Understanding Earth's Deep Past Lessons for Our Climate Future (2011) / Chapter Skim
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2 Lessons from Past Warm Worlds
2 Lessons from Past Warm Worlds
Pages 26-62
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From page 26... ...
Although most deep-time greenhouse climates occurred when there were distinctly different continental configurations, and thus are not direct analogues for the future, past warm climates and abrupt transitions into even hotter states (known as hyperthermal events; Thomas et al., 2000) provide important insights into how physical, biogeochemical, and biological processes operate under warm conditions more analogous to what is anticipated for the future than the moderate and stable climates of the Holocene (past 10,000 years)
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From page 27... ...
. For example, the eruptions of large igneous provinces in the mid-Cretaceous and the subduction of the carbonate-rich tropical Tethys Sea in the early Cenozoic are the most likely cause of the high-CO2 equilibrium climates of the Cretaceous and Eocene greenhouses.
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From page 29... ...
. The following sections describe the insights provided by understanding past warm periods, including the role of greenhouse gases in controlling -- or "forcing" -- global warming; the impact of warming on ice sheet stability, sea level, and oceanic and hydrological processes; and the consequences of global warming for eco systems and the global biosphere.
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From page 30... ...
promoted by global warming can substantially magnify an initial temperature increase. As Earth moves toward a warmer climate state, it is important to understand the extent to which climate sensitivity will change due to processes
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From page 31... ...
, a system with high climate sensitivity to CO2 will warm more in the future than a world with low climate sensitivity. Thus, if the climate sensitiv ity is high, restricting future global warming will require a larger reduction in future CO2 emissions than if climate sensitivity is lower.
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From page 32... ...
For example, the destabilization of continental ice sheets resulting from warming of polar regions can potentially lead to a decrease in deep-water formation, thereby affecting global ocean circulation, stratification, and carbon cycling, leading to higher climate sensitivity than indicated by present estimates. In sufficiently warm climates, even water vapor has a nonlinear dependence on temperature, and this can introduce new and potentially rapid feedbacks, operating at a subdecadal scale, into the climate system.
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From page 33... ...
Hence, an initial warming from greenhouse gases released by burning fossil fuels could end up releasing even more greenhouse gases from natural sources, exacerbating the original warming of the atmosphere. TROPICAL AND POLAR CLIMATE STABILITY AND LATITUDINAL TEMPERATURE GRADIENTS IN A WARMER WORLD With more than half of Earth's surface lying within 30° latitude of the equator, the response of tropical climates to increased greenhouse gas forcing is critically important.
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From page 34... ...
for the terrestrial tropics of past warmer worlds. Additionally, coupled climate model simulations with large radiative forcings and/or paleoclimate simulations for elevated greenhouse gases do not produce a thermostatic regulation of tropical temperatures (e.g., Boer et al., 2005; Poulsen et al., 2007b; Cherchi et al., 2008)
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From page 35... ...
Dashed lines represent deep-sea temperatures. For the Cretaceous, the dotted line is the modern observed zonal surface temperature, while the symbols indicate a compilation of empirical data for specific periods.
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From page 36... ...
Since significant changes in tropical and polar surface temperatures and pole-to-equator temperature gradients occurred in the past, and could occur in a future warmer world, it is imperative to understand the mechanisms and feedbacks that lead to such changes and their consequences for atmospheric and oceanic circulation (Hay, 2008)
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From page 37... ...
. In contrast, proxy time series of sufficient continuity, together with complementary climate models of past sustained warm periods illustrate that small, long-term changes in Pacific sea surface temperatures can have a substantial effect on ENSO phenomena, as well as planetary albedo, regional rainfall, and increased atmospheric levels of water vapor -- a powerful greenhouse gas (see Box 2.3)
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From page 38... ...
As temperatures in the eastern equatorial region cooled gradually after ~2 Ma, the equatorial sea surface temperature gradient increased dramatically (post-1.8 Ma on Figure 2.6) , ultimately establishing strong Walker circulation and the present-day ENSO 1.5 warm period cold period 2.0 Less ice 2.5 δ18O(‰)
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. FIGURE 2.6 Changes in equatorial sea surface temperature gradients over time.
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From page 40... ...
. Accordingly, deep-time data-model comparisons of past warm and transitional periods provide the ultimate test of science's ability to forecast the geographic patterns of hydrologi cal response to CO2-forced global warming and associated precipitation, evaporation, and latent heat fluxes.
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From page 41... ...
Despite such uncertainty, the geological record is the only repository that can place constraints on the sensitivity of ice sheets and equilibrium sea level to rapid (millennia or less) climate change during past warmings that led to collapse of ice sheets of the scale of, or larger than, the Greenland or Antarctic ice sheets.
