Hay,W.W.(2011): Can humans force a return to a ‘Cretaceous’ climate? Sedimentary Geology, 235, 5-26.


 The modern pole-to^equator sea-level temperature difference is about 50℃; that of the mid-Cretaceous ranged from 30℃ to as little as 24℃, implying a much equable climate. This may have been caused by 1) reduction of the ice-forced albedo of the polar regions, ) more efficient meridional energy transport by the atmosphere and ocean, and 3) increased atmospheric greenhouse gas concentrations.
 Earth's icy polar regions stabilize its present ‘inequable’ climate through the ice-albedo feedback effect. The polar ice results in permanent atmospheric highs that stabilize Earth's wind systems. In turn the stable winds drive the ocean currents and determine the location of the frontal systems that separate the low- and high-latitude oceanic gyre systems and bound the region where water sinks into the ocean interior as thermocline and intermediate water masses.
 Increased ocean heat transport can assist in making a more equable climate, but unrealistic volume transports would be required to warm the polar regions to Cretaceous levels. The major factor forcing the equable climate of the Cretaceous is now thought to be increased greenhouse gas concentrations, dominated by CO2. The modern rate of change in atmospheric concentration is greater than 200 ppmv per century and increasing. This compares with 1 ppmv per century during the last deglaciation. At current rates of fossil fuel burning, atmospheric CO2 levels will reach Cretaceous levels of 2 times the pre-industrial level about 2070 and 8 times the pre-industrial levels shortly after 2300. It is likely that Cretaceous atmospheric CO2 concentrations will last for many thousands to tens of thousands of years.
 In addition to increased atmospheric greenhouse gas concentrations, a return to climatic conditions resembling those of the Cretaceous would require ice-free poles and large changes in atmospheric and oceanic circulation. Arctic sea-ice is melting much more rapidly than had been expected, and the Arctic Ocean will soon be free of sea-ice in summer. The Greenland ice sheet is melting more rapidly than expected because of greenhouse warming. Surface meltwater forms lakes, and then flows down through crevasses and holes in the ice to lubricate the base, allowing ice streams to flow much more rapidly. The lifetime of the Greenland ice sheet may be only a few hundred years. The West Antarctic ice sheet is inherently unstable, being grounded on rock well below sea level. The ice shelves blocking ice streams off West Antarctica have begun to melt from beneath and break up as the southern ocean warms. The East Antarctic ice sheet has been regarded as highly stable but discovery of lakes beneath the ice and fast-flowing ice streams raises questions about whether the ice sheet will ultimately succumb to global warming and disintegrate. I conclude that a return to climatic conditions resembling those of the mid-Cretaceous is not only possible but also likely unless humanity can organize an effective campaign to stop CO2 emissions to the atmosphere and remove some of the excess CO2 already introduced.

Keywords: Cretaceous; Paleoclimate; Greenhouse gases; Future climate; Atmospheric circulation; Ocean structure』

1. Introduction
2. How did the Cretaceous Earth differ from the modern Earth?
 2.1. How did the idea of a warmer Earth during the Cretaceous arise?
 2.2. How much do we know about the actual temperatures of the Cretaceous?
 2.3. How different were the oceans of the Cretaceous?
 2.4. What is the evidence for ice in the Cretaceous?
 2.5. How high was the Cretaceous sea-level highstand?
 2.6. What else do we know with certainty about the Cretaceous climate?
 2.7. What we think we know about the Cretaceous
3. What was the root cause of the warmer, more equable climate of the Cretaceous?
 3.1. Why the Earth has its present ‘inequable’ climate
 3.2. How could a more equable climate exist?
 3.3. What do we know about the atmospheric and oceanic circulation of the Cretaceous?
 3.4. What do we know about Cretaceous greenhouse gases?
4. What do we know about the rates of past climate change?
5. Would it be possible for the ongoing human perturbation of climate to return Earth to a condition resembling that of the Cretaceous?
 5.1. Greenhouse gas concentrations
 5.2. Polar ice
  5.2.1. Loss of Arctic Ocean sea-ice
  5.2.2. Melting of the Greenland ice sheet
  5.2.3. The Antarctic West Antarctica and the antarctic Peninsula East Antarctica
 5.3. The thermohaline circulation
6. Some things that could go wrong
 6.1. Release of methane from permafrost and clathrates
 6.2. Mass death of ocean plankton
7. Summary and conclusions

Fig. 1. Paleogeographic comparison of the modern and mid-Cretaceous worlds. Note
the much larger number of land areas in the Cretaceous, although the total land area is reduced. The Cretaceous Tethys Seaway provided a low-latitude communication between all of the major ocean basins, and there were several meridional seaways connecting the polar regions with the tropics.

Fig. 7. Some estimates of Cenozoic and Cretaceous atmospheric CO2 levels, based on different proxies and models. Proxy values often represent special extreme variations (e.g. sudden CO2 rise after release of methane from decomposition of clathrates; sharp CO2 decline after burial of organic carbon). Note that estimates based on paleosol carbonates are generally higher than estimates based on other proxies. Many proxies agree on Neogene values, but the spread of estimates increases markedly in the Paleogene and Cretaceous. Most researchers assume that during most of the Cretaceous atmospheric CO2 levels were between 2 and 8 times present. R values on right axis are ‘times pre-industrial.’ Numbers in
parentheses are estimates for the time in the future when these levels will be reached, assuming the entire 5000 Gt reserves of petroleum, natural gas, and coal will be used at the increasing rate determined from past history (1958.2008). Data from Cerling (1991), Ekart et al. (1999), Freeman and Hayes (1992), Haworth et al. (2005), Pagani et al. (1999), Pearson and Palmer (2000), Retallack (2001), Royer et al. (2001b), Berner and Kothavala (2001), Wallmann (2001), and Yapp and Poths (1996).

Fig. 9. Results of Archer and Brovkin (2008) CLIMBERmodel for atmospheric CO2 and global temperature, assuming burning of the 5000 Gtons of fossil fuels in known reserves by 2300. A) Past and future levels of atmospheric CO2 spliced into a historical context back to 24,000 years ago based on the Antarctic Byrd (Blunier et al., 1998) and LawDome (Etheridge et al., 1996) ice cores. B) Past and future globalmean annual surface temperatures spliced into a historical context back to 24,000 years ago based on the Greenland GISP ice cores (Smith et al., 1997). Vertical dotted lines connecting the two diagrams are 2000 CE and some of the importantmilestones in human history indicated. The threemost importantwarming episodes of the deglaciation (Windimere, Bolling, and Allerod Interstadials) are indicated along with the major frigid episodes (Oldest Dryas, Older Dryas, and Younger Dryas).

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