wells, methane can be burned to immediately convert the gas to carbon dioxide, reducing its GWP. To reduce the impact from livestock, certain diets are being explored, and reductions in livestock populations would also reduce methane emissions.
Ozone occurs at both the troposphere (ground level) and in the stratosphere. Tropospheric ozone is not an emitted pollutant, but is created from the chemical reactions of methane, carbon monoxide, nitrogen oxides, and volatile organic compounds, known as precursors. Its lifetime in the atmosphere varies from hours to weeks. In order to reduce the warming effect of tropospheric ozone it is necessary to reduce these precursor emissions.
Methane is responsible for about half of the increase in tropospheric ozone levels, so actions to reduce methane emissions would have the added benefit of reducing tropospheric ozone levels. Ground-level ozone, carbon monoxide, and nitrogen dioxide are all known as criteria air pollutants, regulated by the EPA due to their harmful health effects. They are found at the source of any fossil fuel combustion, and reducing fossil fuel use at power plants, industrial facilities, and vehicles can reduce tropospheric ozone levels.
Hydrofluorocarbons (HFCs) are good substitutes to ozone-depleting substances chlorofluorocarbons and hydrochlorofluorocarbons (CFCs and HCFCs), which were phased out under the Montreal Protocol.
There are four types of fluorinated gases (F-gases), most of which are human-made: hydrofluorocarbons, perfluorocarbons, nitrogen trifluoride, and sulfur hexafluoride. Their use is expected to grow dramatically over time (see figure below). Though the F-gases are all climate forcers, hydrofluorocarbons are the only short-lived species. HFCs are used in air conditioning, refrigeration (in grocery stores and food processing facilities, for example), foam blowing, aerosol propellants, and as solvents. Emissions generally come from faulty equipment, poor maintenance, and leakage from improper disposal.
The global warming potential of HFCs can be thousands of times greater than that of carbon dioxide and they have a significant impact on climate change. This warming impact is projected to grow in the absence of controls on global production. Given their high emission rates and, on average, their relatively short atmospheric lifetime (compared to carbon dioxide), efforts to reduce hydrofluorocarbon emissions in the near term will significantly reduce projected temperature increases in the coming decades. HFC-143a, the most common hydrofluorocarbon used in refrigeration and air condition systems, leaks into the atmosphere as a result of faulty equipment or improper disposal. To combat this, recycling systems can be set up and alternative gases with lower global warming potential can be mandated in new systems.