1 Nature 2008 Vol: 451(7176):279-283. DOI: 10.1038/nature06588

An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics

Past episodes of greenhouse warming provide insight into the coupling of climate and the carbon cycle and thus may help to predict the consequences of unabated carbon emissions in the future.

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References
  1. Caldeira, K. & Wicket, M. E. Anthropogenic carbon and ocean pH. Nature 425, 365-365 , (2003) .
    • . . . By the year 2400, it is predicted that humans will have released about 5,000 gigatonnes of carbon (Gt C) to the atmosphere since the start of the industrial revolution if fossil-fuel emissions continue unabated and carbon-sequestration efforts remain at current levels1 . . .
    • . . . A greater portion entering the ocean would decrease the atmospheric burden but with a consequence: significantly lower pH and carbonate ion concentrations of ocean surface layers1 (Fig. 1). . . .
    • . . . As the small surface reservoir takes up CO2, its pH decreases1, slowing the additional absorption of CO2 . . .
  2. Archer, D. Fate of fossil fuel CO2 in geologic time. J. Geophys. Res. Oceans 110, C09S05, doi:10.1029/2004JC002625 , (2005) .
    • . . . This anthropogenic carbon input, predominantly carbon dioxide (CO2), would eventually return to the geosphere through the deposition of calcium carbonate and organic matter2 . . .
    • . . . Projected changes in deep-ocean temperature in d assume a homogeneous warming of the ocean with a time lag of 1,000 years relative to atmospheric CO2 (ref. 2) and the following temperature sensitivities to a doubling of CO2 concentration: short-dashed line, 4.5 °C; solid line, 3.0 °C; long-dashed line, 1.5 °C. . . .
  3. Friedlingstein, P. et al. Climate-carbon cycle feedback analysis: Results from the (CMIP)-M-4 model intercomparison. J. Clim. 19, 3337-3353 , (2006) .
    • . . . Of particular concern are potential positive feedbacks that could amplify increases in the concentrations of greenhouse gases — water, CO2, methane and nitrous oxide (N2O) — effectively escalating climate sensitivity to initial anthropogenic carbon input3 . . .
    • . . . Warming and freshening of high-latitude surface water can slow the rate of convective overturning, and increased thermal stratification makes it more difficult for wind-driven mixing to return nutrients from the deep ocean to organisms in the photic zone (the upper 200 m or so of the water column, which is penetrated by sunlight, thereby allowing organisms to photosynthesize)3 . . .
  4. Doney, S. C. & Schimel, D. S. Carbon and climate system coupling on timescales from the Precambrian to the Anthropocene. Annu. Rev. Environ. Resources 32, 14.1-14.36 , (2007) .
    • . . . To evaluate climate theories more thoroughly, particularly with regard to feedbacks and climate sensitivity to pCO2, it is desirable to study samples obtained when CO2 concentrations were high (approaching or exceeding 1,800 p.p.m.v.) and to make observations for intervals longer than those of ocean overturning and carbon cycling (more than 1,000 years)4 . . .
  5. Royer, D. L. CO2-forced climate thresholds during the Phanerozoic. Geochim. Cosmochim. Acta 70, 5665-5675 , (2006) .
    • . . . In contrast to the present day, much of the early Cenozoic was characterized by noticeably higher concentrations of greenhouse gases, as well as a much warmer mean global temperature and poles with little or no ice5, 6 (Fig. 2) . . .
    • . . . Data are a compilation of marine (see ref. 5 for original sources) and lacustrine24 proxy records . . .
  6. Zachos, J., Pagani, M., Sloan, L., Thomas, E. & Billups, K. Trends, rhythms, and aberrations in global climate 65 Ma to present. Science 292, 686-693 , (2001) .
    • . . . In contrast to the present day, much of the early Cenozoic was characterized by noticeably higher concentrations of greenhouse gases, as well as a much warmer mean global temperature and poles with little or no ice5, 6 (Fig. 2) . . .