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South Florida. Because of the time needed for atmospheric CO2 and surface warming to reach equilibrium, this amount of sea level rise might be anticipated even if anthropogenic sources of CO 2 were to cease today.
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From page 43... ...
Evidence for continental ice sheets during past long-lived greenhouse periods defines a "climate-glaciation paradox" which in turn provides an opportunity to uniquely address two issues that cannot be tested in studies of the more recent glacial-interglacial fluctuations of the Ceno zoic. First, the geological records of glacial events during past greenhouse warm periods illustrate ice sheet stability and long-term equilibrium sea level change during times of substantially elevated atmospheric pCO2, major climate perturbations, and in many cases, complete deglaciation.
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From page 44... ...
. Rapid sea level changes during past warm periods also raise the question of whether ephemeral ice sheets were common to all past warm periods.
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From page 45... ...
It is also possible that there are nonglacioeustatic -- and perhaps nonclimatic -- causal mechanisms that could influence ice sheet dynamics and sea level response with continued warming. EXPANSION OF OCEANIC HYPOXIA IN A WARMER WORLD One forecast consequence of global warming is the widespread expansion of oceanic hypoxia.
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From page 46... ...
-- with their attendant changes in surface temperatures and continental weathering (i.e., phosphorus fluxes to the ocean) -- have been linked to increased levels of atmospheric greenhouse gases brought on relatively slowly through volcanism or by more rapid release following magmatic intrusion into organic-rich sediments (Tejada et al., 2002; Svensen et al., 2004; Turgeon and Creaser, 2008; Barclay et al., 2010)
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From page 47... ...
timescales by the release of large amounts of CO2 that boosted atmospheric CO2 levels. The abrupt and prolonged global warming would have been stressful to biota by itself, but for marine organisms the resulting reduction in oxygen solubility and reduced oxygen delivery to the deep ocean led to wide spread seafloor anoxia and permitted the accumulation of toxic substances, including hydrogen sulfide, in the deep ocean (Meyer et al., 2008)
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From page 49... ...
. The rapid response and strong variability recorded by these Cretaceous OAE intervals demonstrates the sensitivity of oceanic conditions to perturbation of atmospheric circula tion and continental weathering brought on by global warming.
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50 UNDERSTANDING EARTH'S DEEP PAST BOX 2.5 Continued FIGURE 2.10 High-resolution plots showing multiple precessional cy cles during Cretaceous OAE 3 (~85 Ma) from Ocean Drilling Program Site 959 in the eastern equatorial Atlantic.
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From page 51... ...
. Ecosystem changes have already accompanied the rapid loss of snowfields and sea ice, with consequent decrease in surface albedo in high-latitude regions, as a result of the global warming of the past century (ACIA, 2004; Chapin et al., 2005)
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SOURCE: Images courtesy Integrated Ocean Drilling Program Science Services.
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From page 53... ...
, further perturbing biota-climate feedbacks compared with those reconstructed from the recent past. The deep-time geological record, in particular the record of warm periods of higher atmospheric pCO2 and including the transitions into and out of these periods, has the potential to yield unique insights into the nature and rate of biotic response to climate perturbation as well as into the biota-climate feedbacks accompanying global warming.
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From page 54... ...
tropical sea surface temperatures (Scheibner and Speijer, 2008) (Figure 2.13)
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From page 55... ...
The atmospheric CO 2 and temperature increases shown are those for the scenarios and do not refer to the particular locations photographed. SOURCE: Photographs by and with permission of Ove Hoegh-Guldberg, Global Change Institute, University of Queensland.
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From page 56... ...
. Notably, the deep-time record of this major reef crisis uniquely captures the consequences on larger-scale marine ecosystems that might be anticipated with the future loss of reefs.
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From page 57... ...
. Such dramatic floristic changes occurred with each climate transition during the final stage of the Late Paleozoic Ice Age.
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58 UNDERSTANDING EARTH'S DEEP PAST BOX 2.7 Continued
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From page 59... ...
estimated pCO2 (blue line) and paleo-sea surface temperatures (red band)
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From page 60... ...
, in striking contrast to the modern mean annual temperatures of minus 30ºC. The fossil floral record indicates that in this warmer world, both subtropical and tropical rainforests had greatly expanded latitudinal ranges (Figure 2.15)
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From page 61... ...
LESSONS FROM PAST WARM WORLDS FIGURE 2.15 Reconstruction showing the large latitudinal range of tropical rainforests during the Paleocene greenhouse. SOURCE: Morley (2000)
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From page 62... ...
. Most importantly, the geologi cal record uniquely captures past climate-ecological interactions that are fully played out and thereby archive the impact, response, interaction, and recovery from past global warming and major climate transitions.
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