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 ...
