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

Climate sensitivity—the mean global temperature response to a doubling of atmospheric CO2 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 aerosols2. 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 CO2. Here we reconstruct atmospheric carbon dioxide concentrations for the early and middle Pliocene, when temperatures were about 3–4 ◦C warmer than preindustrial values3–5, to estimate Earth-system climate sensitivity from a fully equilibrated state of the planet.We demonstrate that only a relatively small rise in atmospheric CO2 levels was associated with substantial global warming about 4.5 million years ago, and that CO2 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 ∼4–5million years ago (Myr). Mean annual temperatures during the middle Pliocene (∼3.0–3.3Myr) and early Pliocene (4.0–4.2Myr) 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 Niñolike conditions6. Meridional5,7 and vertical ocean temperature gradients8 were reduced, and deep-ocean ventilation enhanced, relative to today9,10. Deterioration in Earth’s climate state from 3.5 to 2.5Myr led to an increase in Northern Hemisphere glaciation11. By ∼2Myr, subtropical Pacific meridional SST gradients resembled modern conditions5, and the Pacific zonal SST gradient (∼5 ◦C) was similar to the gradient observed today, with a strong Walker circulation6. Tectonics and changes in ocean12–14 and atmospheric circulation15,16 were potentially important factors in climate evolution during this time. However, an assessment of the timing of oceanographic and climate changes17, and the stability of the Greenland ice sheet to a range of possible forcings18, implicate atmospheric CO2 as the primary factor driving the warmth of the early Pliocene and the onset ofNorthernHemisphere glaciation. For this study, we evaluate the magnitude of CO2 change and Earth-system climate sensitivity during the Pliocene by using the alkenone–CO2 method to reconstruct Pleistocene–Pliocene pCO2 histories from six ocean localities. Ocean sites used in this study