Beyond the Next Few Centuries - Climate Stabilization Targets - page 227

Fenno-Scandian ice sheets, but going forward into warmer climates, it is the
fate of the Greenland and Antarctic ice sheets that is of prime interest.
The loss of Greenland would make the Anthropocene look something
like the Pliocene world, before the Pleistocene glacial/interglacial cycles set
in, and there is strong evidence that this could happen in response to an-
thropogenic CO 2 emissions. Using coupled GCM/ice-sheet models, Ridley
et al. (2005) find that Greenland deglaciates almost completely when the
global mean warming exceeds 5°C for 1,500 years, and (Vizcaino et al.,
2008) find near-total deglaciation when the global mean warming remains
above 3.5°C for 4,000 years. Gregory et al. (2004) argue that even lesser
warming could lead to deglaciation and cite indications that, once deglaci-
ated, the Greenland ice cap might not recover even if CO 2 were restored
to pre-industrial levels. The deglaciation of Greenland is primarily sensitive
to Greenland regional summer warming, and models differ greatly as to
the relationship of global mean temperature to the local summer warming.
Estimates of the global mean warming needed for deglaciation range from
a low of 1.9°C to a high of 4.6°C (Meehl et al., 2007). The vulnerability
of Greenland found in models is consistent with the absence of Northern
Hemisphere glaciation in the Pliocene. Calculated Greenland melt is shown
in Figure 6.2. During the Eemian about 130,000 years ago, Arctic tempera-
tures were about 3-5°C warmer than present, and it has been estimated that
the loss of ice from Greenland and other Arctic ice fields contributed up to
4 m to sea level rise (Jansen et al., 2007).
In protracted warm conditions, the deglaciation of Greenland proceeds
from robust melt ablation, with little need to involve the less well understood
aspects of ice flow. Various aspects of ice dynamics that are not currently
well represented in glacier models have the potential to allow the deglacia-
tion to proceed much more rapidly, but nothing definitive can be said about
the minimum time scale at present.
It is sometimes asserted that anthropogenic CO 2 emission would be
beneficial because it could avoid an impending ice age, but this is incor-
rect. In fact, Berger et al. (2003) and Berger and Loutre (2002) project that
Earth's current orbital configuration would make the Holocene interglacial
unusually long even without bringing anthropogenic CO 2 into the mix.
Earth's orbit would not be expected to produce another ice age for at least
30,000 years (Jansen et al., 2007). Anthropogenic warming is piling warming
on top of an interglacial that is already projected to be unusually long, and
moreover doing it at a time when the precessional cycle will be swinging
into a hot Northern Hemisphere phase over the next 5,000 years. This adds
to the prospect that anthropogenic CO 2 emissions could lead to a major