During the warm early Pliocene (approximately 4.5 to 3.0 million years ago), the most recent interval with a climate warmer than today, the eastern Pacific thermocline was deep and the average west-to-east sea surface temperature difference across the equatorial Pacific was only 1.5 +/- 0.9 degrees C, much like it is during a modern El Niño event. Thus, the modern strong sea surface temperature gradient across the equatorial Pacific is not a stable and permanent feature. Sustained El Niño-like conditions, including relatively weak zonal atmospheric (Walker) circulation, could be a consequence of, and play an important role in determining, global warmth.

The Earth's climate has undergone a global transition over the past four million years, from warm conditions with global surface temperatures about 3 degrees C warmer than today, smaller ice sheets and higher sea levels to the current cooler conditions. Tectonic changes and their influence on ocean heat transport have been suggested as forcing factors for that transition, including the onset of significant Northern Hemisphere glaciation approximately 2.75 million years ago, but the ultimate causes for the climatic changes are still under debate. Here we compare climate records from high latitudes, subtropical regions and the tropics, indicating that the onset of large glacial/interglacial cycles did not coincide with a specific climate reorganization event at lower latitudes. The regional differences in the timing of cooling imply that global cooling was a gradual process, rather than the response to a single threshold or episodic event as previously suggested. We also find that high-latitude climate sensitivity to variations in solar heating increased gradually, culminating after cool tropical and subtropical upwelling conditions were established two million years ago. Our results suggest that mean low-latitude climate conditions can significantly influence global climate feedbacks.

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 ...

About five to four million years ago, in the early Pliocene epoch, Earth had a warm, temperate climate. The gradual cooling that followed led to the establishment of modern temperature patterns, possibly in response to a decrease in atmospheric CO2 concentration, of the order of 100 parts per million, towards preindustrial values. Here we synthesize the available geochemical proxy records of sea surface temperature and show that, compared with that of today, the early Pliocene climate had substantially lower meridional and zonal temperature gradients but similar maximum ocean temperatures. Using an Earth system model, we show that none of the mechanisms currently proposed to explain Pliocene warmth can simultaneously reproduce all three crucial features. We suggest that a combination of several dynamical feedbacks underestimated in the models at present, such as those related to ocean mixing and cloud albedo, may have been responsible for these climate conditions.

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