Understanding Earth's Deep Past Lessons for Our Climate Future (2011) / Chapter Skim
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3 Climate Transitions, Tipping Points, and the Point of No Return
3 Climate Transitions, Tipping Points, and the Point of No Return
Pages 63-80
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From page 63... ...
. Components of the climate system that are particularly vulnerable to being forced by increasing atmospheric CO2 across a threshold into a new state include the loss of Arctic summer sea ice, the stability of the Greenland and West Antarctic ice sheets, the vigor of the meridional overturning circulation in the North Atlantic and around Antarctica, the extent of Amazon and boreal forests, and the variability of the El Niño-Southern Oscillation (ENSO)
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From page 64... ...
. Examples of threshold behaviors include thermohaline circulation modifications, ice sheet instabilities, sea ice instabilities, soil-moisture feedbacks, and the onset of high-latitude convection and associated high-level cloud forcing.
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From page 65... ...
climate forcing across a critical threshold that led to abrupt and highly variable climate responses, as examples of what can be gleaned from the deep-time geological record of climate change and the scientific challenges that persist. Initiation of the Cenozoic Icehouse The early Cenozoic greenhouse Earth was plunged from a protracted state of warmth into its current glacial state 33.7 million years ago (Ma)
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From page 66... ...
at the Eocene-Oligocene boundary (referred to as the "Oi-1 overshoot"; Zachos et al., 2001b) , with this transient cooling a result of some combination of rapid East Antarctic ice sheet growth and global cooling (Zachos et al., 2001a; Coxall et al., 2005)
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From page 67... ...
2,000 1,000 0 B –1 Partial or ephemeral Full scale and permanent 12 Antarctic ice sheets 0 Ice-free temperature (°C) Northern Hemisphere ice sheets 8 ?
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From page 68... ...
will persist through the anthropogenic perturbation, so it is reasonable to anticipate that the climate -- following the current transient warming -- will cool over the subsequent few tens of millennia. Eventually, conditions for the reinitiation of the Antarctic and Greenland ice sheets will be achieved, but these may require atmospheric pCO2 levels similar to preindustrial values and a favorable orbital state (Berger et al., 2003; Pollard and DeConto, 2005)
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From page 69... ...
. The oxygen isotopic compositions of planktonic and benthic foraminifera record rapid warming of ~5°C in tropical surface and deep oceans, and as much as 9°C warming at the poles, that persisted for ~170 ky (Sluijs et al., 2006; Zachos et al., 2006; Röhl et al., 2007)
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From page 70... ...
FIGURE 3.3 Marine stable isotope and seafloor sediment CaCO3 records compiled using several ocean drilling sites for the PETM, a hyperthermal with some parallels to modern greenhouse gas-driven global change.
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From page 71... ...
Despite substantially different land mass-height distributions, ocean circulation patterns, and marine and terrestrial ecosystems from those of today, the geological record of these deglaciations -- specifically the repeated major transitions between glaciations and glacial minima including their terminal epic deglaciations -- provide the only "icehouse" perspective of the response of the climate system and ecosystems to perturbation beyond the range archived in the more recent glacial records. The Late Paleozoic Deglaciation Much of the scientific understanding of feedbacks and thresholds in the current glacial climate system, and their influence on the biosphere,
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From page 72... ...
It appears that a combination of persistent warming from the late Paleocene to early Eocene, punctuated by deep ocean acidification at the PETM, defined a threshold for coral-algal reefs that led to rapid loss and only gradual recovery. Notably, the lack of evidence for surface water acidification probably indicates that the rate of carbon addition was slower -- perhaps by an order of magnitude -- than projected fossil fuel emission rates under the least optimistic scenarios for the future (e.g., the A1 family of scenarios considered by IPCC [2007]
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From page 73... ...
, estimated to be 3,000 Pg carbon using published carbon isotope and observed deep sea carbonate dissolution records, and a carbon cycle model (LOSCAR; Zeebe et al., 2008, 2009)
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From page 74... ...
. For the final stages of this protracted ice age, covariance between shifts in pCO2 and continental and marine surface temperatures inferred from isotopic proxies of soil-formed minerals and marine fossil brachio pods, and ice sheet extent reconstructed from southern Gondwanan glacigenic deposits, indicates a strong linkage of pCO2-climate-ice-mass dynamics that is consistent with greenhouse gas forcing (Montañez et al., 2007)
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From page 75... ...
, abruptly terminating what was probably the longest-lived (~135 Ma; Macdonald et al., 2010) and coldest icehouse period of Earth history, where at times ice sheets extended to sea level in equatorial latitudes -- a climate state popularly referred to as the "Snowball Earth" (Hoffman et al., 1998)
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From page 76... ...
Although CO2 produced by fossil fuel burning is taken out of the atmosphere within decades of its production, the oceans, soils, and vegetation continue to exchange greenhouse gases back into the atmosphere for far longer. Greenhouse gases continue to affect climate and ocean acidity until they are buried as organic matter or converted to mineral forms of inorganic carbon through rock weathering (Box 3.4)
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From page 77... ...
. In addition, the equilibra tion timescale for a pulse of CO2 emission to the atmosphere, such as the current release by fossil fuel burning, scales up with the magnitude of the CO2 release.
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From page 79... ...
Carbon cycle models indicate that even after 100,000 years, the anthropogenic perturbation to the carbon cycle will still be important, especially if the total amount of carbon emitted is large. Consequently, Milankovitch forcings that have so dominated the pacing and extent of climate variations, and especially ice sheets, over the last 2 million years will -- as they were prior to the onset of the current
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From page 80... ...
. By any measure, exploitation of much of the fossil fuel reservoir over only 300 years will clearly leave a far longer lasting legacy.
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