Studies of Pleistocene glacial-interglacial cycles based on sediments and ice cores back to 640 000 years (640 kyr) document abrupt initiation of global warming and cooling events over short time scales of decades to a few years, implying extreme instability of the Earth’s atmosphere, with implications for 21st century climate change projections.
Current rise rates of atmospheric radiative forcing toward ~450 ppm CO2 are tracking toward an ice-free Earth.
Time tables of carbon emission reduction targets which take little account of the rates of ice sheet melt/water feedbacks and carbon cycle feedbacks, including release of methane hydrates from sea bottom sediments and from bogs, are unlikely to be able to prevent runaway global warming on a scale similar to the last glacial terminations.
Extreme climate change events in the recent history of Earth include:
A. Intra-glacial warming cycles, termed Dansgaard-Oeschger (D-O cycles), between 80 kyr and 20 kyr, including 21 ~1470 years-long cycles, each initiated over decades-scale time frames (Figure 1). The D-O cycles are attributed to interactions between weak solar radiation maxima and ocean current systems [1, 2].
Figure 1. An example of a Dansgaard-Oeschger abrupt climate event. 1 of 21 cycles during the alst ice age, 80 000 – 20 000 years-ago. Greenland ice core. From Rahmstorf, 2004.
B. Evidence for the last glacial termination based on deuterium and oxygen isotopes from the Greenland NGRIP ice core indicates sharp 3-years-long warming by 2 to 4oC at 14.7 kyr, sharp 1 year-long cooling at 12.9 kyr, and sharp 3 years-long warming at 11.7 kyr (Figure 2) .
Figure 2. deuterium-drived determinations of temperatures from Greenland NGRIP ice core for the period 14 740 – 11 660 years-ago, displaying abrupt warming and cooling changes between the ‘oldest dryas’ cold period, Allerod and Bolling warm periods, youngest dryas cold period and the Holocene. Note transitions occur over periods of 1 – 3 years. From Steffensen et al., 2008.
C. Evidence for the last glacial termination from the Greenland GISP-2 ice core, based on Nitrogen and Argon isotopes, indicates abrupt warming by 10±4oC at 12.8 kyr over a period of ~100 years and abrupt warming by 4±1.5 oC at 11.27 kyr over period of 70 years . Sea level rose by 40 metres following the termination up to about 8500 years-ago .
Mean global temperature changes are estimated as about half the polar temperatures.
The origin of the D-O cycles is interpreted in terms of interaction between weak insolation signals and the thermohaline current system . Glacial terminations at intervals of about 100 kyr, 41 kyr and 23 kyr (Milankovich cycles) were triggered by axial tilt toward the poles, elevating mid-June insolation by up to <60 Watt/m2 at latitude 65N . The glacial terminations involved mean global solar radiation anomalies of 4 to 5 Watt/m2, triggering ice melt feedback loops and greenhouse gas release loops . The intertwined synergy of these processes resulted in: (1) Ice sheet and glacier melt, reduced short-wave reflection (albedo) by sea ice and ice sheets, exposure of water surfaces absorbing infrared, further melting of ice by warming water, migration of boreal forest northward causing decrease in albedo. (2) Carbon gases (CO2, CH4) released from warming oceans, drying biosphere and fires; methane released from sea-bottom methane hydrates (clathrates: water-CH4 molecules) in sea bottom sediments and from drying bogs. Rapid release of methane hydrates is invoked as a mechanism for a runaway greenhouse effect and consequent mass extinctions through the history of Earth, specifically the Permian-Triassic (251 Ma) mass extinction and the Paleocene-Eocene (55 Ma) extinction (~ +6oC global warming)  The combined radiative synergy of ice melt-feedback and greenhouse gases-feedback triggering rapid polar meltdown affected pole-ward migration of the inter-tropical convergence zone (ITCZ), subtropical arid zone and mid-latitude zones, affecting the ocean thermohaline circulation. The Greenland ice-melt flow result in abortion of the Gulf Stream which warms Europe and northeast America. Warming of the southern oceans weakens the Humboldt current west of South America and the trade winds, indirectly enhancing El-Nino events which result in droughts in the southwestern Pacific, India and Africa . The rise in mean global temperature by several degrees Celsius over time scales of a few years to a century testifies to a high susceptibility of the atmosphere to minor to moderate energy forcings. According to Hansen et al. 2007  the solar energy pulse from orbital variations which triggered the glacial terminations is up to 0.25 Watt/m2. Mean global atmospheric energy rise associated with the glacial terminations of +4 to +5 Watt/m2 (~ +5 to +7oC) are consistent with the upper range of the IPCC projections for the 21st century, +1.1 to +6.4oC. Comparisons between abrupt glacial-interglacial terminations and 21st century projections are complicated by the lower mean global temperatures at which the glacial terminations commenced and the large volumes of ice compared to the Holocene, including the Laurentian and Fennoscandian ice sheets. The fast rise of the greenhouse gas forcing component since the mid-1800s, at rates since 1960 reaching 387 ppm in 2007 at rates of >1.6 ppm/year, are two order of magnitude higher than CO2 rise rates of 0.012 ppm/year at the last termination. Where 1 ppm CO2 induces an atmospheric energy rise of ~0.02 Watt/m2, this equates to an increase of 1.7 Watt/m2 in atmospheric radiative energy since 1750, not counting carbon cycle and ice melt feedbacks.
The non-linear nature of atmospheric CO2 and CH4 rise, from 1.3%/year in 1990-1999 to 3.3%/year in 2000-2006 , combined with further ice melt and albedo decline and carbon cycle feedback effects, including release of CH4 hydrates, drying/burning biosphere, reduced CO2 sequestration by the oceans, threatens to move the Earth’s atmosphere into glacial termination-like conditions. Rapid ice melt rates in Greenland, the Arctic Sea and west Antarctica, the latter continuing through the southern winter, are documented from satellite and on the ground. The polar ice sheets, initiated about 34 million years ago, when CO2 levels declined below 450 ppm, are in danger.
The Earth’s climate is tracking into uncharted territory.
Andrew Glikson undertakes earth and paleo-climate research at the Research School of Earth Science, Australian National University.
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