    • . . . The climate curve is a stacked deep-sea benthic foraminiferal oxygen-isotope curve based on records from Deep Sea Drilling Project and Ocean Drilling Program sites6, updated with high-resolution records for the interval spanning the middle Eocene to the middle Miocene25, 26, 27 . . .
    • . . . During the most prominent and best-studied hyperthermal, the Palaeocene–Eocene Thermal Maximum (PETM; about 55 million years ago), the global temperature increased by more than 5 °C in less than 10,000 years6 (Fig. 3) . . .
    • . . . The carbon isotope (a) and oxygen isotope (b) records are based on benthic foraminiferal records (see ref. 6 for original sources), and the CaCO3 records (c) are from drill holes in the South Atlantic8 . . .
  7. Walker, J. C. G., Hays, P. B. & Kasting, J. F. A negative feedback mechanism for the long-term stabilization of Earth's surface-temperature. J. Geophys. Res. Oceans Atmos. 86, 9776-9782 , (1981) .
    • . . . Changes in chemical weathering of silicate rocks were also important7 . . .
    • . . . Whereas other processes (such as the oxidation and burial of organic carbon) change CO2 concentrations, the negative weathering feedback loop maintains Earth's climate within a habitable range over millions of years and longer7. . . .
  8. Zachos, J. C. et al. Rapid acidification of the ocean during the Paleocene-Eocene Thermal Maximum. Science 308, 1611-1615 , (2005) .
    • . . . The carbon isotope (a) and oxygen isotope (b) records are based on benthic foraminiferal records (see ref. 6 for original sources), and the CaCO3 records (c) are from drill holes in the South Atlantic8 . . .
    • . . . This includes a rapid and pronounced decrease in the 13C/12C ratio of carbonate and organic carbon across the globe (that is, a negative carbon isotope excursion) and a prominent drop in the carbonate content of marine sediment deposited at several thousands of metres water depth (that is, a deep-sea dissolution horizon)8 . . .
    • . . . This is followed by an increase in carbonate accumulation at many locations, presumably reflecting a recovery of carbonate ion concentration8 . . .
  9. Lourens, L. J. et al. Astronomical pacing of late Palaeocene to early Eocene global warming events. Nature 435, 1083-1087 , (2005) .
    • . . . Several other early Eocene hyperthermals have been documented recently9, including the Eocene Thermal Maximum 2 (Fig. 2) . . .
  10. Svensen, H. et al. Release of methane from a volcanic basin as a mechanism for initial Eocene global warming. Nature 429, 524-527 , (2004) .
    • . . . Carbon might have come from deeply buried rocks, perhaps liberated as methane and CO2 during intrusive volcanism10 . . .
  11. Dickens, G. R. Rethinking the global carbon cycle with a large, dynamic and microbially mediated gas hydrate capacitor. Earth Planet. Sci. Lett. 213, 169-183 , (2003) .
    • . . . For example, a rise in deep-sea temperature might have triggered the decomposition of gas hydrates on continental margins, releasing substantial amounts of methane and fuelling additional warming11 . . .
    • . . . The positive feedbacks of greatest concern for understanding overall global warming may be those that could release hundreds to thousands of gigatonnes of carbon after initial warming11, 12, 13 . . .
    • . . . However, the amount of methane that could be liberated is enormous, and after gas hydrate dissociation was initiated, the flux might proceed rapidly as overpressured pore waters triggered fluid expulsion or sediment slides on the sea floor11. . . .
  12. Kurtz, A. C., Kump, L. R., Arthur, M. A., Zachos, J. C. & Paytan, A. Early Cenozoic decoupling of the global carbon and sulfur cycles. Paleoceanography 18, 1090, doi:10.1029/2003PA000908 , (2003) .
    • . . . Another such source is the oxidation of organic matter in terrestrial environments12, 13 . . .
    • . . . The positive feedbacks of greatest concern for understanding overall global warming may be those that could release hundreds to thousands of gigatonnes of carbon after initial warming11, 12, 13 . . .
  13. Higgins, J. A. & Schrag, D. P. Beyond methane: Towards a theory for the Paleocene-Eocene Thermal Maximum. Earth Planet. Sci. Lett. 245, 523-537 , (2006) .
