Environmental Engineering Reference
In-Depth Information
2
Emissions, Concentrations,
and Related Factors
2.1 CONTRIBUTION OF DIFFERENT CHEMICALS TO CO
2
EQUIVALENT LEVELS AND CLIMATE CHANGES
A range of anthropogenic chemical compounds contribute to chang-
ing Earth's energy budget, thereby causing the planet's global climate to
change. For example, increases in greenhouse gases absorb infrared energy
that would otherwise escape to space, acting to warm the planet, while
some types of aerosol particles can contribute to cooling the planet by
reflecting incoming visible light from the Sun. These components of our
atmosphere are emitted from a variety of human activities, including for
example fossil fuel burning, land-use change, industrial processes such
as cement production, and agriculture. The gases and particles involved
are frequently referred to as drivers of climate change, or radiative forcing
agents. Detailed reviews of radiative forcing is presented in Forster et al.
(2007) and Denman et al. (2007). Radiative forcing due to various climate
change agents can be converted to equivalency with the concentration of
CO
2
(CO
2
equivalent), one frame of reference for this report (see Figure 2.1).
Here we briefly summarize how major forcing agents contribute to current
and future CO
2
-equivalent target levels and explore implications for global
mean temperature increases.
Some greenhouse gases and aerosols are retained for days to years in
the atmosphere after emission. The concentrations of such compounds in the
atmosphere are tightly coupled to the rate of emission. Their concentrations
would drop rapidly if emissions were to cease. Increasing emissions lead
to increases in concentrations of such gases, while constant emissions are
required for their concentrations to be stabilized. Methane is a key green-
house gas with an atmospheric lifetime of about 10 years whose concentra-
tion has approximately doubled since the pre-industrial era (1750), and it
is the second most important greenhouse gas, currently contributing about
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