Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 35

2.3.3 The transition from the Last Glacial Maximum (LGM) to
the pre-industrial era
A fundamental dictum in climatology is that the Earth's climate does not change
arbitrarily and capriciously but, rather, as a consequence of changes in its heat
balance caused by either external changes such as changes in the Sun's power
output, changes in the Earth's orbit around the Sun, changes in the Earth's
surface producing changes in albedo, and changes in the atmosphere, principally
greenhouse gas concentrations. Each of these potential changes is described by
an equivalent ''forcing'' which is a hypothetical heat flux uniformly distributed
across the spherical Earth, measured in W/m 2 . According to this dictum, the
Earth will only settle into an equilibrium climate when there is no net forcing and
the heat flux radiated out from the top of the atmosphere balances the solar heat
input to the Earth's surface. When a change takes place, producing a net forcing,
the Earth will respond by warming or cooling (depending on the sign of the
forcing) until a new radiative balance is achieved. There is also a possibility of
self-induced climate change due to internal variability such as changes in ocean
flows and winds that may produce internal feedbacks resulting in climate change.
These changes are generally thought to be smaller than the major climate changes
of the past such as ice age-interglacial transitions, and the great cooling that took
place since about 50 million years ago. Over time spans of millions of years, or
even tens of thousands of years, the Earth's climate will have time to reach a
series of quasi-equilibrium states in response to forcings. Over short periods such
as decades, the Earth may lag in its response to forcings, or the effect of forcings
may be hidden by short-term fluctuations due to various feedbacks or internal
fluctuations.
About 20,000 years ago, the most recent ice age was at its maximum extent
with gigantic ice sheets in the higher latitudes of the Northern Hemisphere. There
is reliable evidence from ice cores that the CO 2 concentration at that time was
roughly 170-180 ppm. The first question is what was T G at the LGM?
Dwyer et al. (1995) utilized the ratio of magnesium to calcium (Mg/Ca) in
fossil ostracods from Deep Sea Drilling Project Site 607 in the deep North
Atlantic to infer that bottom water temperature changed by 4.5 C in going from
the LGM to pre-industrial times.
According to Leroux (2005), the difference in temperature between an ice age
and an interglacial was about 10 C in the Antarctic and about 6 C globally.
Taylor et al. (2001) carried out an analysis in which they took into account
the reduced CO 2 concentration and the extended ice sheets of the LGM in climate
models to estimate the amount of global average cooling at the LGM compared
with pre-industrial times. Using six different climate models, they obtained values
of 3.5, 3.7, 3.8, 4.4, 5.2, and 5.9, for an average of 4.4 C.
Crucifix (2006) provided a less optimistic view of the precision to which this is
known: ''The global temperature change is therefore estimated to be comprised
between 3 C and 9 C with 95% confidence.'' He also estimated that the tropical
ocean sea surface temperature decreased between 1.7 and 2.7 C, and Antarctic