    • . . . Another such source is the oxidation of organic matter in terrestrial environments12, 13 . . .
    • . . . The positive feedbacks of greatest concern for understanding overall global warming may be those that could release hundreds to thousands of gigatonnes of carbon after initial warming11, 12, 13 . . .
  14. Wing, S. L., Gingerich, P. D., Schmitz, B. & Thomas, E. (eds). Causes and Consequences of Globally Warm Climates in the Early Paleocene (Geol. Soc. Am. Spec. Pap. 369, Boulder, Colorado, 2003) , .
    • . . . Already, recent studies of the PETM seem to validate some forecasts about future first-order changes in climate: extreme ocean warming of more than 5 °C extended to the North Pole; shifts in regional precipitation occurred, resulting in greater discharge from rivers at high latitudes and freshening of surface waters in the Arctic Ocean; and global ecosystems changed markedly, with major latitudinal and intercontinental migrations in terrestrial plants and mammals and with the sudden appearance of 'exotic' phytoplankton and zooplankton in open and coastal ocean environments (see refs 14 and 15 for reviews). . . .
  15. Sluijs, A., Bowen, G. J., Brinkhuis, H., Lourens, L. J. & Thomas, E. in Deep-Time Perspectives on Climate Change: Marrying the Signal from Computer Models and Biological Proxies (eds Williams, M. et al.) 323-349 (Geological Society of London, London, 2007) , .
    • . . . Already, recent studies of the PETM seem to validate some forecasts about future first-order changes in climate: extreme ocean warming of more than 5 °C extended to the North Pole; shifts in regional precipitation occurred, resulting in greater discharge from rivers at high latitudes and freshening of surface waters in the Arctic Ocean; and global ecosystems changed markedly, with major latitudinal and intercontinental migrations in terrestrial plants and mammals and with the sudden appearance of 'exotic' phytoplankton and zooplankton in open and coastal ocean environments (see refs 14 and 15 for reviews). . . .
    • . . . One feature common to all greenhouse periods, whether transient or long-lived, is exceptionally warm poles15 . . .
  16. Thomas, D. J., Zachos, J. C., Bralower, T. J., Thomas, E. & Bohaty, S. Warming the fuel for the fire: Evidence for the thermal dissociation of methane hydrate during the Paleocene-Eocene Thermal Maximum. Geology 30, 1067-1070 , (2002) .
    • . . . Is there any evidence of similar transient responses during past episodes of abrupt warming? High-resolution single-shell foraminiferal isotope records from the PETM suggest a delay of several thousand years in the propagation of the carbon isotope excursion from the surface ocean to the deep sea16, a pattern that could reflect a transient slowing of overturning circulation . . .
  17. Thomas, E. & Shackleton, N. J. in Correlation of the Early Paleogene in Northwest Europe (eds Knox, R. W. O. B., Corfield, R. M. & Dunay, R. E.) 401-441 (Geol. Soc. Lond. Spec. Publ. 101, London, 1996) , .
    • . . . Moreover, the PETM coincided with a major extinction of benthic foraminiferans, with widespread oxygen deficiency in the ocean as a possible cause17. . . .
  18. Pancost, R. D. et al. Increased terrestrial methane cycling at the Palaeocene-Eocene Thermal Maximum. Nature 449, 332-335 , (2007) .
    • . . . By contrast, regions that once were dry might emit methane as they become wetter18 . . .
    • . . . Records of geochemical or physical fingerprints, such as hopanoids from methanotrophs or charcoal from wildfires, would help18 . . .
  19. Zeebe, R. E. & Zachos, J. C. Reversed deep-sea carbonate ion basin gradient during Paleocene-Eocene Thermal Maximum. Paleoceanography 22, PA3201, doi:10.1029/2006PA001395 , (2007) .
    • . . . Constraining the rate and mass of carbon released, for example by quantifying changes in ocean carbonate chemistry, is also essential for identifying sources19. . . .
