Environmental Engineering Reference
In-Depth Information
fect of CO
2
is logarithmic in CO
2
concentration, so that if one starts with
a high level of CO
2
in order to explain the warm Paleocene, one needs an
unrealistically large amount of additional CO
2
to double or quadruple CO
2
and give the requisite warming. On the other hand, if the sensitivity of cli-
mate to CO
2
is very high, then one can explain the Paleocene temperature
with a smaller CO
2
concentration, and also get the required PETM warm-
ing from the release of a smaller amount of carbon to the atmosphere. This
appears to demand a very high climate sensitivity, at least at the top of the
IPCC range, and perhaps beyond (Pagani et al., 2006). Methane or other,
currently unknown, radiative forcing agents may have affected the Pliocene
climate and the PETM warming, but for now the simplest explanation of the
PETM would appear to be that climate sensitivity is very high.
Impacts of anthropogenic global warming are quite sensitive to tropical
warming, and the warm climates of the past shed some light on this issue
as well. It has occasionally been proposed that the tropics are subject to a
thermostat of one sort or another that limits tropical warming, but such pro-
posals have been found to have no basis in physics (Williams et al., 2009).
Moreover, the paleoclimate record of the Eocene and Paleocene provides
direct support for the possibility of tropical temperatures considerably in
excess of those prevailing today (Huber, 2008). Uncertainties in past CO
2
concentrations, however, make it impossible to say whether current general
circulation models overestimate or underestimate tropical climate sensitivity.
It is generally recognized, however, that general circulation models have
difficulty reproducing the low meridional temperature gradient prevail-
ing in past warm climates (Pierrehumbert, 2002; Huber and Sloan, 2001).
This suggests that the Earth system is subject to feedbacks amplifying polar
warming, which are not adequately represented in current models (Abbot
et al., 2009a).
The Potential for Large Biogeochemical
Emissions: Evidence and Time Scales
Emissions of greenhouse gases could be augmented in a warmer world
due to releases of gases from biogeochemical processes, such as methane
from methane hydrates both in permafrost at high latitudes and under the
deep ocean, enhanced nitrous oxide emissions from soils, and increased
release of carbon dioxide from warming peat, soils, and the biosphere (Den-
man et al., 2007). Some of these sources could be very large, raising the issue
of the risk of substantial contributions to climate change. For example, some
estimates suggest that the carbon reservoir in the form of methane stored
