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.
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