Environmental Engineering Reference
In-Depth Information
interest: the Pliocene period extending from 5.3 to 2.6 million years ago
and preceding the initiation of the Pleistocene glacial/interglacial cycles;
the Miocene period from 23 to 5.3 million years ago, during which Ant-
arctic glaciation was initiated; and the Paleocene/Eocene boundary about
56 million years ago, at which point a massive release of carbon dioxide
(presumably from land ecosystems) caused a massive warming of an already
warm and largely ice-free climate.
The importance of the Pliocene is that the carbon dioxide concentra-
tions at the time were only moderately greater than at present (380-425
ppm), but the climate was substantially different from today's climate (Lunt
et al., 2010). Antarctica was already glaciated, but the Northern Hemisphere
(particularly Greenland) was free of large ice sheets. Northern Hemisphere
mid-latitude and high-latitude temperatures were considerably greater than
today's. Tropical temperatures were not much warmer than modern values,
although the characteristic east-west temperature gradient of the Pacific may
have been much weaker, consisting of a permanent El Niño pattern (Wara
et al., 2005). It is difficult to estimate global mean temperatures directly
from proxy data, but using a combination of simulations and marine prox-
ies, Lunt et al. (2010) estimate that the Pliocene global mean temperature
was 3°C warmer than at present. The Pliocene provides at least a hint that
the Earth System Sensitivity is such that a doubling of atmospheric CO
2
, or
perhaps even less, could cause a transition to a largely ice-free Northern
Hemisphere, provided the CO
2
remains high long enough. In a model-based
diagnosis of Pliocene climate feedbacks, Lunt et al., 2010 estimate that Earth
System Sensitivity (not counting carbon cycle feedbacks) is 1.45 times the
basic climate sensitivity.
The Paleocene-Eocene Thermal Maximum (PETM) provides one of the
most worrying indicators of Earth System Sensitivity. This event begins at the
end of the Paleocene, during which Earth was already in a warm, globally
ice-free state. Carbon isotope data indicate that at this time a rapid release of
isotopically light carbon occurred, and that the climate warmed globally by
about 4°C. The most consistent picture at present is that the isotopically light
carbon comes from a release of about 3,000 GtC of presumably land-based
organic carbon, rather than from a destabilization of methane clathrates. For
a review of the PETM and estimates of the amount of organic carbon release,
the reader is referred to Zeebe et al., 2009. The essential challenge posed
by the PETM is that one must explain an already warm Paleocene climate
(presumably caused by elevated CO
2
), at the same time as accounting for
the additional warming caused by release of additional carbon, which must
not exceed the limits allowed by data. The problem is that the radiative ef-
