Short-lived climate forcers

 

Climate forcers, also referred to as climate forcing agents, act directly to change climate and include both natural and human contributors. They are often distinguished as short- or long-lived, according to their lifetime in the atmosphere. For example, carbon dioxide (CO2), the largest climate forcer from human activity, is considered long-lived. Although often described as having a lifetime of a century or more, a single lifetime value is not strictly applicable (owing to its complex interactions with the Earth system), but an estimated 15%– 40% of CO2 emitted by the year 2100 will remain in the atmosphere, and continue to exert a climate warming effect, for more than 1000 years (Ciais et al., 2013). Short-lived climate forcers are those with a lifetime of a few days to a few decades and include sulphate aerosols and black carbon (soot) with lifetimes of a few days; tropospheric ozone and various hydrofluorocarbons, with a lifetime of a few weeks; and methane, with a lifetime of a decade or so. Reducing emissions of short-lived substances leads to lower atmospheric concentrations of these substances shortly thereafter. Many of these short-lived species contribute to poor air quality. Those that have a climate warming effect are also referred to as short-lived climate pollutants (http://www. ccacoalition.org/en/science-resources) and include black carbon, methane, and tropospheric ozone. In some cases, aerosols that have a cooling effect are co-emitted with short-lived warming agents (Arctic Council, 2011), complicating estimates of the near-term effectiveness of emission reductions. Short-lived climate forcers are important in climate policy discussions because targeted mitigation of those with warming effects can both slow global temperature increase and improve human health by improving air quality.