Environmental Engineering Reference
In-Depth Information
In sharp contrast, some greenhouse gases have biogeochemical proper-
ties that lead to atmospheric retention times (lifetimes) of centuries or even
millennia. These gases can accumulate in the atmosphere whenever emis-
sions exceed the slow rate of their loss, and concentrations would remain
elevated (and influence climate) for time scales of many years even in the
complete absence of further emission. Like the water in a bathtub, concen-
trations of carbon dioxide are building up because the anthropogenic source
substantially exceeds the natural net sink. Even if human emissions were
to be kept constant at current levels, concentrations would still increase,
just as the water in a bathtub does when the water comes in faster than it
can flow out the drain. The removal of anthropogenic carbon dioxide from
the atmosphere involves multiple loss mechanisms, spanning the biosphere
and ocean (see Section 2.4), and carbon dioxide removal cannot be char-
acterized by any single lifetime. Although some carbon dioxide would be
lost rapidly to the terrestrial biosphere and to the shallow ocean if human
emissions cease, some of the enhanced anthropogenic carbon will remain
in the atmosphere for more than 1,000 years, influencing global climate
(Archer and Brovkin, 2008). The warming induced by added carbon dioxide
is expected to be nearly irreversible for at least 1,000 years (Matthews and
Caldeira, 2008; Solomon et al., 2009), see Section 3.4.
Figure 2.1 shows that carbon dioxide is the largest driver of current
anthropogenic climate change. Other gases such as methane, nitrous oxide,
and halocarbons also make significant contributions to the current total
CO
2
-equivalent concentration, while aerosols (see Section 2.3) exert an
important cooling effect that offsets some of the warming. The best estimate
of net total CO
2
equivalent concentration of the sum across these forcing
agents in the year 2005 is about 390 ppmv (with a very likely range from
305 to 430 ppmv). Global carbon dioxide emissions have been increasing at
a rate of several percent per year (Raupach et al., 2007). If there were to be
no efforts to mitigate its emission growth rate, scenario studies suggest that
carbon dioxide could top 1,000 ppmv by the end of the 21st century. Carbon
dioxide alone accounts for about 55% of the current total CO
2
-equivalent
concentration of the sum of all greenhouse gases, and it will increase to
between 75 and 85% by the end of this century based on a range of future
emission scenarios (see Section 2.2). Thus carbon dioxide is the main forcing
agent in all of the stabilization targets discussed here, but the contributions
of other gases and aerosols to the total CO
2
-equivalent remain significant,
motivating their consideration in analysis of stabilization issues.
How large a reduction of emissions is required to stabilize carbon di-
oxide concentrations, and does it depend upon when it is done or on the
