Ice core data - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 149

attribute their claim that ''variations in greenhouse gas concentrations are too
weak to explain the interhemispheric link'' to Lorius et al. (1990), because the
latter paper provides exactly the opposite viewpoint. Lorius et al. said ''the
climate change during the past few hundred thousand years was linked with
changes of greenhouse-gas concentration'' and ''changes in the CO 2 and CH 4
content have played a significant part in the glacial-interglacial climate changes by
amplifying, together with the growth and decay of the Northern Hemisphere ice
sheets, the relatively weak orbital forcing and by constituting a link between the
Northern and Southern Hemisphere climates.'' Lorius et al. (1990) concluded
'' 50 10% is a reasonable estimate for the overall contribution of the green-
house gases to the Vostok temperature change over the last climate cycle. This
means that 3 C of the 6 C in the glacial-interglacial change at Vostok could be
attributed to the greenhouse effect.''
Another weakness of the theory of Laeppele et al. (2011) is that the
connection of length of austral winter to ice core isotope results is almost non-
existent. Furthermore, as shown in Section 4.3.4, Hansen et al. (2010) have
provided a credible estimate of forcings in worldwide glacial-interglacial transi-
tions based on the assumption that the trigger to stimulate transitions occurs via
solar variability in the NH, and then extends worldwide due to various secondary
effects.

4.3.4 Worldwide effects of changes originating in the NH

We can propose the following sequence of events leading to ice age-interglacial
transitions. A trend is initiated via peak summer solar intensity at latitudes
roughly in the range 60 Nto70 N, which begins either a cooling or a heating
cycle, depending on the sign. This impacts the ability of surface snow and ice to
survive the summers and expand over periods of thousands of years. Once started,
a cooling trend produces secondary effects. Nascent ice sheets begin to form,
increasing global albedo. Ice packs expand. Sea level begins to drop, replacing sea
by land, thus increasing global albedo further. Vegetation in the NH suffers, also
contributing to increased albedo. The concentrations of greenhouse gases (water
vapor, CO 2 , and CH 4 ) decrease, producing further cooling worldwide. These
effects spread southward, gradually providing cooling to the entire planet. Hansen
et al. (2010) estimated that the forcings in going from a full interglacial to a glacial
maximum would be:

. Changes in albedo due to ice sheets, replacement of ocean by land, and changes
in vegetation 3.5W/m 2

.

Changes in CO 2 ,CH 4 , and N 2 O concentrations 3W/m 2 .

However they did not take into account changes in humidity, dust levels, or
cloudiness. Nevertheless, there is enough forcing to account for worldwide climate
changes without invoking the theory of Laeppele et al. (2011).