B. Arctic Sea ice melt rates of ~5.4% per-decade since 1980, increasing to >10% per year during 2006–2007 (NSIDC, 2008);

C. West Antarctica sea ice melt rates >10% per decade culminating in mid-winter ice shelf breakdown (Wilkins ice shelf; June, 2008, NSIDC, 2008);

D. Advanced melt of Greenland ice;

E. Slow-down of the North Atlantic thermohaline conveyor belt and down-welling water columns (NASA, 2004; Bryden et al., 2005), with attendant danger of its cessation analogous to conditions ~8.2 kyr ago (Alley et al., 1997), considered in a Pentagon inquiry (Stipp, 2004);

F. Temperature projections for the North Atlantic Ocean (Keenlyside et al., 2008) may reflect the effect of Greenland ice melt waters;

G. Increased frequency and intensification of categories 4 and 5 hurricanes (Webster et al., 2005) and, not least, elevated methane release from Arctic Sea sediments and sub-Arctic permafrost (Walter et al., 2006; Rigby, 2008).

Increasingly an analogy emerges between these developments and aspects of abrupt climate changes associated with the last glacial termination. As stated by Alley et al. (2003) “Large, abrupt, and widespread climate changes with major impacts have occurred repeatedly in the past, when the Earth system was forced across thresholds.” Ice core and sedimentary evidence for the Pleistocene (1.8 Ma – 10,000 years ago) demonstrate abrupt glacial terminations, intra-glacial global warming events (Dansgaard–Oeschger cycles; Broecker, 2000; Ganopolski & Rahmstorf, 2002; Braun et al., 2005) as well as sharp to protracted cooling periods. The latest glacial termination includes a number of tipping points which involve sharp rise and fall of temperatures by several degrees C over time scales of centuries, decades, or even a few years (Clark et al., 2003; Kobashi et al., 2008; Steffensen et al., 2008), affecting both high latitudes and tropical zones (Hughen et al., 1996).

Comparisons between CO2, CH4, temperature and sea level changes during glacial terminations, post-1850 and 20th–21st century climate change rates (Table 1; Glikson, 2008) suggest:

1. CO2 rise rates: Late 20th century and early 21st century rates averaging 1.45 ppm/yr and rising to 1.8 ppm/yr in 2006 and 2.2 ppm/yr in 2007, exceed 1850–1970 rates by factors of ~4–5 and are two orders of magnitude higher than mean CO2 rise rates of the last glacial termination (~0.014 ppm/yr) (Rahmstorf et al., 2006; Global Carbon Project, 2008).

2. CH4 rise rates: A 10 ppb/yr rise in methane during 2007 (http://web.mit.edu/newsoffice /2008/techtalk53-7.pdf), exceeding the 1850–1970 rise (~5.4 ppb/yr), is orders of magnitude higher than during the last glacial termination. Methane deposits potentially vulnerable to climate change reside in permafrost (~900 GtC), high latitude peat lands (~400 GtC), tropical peat lands (~100 GtC), vulnerable vegetation (~650 GtC) and methane hydrates and clathrates in the ocean and ocean floor sediments (>16,000 GtC). The total exceeds the atmospheric level of carbon (~750 GtC), carbon emissions to date (~305 GtC) and known economic carbon reserves (>>4000 GtC).

3. Temperature rise rates: Mean temperature rise rates of 0.016 oC during 1970–2007 were about an order of magnitude higher than during 1850–1970 (0.0017 oC) and the last glacial termination. As indicated by deuterium studies of Greenland ice cores, abrupt tipping points during the last termination (14.7–11.7 kyr) resulted in extreme temperature changes on the scale of several degrees C in a few years (Steffensen et al., 2008).

4. Sea level rise rates: Mean sea level rise rate of ~0.32 cm/yr during 1988–2007 more than doubled relative to the mean ~0.14 cm/yr rate of 1973–1988 and three times those of 1850–1970. In so far as doubling of sea level rise rates continues at this rate through the 21st century, they may approach rates similar to those of the last glacial termination (1.3–1.6 cm/yr) before mid-century, with sea level rise by several metres toward the end of the century as estimated by Hansen et al. (2007).

Whereas larger ice sheets existed on Earth at the outset of the last glacial termination, when the large Laurentian and Fennoscandian ice sheets began to melt, than during the Holocene, comparisons between climate forcings during the glacial termination and those operating since about 1750 may be instructive:

1. The last glacial termination, triggered by insolation peaks, involved total radiative forcing rise of about 6.5 Watt/m2, including ~3.0±0.5 Watt/m2 induced by rising greenhouse gases (GHG: CO2, CH4, NxO) and 3.5±1.0 Watt/m2 induced by lowered albedo associated with melting of ice sheets and spread of vegetation. Both factors, including their feedback effects, result in mean global temperature rise of ~5.0±1.0 oC (Hansen et al., 2008).

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 ~0.9 oC for the Northern Hemisphere (mean CO2 rise ~1.7 ppm/yr; temperature rise 0.027 oC/yr; 0.016 oC 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 1.0 oC 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±0.5 oC, where a trajectory toward 550 ppm threatens to raise temperatures to about 2.6 oC 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 ~6 oC 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 & 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% in emissions entitlements from 2000 levels, or one-third from Kyoto compliance levels over 2008–2012, or 40% per capita from 2000 levels. For 2050, reductions would be 90% from 2000 levels (95% per capita)”.

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 0.1 oC:

1. A CO2-e rise of 450 ppm by 2050 would raise temperature by +1.6 oC relative to 1990

2. A CO2-e rise of 550 ppm by 2050 would raise temperature by +1.7 oC 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±1.5 oC 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 0.1 oC 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 +2–3 oC, ice-free Arctic Sea, tens of metres sea level rise and a permanent El-Nino (Dowsett et al., 2005; Haywood & 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 ~6 oC, development of subtropical conditions in the Arctic circle (sea temperatures 18–23 oC (Sluijis et al., 2007), ocean acidification and mass extinction of 3035% 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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