Changing atmospheric CO2 concentration was the primary driver of early Cenozoic climate

Abstract

The Early Eocene Climate Optimum (EECO, which occurred about 51 to 53 million years ago) 1 , was the warmest interval of the past 65 million years, with mean annual surface air temperature over ten degrees Celsius warmer than during the pre-industrial period 2-4 . Subsequent global cooling in the middle and late Eocene epoch, especially at high latitudes, eventually led to continental ice sheet development in Antarctica in the early Oligocene epoch (about 33.6 million years ago). However, existing estimates place atmospheric carbon dioxide (CO 2 ) levels during the Eocene at 500-3,000 parts per million [5][6][7] , and in the absence of tighter constraints carbon-climate interactions over this interval remain uncertain. Here we use recent analytical and methodological developments 8-11 to generate a new high-fidelity record of CO 2 concentrations using the boron isotope (δ 11 Β) composition of well preserved planktonic foraminifera from the Tanzania Drilling Project, revising previous estimates 6 . Although species-level uncertainties make absolute values difficult to constrain, CO 2 concentrations during the EECO were around 1,400 parts per million. The relative decline in DOI: 10.1038/nature17423 Page 2 of 38 CO 2 concentration through the Eocene is more robustly constrained at about fifty per cent, with a further decline into the Oligocene 12 . Provided the latitudinal dependency of sea surface temperature change for a given climate forcing in the Eocene was similar to that of the late Quaternary period 13 , this CO 2 decline was sufficient to drive the well documented high-and low-latitude cooling that occurred through the Eocene 14 . Once the change in global temperature between the pre-industrial period and the Eocene caused by the action of all known slow feedbacks (apart from those associated with the carbon cycle) is removed 2-4 , both the EECO and the late Eocene exhibit an equilibrium climate sensitivity relative to the pre-industrial period of 2.1 to 4.6 degrees Celsius per CO 2 doubling (66 per cent confidence), which is similar to the canonical range (1.5 to 4.5 degrees Celsius 15 ), indicating that a large fraction of the warmth of the early Eocene greenhouse was driven by increased CO 2 concentrations, and that climate sensitivity was relatively constant throughout this period.Over the past 540 million years, Earth's climate has oscillated between a globally warm 'greenhouse state' and an 'icehouse state' with substantial continental glaciation 16 . The most recent of these transitions occurred between the warmest time interval of the last 65 million years-the EECO (about 14 ± 3 °C warmer than preindustrial times 2 )-and the rapid growth of ice on Antarctica in the earliest icehouse state of the Oligocene (~33.6 Myr ago 1 ). It has been suggested that variations in the concentration of the greenhouse gas CO 2 were responsible for both the overall warmth of the Eocene and the subsequent cooling 17 . Recent studies have documented the importance of CO 2 decline for the final step into the icehous...

Equatorial heat accumulation as a long-term trigger of permanent Antarctic ice sheets during the Cenozoic

“…S5A for an edited version and the scale bar). (14,16,28,29,35,36). For reference, choosing a pH of 7.9 (37) would offset our curve by a factor of approximately +400 ppm without altering the trend (Fig.…”

Climate sensitivity and meridional overturning circulation in the late Eocene using GFDL CM2.1

“…This includes the gradual opening of the Tasman Gateway and Drake Passage, and the widening of the Atlantic. 5 Some recent climate model studies have used reconstructions of late Eocene and early Oligocene topography for their control runs (Kennedy et al, 2015;Ladant et al, 2014;Lefebvre et al, 2012;Lunt et al, 2016;Zhang et al, 2010). Kennedy et al (2015) found that when an ice sheet is placed over Antarctica, the Pacific sector of the Southern Ocean surface warms significantly when the Tasman 10 Seaway is constricted as in the late Eocene, whereas when the Tasman Seaway is wider as in the early Oligocene, there is instead a weak cooling.…”

“…A long-term decline in atmospheric CO 2 has been found to be a major driver of Eocene cooling (Anagnostou et al, 2016), and a plausible trigger of the EOT glaciation (DeConto and Pollard, 2003).…”

Boron Isotopes in the Ocean Floor Realm and the Mantle

“…However, several studies have used marine carbonate to reconstruct seawater d 11 B for various stages during the late Neoproterozoic and the Phanerozoic ( Fig. 8.7; Joachimski et al 2005;Kasemann et al 2010;Foster et al 2012;Clarkson et al 2015;Anagnostou et al 2016).…”

“…The stippled line is interpolated for periods where no model exists. Reconstruction of seawater d 11 B based on data from marine carbonate are marked in red boxes (Joachimski et al 2005;Kasemann et al 2010;Foster et al 2012;Clarkson et al 2015;Anagnostou et al 2016). Minimum and maximum seawater values for Eocene estimates are marked by upward-and downward-pointing triangles, respectively.…”