2. Since about 1750 global warming is driven by radiative GHG forcing of near + 3.0 Watt/m2 consequent on rise of GHG (CO2, CH4, NxO, ozone, halocarbons), compensated in part by albedo increase due to land clearing (- 0.2 Watt/m2), aerosols ( - 0.5 Watt/m2) and clouds (- 0.7 Watt/m2). When the albedo loss due to melting of the Arctic and Antarctic sea ice, the margins of Greenland and Antarctic ice sheets and mountain glaciers, is accounted for, the total forcing would be tracking toward values about half those of the last glacial termination of 6.5±1.5 Watt/m2.
Detailed deuterium proxy-based paleo-temperature studies of Greenland ice cores GISP-2 indicate that, far from smooth, the transitions associated with the glacial terminations involved abrupt tipping points where temperatures rose or fell sharply by several degrees C over time scales as short as a few decades or even a few years (Kobashi et al., 2008; Steffensen et al., 2008). A potential onset of such tipping points in the context of 21st century climate change is consistent with observations pertaining to the last glacial termination, current methane release from sediments off-shore Siberia and from permafrost, Arctic Sea ice melt, Antarctic sea ice and ice shelf melt and intensifying Atlantic hurricanes.
A marked climate tipping point is defined about 1975-76, with abrupt rise of temperature and temperature rise rates. 1975 – 2008 climate change developments incurred CO2 rise by 55 ppm (332-387 ppm) and mean temperature rise of ~ +0.9oC for the Northern Hemisphere (mean CO2 rise ~ 1.7 ppm/yr; temperature rise 0.027oC/yr; 0.016oC per 1 ppm CO2). In so far as the relations between CO2 and temperature during 1975-2008 can be used as a baseline, a rise of CO2 levels to 450 ppm by 2050 would result in minimum additional temperature rise by approximately 1.0oC relative to 2008.
Conservative estimate of the "climate sensitivity", estimated at 3 degrees rise per doubling of CO2 for fast climate feedback processes (water vapor, clouds, aerosols, sea ice), implies a rise of CO2 by 100 ppm (from 450 to 550 ppm CO2) will elevate global temperatures by about 1.0±0.5oC, where a trajectory toward 550 ppm threatens to raise temperatures to about 2.6oC later in the 21st century. However, slow climate change feedbacks (reduced continental ice sheets, increased vegetation cover in permafrost-melt areas) ensue in climate sensitivity of ~ 6oC per doubling of CO2 – consistent with the last glacial termination (Hansen et al., 2008).
Given the onset of the Antarctic ice sheet at or below 500 ppm CO2 at ~34 Ma (late Eocene), and of the Arctic Sea ice below 400 ppm at 2.8 Ma (mid-Pliocene) (Haywood and Williams, 2005), the projected consequences of CO2 trajectories toward 550 ppm are likely involve catastrophic climate tipping points.
The IPCC-2007 and Garnaut Review-2008 climate change projections
The termination of glacial periods through insolation maxima associated with Milankovic eccentricity, obliquity and precession cycles, effecting 40 – 60 Watt/m2 spikes at latitude 65N (Roe, 2005), trigger forcing of ~ 6 – 7 Watt/m2 and associated carbon cycle and ice melt/water feedback effects (Hansen et al., 2006, 2007, 2008). However, feedback effects are neglected in the IPCC-2007 report, which states: “The emission reductions to meet a particular stabilization level reported in the mitigation studies assessed here might be underestimated due to missing carbon cycle feed-backs (see also Topic 2.3) AR4 caption to Table 5.1”.
Wigley (1993, 2006) and Wigley et al. (2007) modeled CO2 trajectories, accounting for carbon feedbacks, reversal of atmospheric CO2 overshoots and stabilization, stating: “Stabilization of the climate system requires stabilization of greenhouse-gas concentrations. Most work to date has considered only stabilization of CO2, where there are choices regarding both the concentration stabilization target and the pathway towards that target. Here we consider the effects of accounting for non-CO2 gases (CH4 and N2O), for different CO2 targets and different pathways. As primary cases for CO2 we use the standard “WRE” pathways to stabilization at 450 ppm or 550 ppm. We also consider a new “overshoot” concentration profile for CO2 in which concentrations initially exceed and then decline towards a final stabilization level of 450 ppm, as might occur if an initial target choice were later found to be too high.”
However, the recent history of the atmosphere betrays little evidence for stabilization scenarios. By contrast, glacial-interglacial cycles culminate with runaway warming and tipping points preceding sharp or gradual temperature declines (Broecker, 2000; Alley et al., 1997, 2003; Braun et al., 2005; Roe, 2006; Hansen et al., 2006, 2007, 2008; Steffensen et al., 2008; Kobashi et al., 2008)
Principal alternatives considered in the Garnaut (2008) Climate Change Review include (p. 277):
1. “Australia’s full part for 2020 in a 450 scenario would be a reduction of 25 per cent in emissions entitlements from 2000 levels, or one-third from Kyoto compliance levels over 2008–12, or 40 per cent per capita from 2000 levels. For 2050, reductions would be 90 per cent from 2000 levels (95 per cent per capita)”.
2. “Australia’s full part for 2020 in a 550 scenario would be a reduction in entitlements of 10 per cent from 2000 levels, or 17 per cent from Kyoto compliance levels over 2008–12, or 30 per cent per capita from 2000. For 2050, reductions would be 80 per cent from 2000 levels or 90 per cent per capita.”
3. “If there is no comprehensive global agreement at Copenhagen in 2009, Australia, in the context of an agreement among developed countries only, should commit to reduce its emissions by 5 per cent (25 per cent per capita) from 2000 levels by 2020, or 13 per cent from the Kyoto compliance 2008–12 period.”
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