Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 39

only about two-thirds the amplitude of global mean surface temperature change.''
It is not clear how they arrived at the 2/3 figure.
Hansen and Sato (2011) interpreted oxygen isotope data in deep-sea sediments
to infer deep-ocean temperatures, and then asserted that these were representative
of global average temperatures except when the deep-sea temperature approached
the freezing point of water, whereupon the change in deep-sea temperature was
assumed to represent only 2/3 of the change in global average temperature. Then,
using Dwyer's estimate that bottom waters were 4.5 C colder during the LGM,
Hansen and Sato arrived at an estimate of 6.8 C for D T G based on the 2/3 rule
(see Figure 2.8 ).
The reason the LGM point lies so low is important changes took place on the
surface of the Earth as the ice sheets expanded. These changes go beyond the
purely spectroscopic effect of less absorption of IR by CO 2 in the atmosphere as
the CO 2 concentration was lowered to below 200 ppm. The growth of large ice
sheets from recent pre-industrial times to the LGM resulted in an increase in the
Earth's albedo across the ice sheets, as well as for mountain glaciers. In addition,
the drop in sea level moved shorelines outward, converting ocean to land, thereby
further increasing the Earth's albedo. As the climate got colder, biomass and
vegetation grew less abundantly, increasing the Earth's albedo still further.
Undoubtedly, there were other effects as well (humidity, cloudiness, dust, etc.).
Hansen and Sato (2011) estimated the various forcings in the past, present,
and future, as well as the Earth's response to a forcing. The Earth's response is
expressed as climate sensitivity: C change in average global temperature per unit
change in forcing (W/m 2 ). This assumes that a single term, average global
temperature, captures a description of the Earth's climate, whereas many different
climate distributions across regions could conceivably lead to the same average
global temperature. In some cases, direct data are available. In many cases,
however, direct data are not available and Hansen and Sato (2011) utilized
various models, assumptions, and inferences to derive approximate estimates of
key quantities. They seems to adopt these estimates as if they were factual, which
they are not. Hansen and Sato (2011) evidently believe that the long-term thermo-
static regulator of the Earth's climate is the CO 2 concentration in the atmosphere.
The balance between volcanic emissions (and, more recently, emissions due to
human industrial activity) and absorption by surface carbon reservoirs
(atmosphere, ocean, soil, and biosphere) determines net atmospheric composition.
Hansen and Sato (2011) carried out an analysis in which they attempted to
utilize the data relating the LGM to pre-industrial times as a basis for estimating
the temperature rise in going from pre-industrial times to a doubling of the CO 2
concentration (from 280 to 560 ppm).
According to Hansen and Sato (2011), based on Hansen et al. (2008), the
transition between the LGM and pre-industrial times can be characterized by two
major sources of forcing:

. Changes in concentration of greenhouse gases (about 75% due to CO 2 ).
. Surface changes such as ice sheet area, vegetation distribution, and shoreline

movements.