High Earth-system climate sensitivity determined from Pliocene carbon dioxide concentrations

Abstract

Climate sensitivity-the mean global temperature response to a doubling of atmospheric CO 2 concentrations through radiative forcing and associated feedbacks-is estimated at 1.5-4.5• C (ref. 1). However, this value incorporates only relatively rapid feedbacks such as changes in atmospheric water vapour concentrations, and the distributions of sea ice, clouds and aerosols 2 . Earth-system climate sensitivity, by contrast, additionally includes the effects of long-term feedbacks such as changes in continental ice-sheet extent, terrestrial ecosystems and the production of greenhouse gases other than CO 2 . Here we reconstruct atmospheric carbon dioxide concentrations for the early and middle Pliocene, when temperatures were about 3-4• C warmer than preindustrial values [3][4][5] , to estimate Earth-system climate sensitivity from a fully equilibrated state of the planet. We demonstrate that only a relatively small rise in atmospheric CO 2 levels was associated with substantial global warming about 4.5 million years ago, and that CO 2 levels at peak temperatures were between about 365 and 415 ppm. We conclude that the Earth-system climate sensitivity has been significantly higher over the past five million years than estimated from fast feedbacks alone.The magnitude of Earth-system climate sensitivity can be assessed by evaluating warm time intervals in Earth history, such as the peak warming of the early Pliocene ∼4-5 million years ago (Myr). Mean annual temperatures during the middle Pliocene (∼3.0-3.3 Myr) and early Pliocene (4.0-4.2 Myr) were ∼2.5• C (refs 3, 4), and 4• C (ref. 5) warmer than preindustrial conditions, respectively. During the early Pliocene, the equatorial Pacific Ocean maintained an east-west sea surface temperature (SST) gradient of only ∼1.5• C, which arguably resembles permanent El Niño-like conditions 6 . Meridional 5,7 and vertical ocean temperature gradients 8 were reduced, and deep-ocean ventilation enhanced, relative to today 9,10 . Deterioration in Earth's climate state from 3.5 to 2.5 Myr led to an increase in Northern Hemisphere glaciation 11 . By ∼2 Myr, subtropical Pacific meridional SST gradients resembled modern conditions 5 , and the Pacific zonal SST gradient (∼5• C) was similar to the gradient observed today, with a strong Walker circulation 6 . Tectonics and changes in ocean [12][13][14] and atmospheric circulation 15,16 were potentially important factors in climate evolution during this time. However, an assessment of the timing of oceanographic and climate changes 17 , and the stability of the Greenland ice sheet to a range of possible forcings 18 , implicate atmospheric CO 2 as the primary factor driving the warmth of the early Pliocene and the onset of Northern Hemisphere glaciation.For this study, we evaluate the magnitude of CO 2 change and Earth-system climate sensitivity during the Pliocene by using the alkenone-CO 2 method to reconstruct Pleistocene-Pliocene pCO 2 histories from six ocean localities. Ocean sites used in this study Alkenones are long-chained (C 37 -C ...

Carbon Footprint of Transitional Shelters

“…The amount of CO 2 in the atmosphere has increased rapidly during the industrial era. The content of CO 2 in the atmosphere is now higher than ever during the existence of human kind on earth (Pagani et al 2010). In only a couple of centuries, the atmospheric CO 2 has risen from 300 ppm to around 400 ppm (NOAA 2014).…”

Late Pliocene changes in the North Atlantic Current

“…Atmospheric pCO 2 was approximately 100 ppm higher than pre-industrial levels (Pagani et al, 2010) and ice sheets in the Northern Hemisphere were relatively small. The general surface current system was similar to the present one ), but sea surface temperatures (SSTs) in the North Atlantic Ocean were up to 10°C warmer compared to the present as an intense North Atlantic Current (NAC) led to a reduced meridional SST gradient (e.g., Cronin, 1991;Dowsett et al, 1992;Robinson, 2009).…”

Climate and environment of a Pliocene warm world

“…Pagani et al, 2010;Seki et al, 2010). Past CO 2 -concentrations close to or even higher than modern values along with a near-modern palaeogeography, ocean bathymetry and palaeobiology, suggest that the warm Pliocene climates may provide plausible comparative scenarios for interpreting the path of future climate warming during the 21 st century (e.g.…”