  20. Sloan, L. C. & Pollard, D. Polar stratospheric clouds: A high latitude warming mechanism in an ancient greenhouse world. Geophys. Res. Lett. 25, 3517-3520 , (1998) .
    • . . . In the more extreme cases, the EECO and PETM, high-latitude temperatures were substantially higher than can be simulated by models without unreasonably high pCO2 (refs 20, 21) . . .
    • . . . In contrast, polar stratospheric clouds, which might have been more extensive during the greenhouse intervals because of higher concentrations of methane in the atmosphere, seem to be very effective at trapping heat20 . . .
  21. Beerling, D. J., Hewitt, C. N., Pyle, J. A. & Raven, J. A. Critical issues in trace gas biogeochemistry and global change. Phil. Trans. R. Soc. A 365, 1629-1642 , (2007) .
    • . . . In the more extreme cases, the EECO and PETM, high-latitude temperatures were substantially higher than can be simulated by models without unreasonably high pCO2 (refs 20, 21) . . .
    • . . . Recent theoretical and experimental studies indicate that, under high pCO2, background concentrations of trace gases such as methane and N2O should be higher because of greater production under warmer and wetter conditions (that is, more extensive wetlands) and because of lower rates of oxidation in the atmosphere (resulting from lower emissions of volatile organic compounds by plants)21 . . .
  22. Huber, M. & Sloan, L. C. Heat transport, deep waters, and thermal gradients: Coupled simulation of an Eocene greenhouse climate. Geophys. Res. Lett. 28, 3481-3484 , (2001) .
    • . . . Modified ocean heat transport has been investigated and found to be incapable of transporting heat fast enough to compensate for polar heat loss22 . . .
  23. Schmitz, B. & Pujalte, V. Abrupt increase in seasonal extreme precipitation at the Paleocene-Eocene boundary. Geology 35, 215-218 , (2007) .
    • . . . For example, recent studies show that some regions, such as the western interior of North America, became drier at the onset of the PETM, whereas other regions, such as western Europe, experienced increased extreme precipitation events and massive flooding23 . . .
  24. Lowenstein, T. K. & Demicco, R. V. Elevated Eocene atmospheric CO2 and its subsequent decline. Science 313, 1928-1928 , (2006) .
    • . . . Data are a compilation of marine (see ref. 5 for original sources) and lacustrine24 proxy records . . .
    • . . . The dashed horizontal line represents the maximum pCO2 for the Neogene (Miocene to present) and the minimum pCO2 for the early Eocene (1,125 p.p.m.v.), as constrained by calculations of equilibrium with Na–CO3 mineral phases (vertical bars, where the length of the bars indicates the range of pCO2 over which the mineral phases are stable) that are found in Neogene and early Eocene lacustrine deposits24 . . .
  25. Billups, K., Channell, J. E. T. & Zachos, J. Late Oligocene to early Miocene geochronology and paleoceanography from the subantarctic South Atlantic. Paleoceanography 17, U39-U49 , (2002) .
    • . . . The climate curve is a stacked deep-sea benthic foraminiferal oxygen-isotope curve based on records from Deep Sea Drilling Project and Ocean Drilling Program sites6, updated with high-resolution records for the interval spanning the middle Eocene to the middle Miocene25, 26, 27 . . .
  26. Bohaty, S. M. & Zachos, J. C. Significant Southern Ocean warming event in the late middle Eocene. Geology 31, 1017-1020 , (2003) .
    • . . . The climate curve is a stacked deep-sea benthic foraminiferal oxygen-isotope curve based on records from Deep Sea Drilling Project and Ocean Drilling Program sites6, updated with high-resolution records for the interval spanning the middle Eocene to the middle Miocene25, 26, 27 . . .
  27. Palike, H. et al. The heartbeat of the Oligocene climate system. Science 314, 1894-1898 , (2006) .
    • . . . The climate curve is a stacked deep-sea benthic foraminiferal oxygen-isotope curve based on records from Deep Sea Drilling Project and Ocean Drilling Program sites6, updated with high-resolution records for the interval spanning the middle Eocene to the middle Miocene25, 26, 27 . . .
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