Future prospects - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 358

that a similar trend for solar oscillations occurred around 400,000 years ago (see
Figure 10.3 ) when there was an exceptionally long interglacial period. Hence, they
suggested that we might expect a similarly long interglacial period in our future.
However, Figure 10.3 shows that the period of low-amplitude solar oscillations
began about 450 kybp and continued to about 340 kybp . During that period there
occurred both a long interglacial and a major ice age (which began to develop
around 395 kybp ). Thus, the period from about 450 kybp and continuing to about
390 kybp is supportive of the notion that the amplitude of solar oscillations at
high latitudes is the controlling force in determining glacial-interglacial cycles, but
the period from 390 to 340 kybp provides a contrary trend.
Several significant papers by leading experts suggested that global warming
could interrupt the thermohaline circulation of the Atlantic Ocean and lead to a
variety of consequences, one of which might be premature evolution of the next
ice age (e.g., Broecker, 1997b). Broecker (1999) mentioned that there were sugges-
tions that the ongoing greenhouse buildup might induce a shutdown of the ocean's
thermohaline circulation, raising questions as to how Earth's climate would
change in response. Thornalley et al. (2011) found evidence that thermohaline
circulation has played a role in climate change since the Last Glacial Maximum.
Broecker argued that an extreme scenario is unlikely, because models suggest that
Earth would have to undergo a 4 to 5 C greenhouse warming in order to force an
ocean conveyor shutdown. Broecker also lamented the lack of an atmospheric
model that would lead (from first principles) to the observed large and abrupt
changes in the climatic state of Earth's atmosphere. Broecker summed up his
doubts about climate models succinctly:
''No one understands what is required to cool Greenland by 16 C and the
tropics by 4 1 C, to lower mountain snowlines by 900m, to create an ice sheet
covering much of North America, to reduce the atmosphere's CO 2 content by
30%, or to raise the dust rain in many parts of Earth by an order of magnitude. If
these changes were not documented in the climate record, they would never enter
the minds of the climate dynamics community. Models that purportedly simulate
glacial climates do so only because key boundary conditions are prescribed (the
size and elevation of the ice sheets, sea ice extent, sea surface temperatures,
atmospheric CO 2 content, etc.).''
Rahmstorf (2004) said:
''Threatening scenarios of a breakdown of the Atlantic thermohaline
circulation, a collapse of northern European agriculture and fisheries, and of
glaciers advancing on Scandinavia and Scotland have captured the popular
imagination in recent years, with a number of newspaper reports, magazine
articles and television documentaries covering this topic with a widely varying
degree of accuracy. The risk of critical thresholds in the climate system being
crossed where some irreversible qualitative change sets in (such as a major ocean