The main short-lived climate pollutants (SLCPs) are black carbon (or ‘soot’) particles, methane, tropospheric ozone and some hydrofluorocarbons (HFCs). Controlling emissions of these SLCPs or their precursors could roughly halve projected warming over the next few decades while saving millions of lives and increasing crop yields by tens of millions of tons annually via improved air quality. These benefits would be obtained by reducing emissions of some SLCPs, such as black carbon and methane, that are at historically high levels, whereas emissions of others, such as HFCs, would have to prevented from growing from their current small levels. Additional short-lived compounds affect climate and degrade air quality, but do not clearly lead to warming and so are not typically included as SLCPs.
Black carbon is an important component of particulate matter air pollution. Airborne particulate matter is the major environmental cause of premature death globally and contributes substantially to many other adverse health impacts as well. Methane is a precursor to the formation of ozone in the lower atmosphere which, at ground-level, harms human health, crops and the climate. The ozone precursors carbon monoxide, and volatile organic compounds are also considered SLCPs.
In addition, methane, black carbon and ozone are powerful warming agents. Black carbon and ozone also disturb rainfall and regional circulation patterns and black carbon darkens snow and ice, increasing absorption of sunlight and exacerbating melting.
Though HFCs represent a small fraction of the current total greenhouse gases (less than one percent), their warming impact per molecule is particularly strong and, if left unchecked, growth in HFC emissions could lead to substantial additional warming. HFCs themselves do not have direct impacts as air pollutants, but there are studies that suggest some of their products may have environmental impacts when broken down.
Short-lived pollutants, ones that live in the atmosphere for roughly 10 years or less, will be cleansed from the atmosphere fairly quickly once their emissions cease. Their influence will also go away fairly rapidly after the cessation of their emissions. This is unlike CO2 which lives in the atmosphere for a very long time and whose effect on global warming is almost permanent on the human life timescale.