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Life Arts    H2'ed 11/21/08

21st century climate tipping points

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Message Andrew Glikson
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Recent climate developments in the polar cryosphere and the oceans suggest the atmosphere is tracking toward conditions similar to those of ~ 2.8 Ma (mid-Pliocene: + 2 to 3oC; sea level + 25±12 metres; permanent El-Nino) (Haywood and Williams, 2005; Dowsett et al., 2005) and a possible tipping point. The polar Sea ice and continental ice sheets, which serve as Earth’s climate thermostat, are changing at an accelerated rate. Developments to date include: 

A.      The rise of mean Arctic and sub-Arctic temperatures in 2005-2008 by near +4oC relative to 1951-1980 (NASA-GISS);

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

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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);

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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 and 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).

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3.        Temperature rise rates:  Mean temperature rise rates of 0.016oC during 1970 - 2007 were about an order of magnitude higher than during 1850-1970 (0.0017oC) 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±1.0oC (Hansen et al., 2008).

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Earth and paleo-climate scientist Australian National University Canberra, A.C.T. 0200
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