Environmental Engineering Reference
In-Depth Information
Figure 2.7 shows the CO
2
-equivalent concentrations for these two sce-
narios at three points in time. Notice that in the case of the tighter radiative
forcing goal, there is some “overshoot”. That is, the target is exceeded in the
middle part of the century and then gradually approached. This is due to the
assumption that there will be a “negative” emitting technology, Bioenergy
with Carbon Capture and Sequestration (BECS). Otherwise a faster decline
rate of the capital stock would be required.
2.3 SHORT-LIVED RADIATIVE FORCING AGENTS:
PEAK TRIMMING VERSUS BUYING TIME
The role of CO
2
emissions in Earth's climate future is unique among the
major radiative forcing agents, because the impact of the CO
2
emitted into
the atmosphere will continue to alter Earth's energy budget for millennia to
come. As noted above, Earth's energy budget is also subject to the influence
of a number of short-lived radiative forcing agents whose radiative effect
would decay to zero on a time scale of weeks to decades if their sources
where shut off. These agents include aerosols, black carbon on snow or
ice, and methane.
Aerosols are produced by burning biomass and fossil fuels, but unlike
CO
2
they are not an inevitable by-product of combustion. Some aerosols
reflect energy to space, but other aerosols such as black carbon absorb sun-
light. Reflecting aerosols unambiguously lead to cooling of the surface. Their
effect has offset a portion of the radiative forcing from the anthropogenic
increase of greenhouse gases so far, and any action that reduces reflecting
aerosol emissions will lead to a nearly immediate warming. The effect of
airborne absorbing aerosols is more subtle, because they primarily act to
shift the absorption of solar radiation from the surface to the interior of the at-
mosphere, leaving the top-of-atmosphere energy budget largely unchanged
(Randles and Ramaswamy, 2008). The global mean effect of surface black
carbon is difficult to quantify but is unambiguously a warming, amplified
further by the albedo feedback of melting snow and ice (Flanner et al., 2007;
Hansen and Nazarenko, 2004; McConnell et al., 2007). Aerosol effects can
include direct damage to human health and agriculture, implying that they
should be cleaned up for reasons independent of climate (Agrawal et al.,
2008; Ramanathan et al., 2008). A key question is whether the effort to do
so will help or hurt other efforts to keep warming in check. Although the
discussion of aerosol effects will be based primarily on temperature changes,
it should be kept in mind that the spatially inhomogeneous radiative forcing
from aerosols can lead to regional effects such as changes in clouds and the
