Environmental Engineering Reference
In-Depth Information
especially in view of the great difficulties models have with reproducing
Antarctic regional climate.
A great deal of public attention has focused on 350 ppm as a target for
CO
2
concentration. The identification of 350 ppm as a danger threshold for
CO
2
(Hansen et al., 2008) rests largely on the study of the Miocene initia-
tion of Antarctic glaciation. The assumption here is that Antarctica would
deglaciate at CO
2
concentrations similar to those which permitted the initia-
tion of Antarctic glaciation. The CO
2
concentration at the time of Antarctic
glacier initiation is not very well known, and 350 ppm is drawn from the
lower end of estimates of the concentrations prevailing at the time of initia-
tion of Antarctic glaciation. Beyond that it is likely that the East Antarctic
ice sheet, once formed, can survive considerably higher temperatures than
those prevailing at the time of its initiation. The paleoclimate argument for
350 ppm as a danger threshold must be considered speculative, but the es-
sential difficulty is that there are no past analogues in which a massive East
Antarctic ice sheet has been subjected to temperatures as warm as those
that may prevail in the Anthropocene.
It is important to recognize that the hypothetical dangers from a CO
2
concentration of 350 ppm reside in the very long-term climate feedbacks.
Therefore, the CO
2
would have to remain above this level for thousands of
years in order for these feedbacks to be a major concern. The concentra-
tion can exceed 350 ppm at its peak without incurring a risk of triggering
the long-term feedbacks, so long as it subsides to 350 ppm or less over a
few thousand years. In the carbon cycle model of Eby et al. (2009), cu-
mulative carbon emissions of 850 GtC or less are required to allow the
CO
2
concentration to subside to 350 ppm within 5,000 years, but over the
range of carbon cycle models with long-term climate and sediment dissolu-
tion feedbacks discussed in Archer et al. (2009), two-thirds of the models
recover to under 350 ppm in 5,000 years when the cumulative emissions
are 1,000 Gt.
If uncompensated by increase in water storage in East Antarctica, the
total loss of the Greenland ice sheet would lead to a sea level rise of 7.5 m
(Bamber et al., 2001), while the total loss of the West Antarctic Ice Sheet
would lead to an additional rise of 5 m (Bamber et al., 2009). The latter is
comparable to the contributions of the West Antarctic Ice Sheet to Pliocene
and Pleistocene sea level fluctuations as modeled by Pollard and DeConto
(2009). Melting of all mountain glaciers and ice caps would add at least 0.7
m to sea level (Bahr et al., 2009), although it should be noted there is consid-
erable uncertainty regarding the thickness of many ice caps in mountainous
regions. This would be added to the long-term sea level rise due to thermal
