The differences between the 550 ppm and 450 ppm scenarios are as follows (p.86):
“No-mitigation case. A global emissions case in which there is no action to mitigate climate change—the Garnaut–Treasury reference case—was developed as part of the Review. This emissions case recognizes recent high trends in the emissions of carbon dioxide and other greenhouse gases. Emissions continue to increase throughout the 21st century, leading to an accelerating rate of increase in atmospheric concentrations. By the end of the century, the concentration of long-lived greenhouse gases is 1565 ppm CO2-e, and carbon dioxide concentrations are over 1000 ppm—more than 3.5 times higher than pre-industrial concentrations.
550 mitigation case. Emissions peak and decline steadily, so that atmospheric concentrations stop rising in 2060 and stabilize at around 550 ppm CO2-e—one-third of the level reached under the no-mitigation case.
450 mitigation case. Emissions are reduced immediately and decline more sharply than in the 550 case. Atmospheric concentrations overshoot to 530 ppm CO2-e in mid-century and decline towards stabilization at 450 ppm CO2-e early in the 22nd century.”
The Review (p. 95) acknowledges: “The small change in global average temperature between the 550 and 450 mitigation pathways could have a relatively large impact on sea-ice extent.” Yet Table 11.1 and Figure 44 of Garnaut-2008 Review, suggest the difference by 2050 is no more than 0.1oC:
1. A CO2-e rise of 450 ppm by 2050 would raise temperature by +1.6oC relative to 1990
2. A CO2-e rise of 550 ppm by 2050 would raise temperature by +1.7oC relative to 1990
As indicated above, these estimates are near one order of magnitude low as compared to projections based on climate sensitivity, estimated at 3±1.5oC per doubling of CO2 concentration (Charney, 1979).
To summarize:
1. IPCC-2007 and Garnaut-2008 CO2 stabilization scenarios, derived from modeled equilibrium states (Wigley, 1993, 2006; Wigley et al., 2007; Archer, 2005; Bender et al., 2005; Lenton and Britton, 2006) appear to take little account of methane release, the effects of ice sheet melt and potential tipping points.
2. Garnaut-2008’s choice between a 450 ppm and 550 ppm trajectory for 2050, projected difference of 0.1oC per 100 ppm CO2-e rise for these trajectories, and the assumption of CO2 ‘stabilization’, are difficult to reconcile with extensions of the 1975 – 2008 CO2 trend. These projections take little account of the consequences of non-linear climate feedback processes due to methane release from sediments and permafrost, ice sheet breakup, infrared absorption by exposed sea water, and consequent climate tipping points.
3. The assumption that CO2 levels can be reversed from 550 ppm, once reached, to 450 ppm over acceptable time scales, finds little support in the centuries-scale atmospheric residence time of CO2 and in past atmospheric records.
Climate models, effective in modeling 20th and early 21st century climate change, tend to underestimate the magnitude and pace of global warming (Rahmstorf et al., 2007). According to Hansen et al. (2008) “Climate models alone may be unable to define climate sensitivity more precisely, because it is difficult to prove that models realistically incorporate all feedback processes. The Earth’s history, however, allows empirical inferences of both fast feedback climate sensitivity and long term sensitivity to specified greenhouse gas change including the slow ice sheet feedback.”.
The Earth atmosphere is already tracking toward conditions increasingly similar to the mid-Pliocene ~3.0 Ma, with temperatures higher than mean Holocene temperatures by + 2-30C, ice-free Arctic Sea, tens of metres sea level rise and a permanent El-Nino (Dowsett et al., 2005; Haywood and Williams, 2005; Gingerich, 2006). Additional anthropogenic GHG forcing and methane emission threaten conditions approaching those of the Paleocene-Eocene Thermal Maximum (PETM) 56 Ma, when the eruption of some 1500 GtC (Sluijis et al., 2007), inferred from low δ13C values (-2 to 3‰ 13C), resulted in global warming of ~ 6oC, development of subtropical conditions in the Arctic circle (sea temperatures 18 – 23oC (Sluijis et al., 2007), ocean acidification and mass extinction of 30-35% of benthic plankton (Panchuk et al., 2008). The recent history of the atmosphere, and the presence of thousands of GtC in metastable methane hydrates, clathrates and permafrost, suggests a CO2 trajectory toward 550 ppm may lead toward conditions similar to the PETM.
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