We constructed an 800,000-year synthetic record of Greenland climate variability based on the thermal bipolar seesaw model. Our Greenland analog reproduces much of the variability seen in the Greenland ice cores over the past 100,000 years. The synthetic record shows strong similarity with the absolutely dated speleothem record from China, allowing us to place ice core records within an absolute timeframe for the past 400,000 years. Hence, it provides both a stratigraphic reference and a conceptual basis for assessing the long-term evolution of millennial-scale variability and its potential role in climate change at longer time scales. Indeed, we provide evidence for a ubiquitous association between bipolar seesaw oscillations and glacial terminations throughout the Middle to Late Pleistocene.

The asynchronous relationship between millennial-scale temperature changes over Greenland and Antarctica during the last glacial period has led to the notion of a bipolar seesaw which acts to redistribute heat depending on the state of meridional overturning circulation within the Atlantic Ocean. Here we present new records from the South Atlantic that show rapid changes during the last deglaciation that were instantaneous (within dating uncertainty) and of opposite sign to those observed in the North Atlantic. Our results demonstrate a direct link between the abrupt changes associated with variations in the Atlantic meridional overturning circulation and the more gradual adjustments characteristic of the Southern Ocean. These results emphasize the importance of the Southern Ocean for the development and transmission of millennial-scale climate variability and highlight its role in deglacial climate change and the associated rise in atmospheric carbon dioxide.

a These authors contributed equally to this work. 14 Theory and climate modelling suggest that the sensitivity of Earth's climate to changes in radiative 15forcing could depend on background climate. However, palaeoclimate data have thus far been 16 insufficient to provide a conclusive test of this prediction. Here we present new atmospheric CO 2 17 reconstructions based on multi-site boron-isotope records through the late Pliocene (3.3 to 2.3 18 Myr ago). We find that Earth's climate sensitivity to CO 2 -based radiative forcing (Earth System 19 Sensitivity) was half as strong during the warm Pliocene as during the cold late Pleistocene (0.8 to 20 0 Myr ago). We attribute this difference to the radiative impacts of continental ice-volume 21 changes (ice-albedo feedback) during the late Pleistocene, because equilibrium climate sensitivity 22 is identical for the two intervals when we account for such impacts using sea-level reconstructions. 23 We conclude that, on a global scale, no unexpected climate feedbacks operated during the warm 24Pliocene, and that predictions of equilibrium climate sensitivity (excluding long-term ice-albedo 25 feedbacks) for our Pliocene-like future (with CO 2 levels up to maximum Pliocene levels of 450 26 ppm) are well described by the currently accepted range of 1.5 to 4.5 K per CO 2 doubling. 27Since the start of the industrial revolution, the concentration of atmospheric CO 2 (and other 28 greenhouse gases; GHGs) has increased dramatically (from ~280 to ~400 ppm) 1 . It has been known 29 for over 100 years that changes in GHG concentration will cause the surface temperature of the 30 Earth to vary 2 . A wide range of observations reveals that the sensitivity of Earth's surface 31 temperature to radiative forcing amounts to ~3 K warming per doubling of atmospheric CO 2 32 concentration (with a 66% confidence range of 1.5 to 4.5 K; e.g. ref. 1,3), due to direct radiative 33 forcing by CO 2 plus the action of a number of fast-acting positive feedback mechanisms, mainly 34 related to atmospheric water vapour content and sea-ice and cloud albedo. Uncertainty in the 35 magnitude of these feedbacks confounds our ability to determine the exact equilibrium climate 36 sensitivity (ECS; the equilibrium global temperature change for a doubling of CO 2 on timescales of 37 about a century, when all 'fast' feedbacks have had time to operate; see ref. 3 for more detail). 38Although the likely range of values for ECS is 1.5 to 4.5 K per CO 2 doubling, there is a small but finite 39 possibility that climate sensitivity may exceed 5 K (e.g. ref. 1). Understanding the likely value of ECS 40 clearly has important implications for the magnitude, eventual impact and potential mitigation of 41 future climate change. 42Any long-range forecast of global temperature (i.e. beyond the next 100 years) must also consider 43 the possibility that ECS could depend on the background state of the climate 4,5 . That is, in a warmer 44 world, some feedbacks that determine ECS could become more efficient and/or new feed...

SignificanceConflicting sets of hypotheses highlight either the role of ice sheets or atmospheric carbon dioxide (CO2) in causing the increase in duration and severity of ice age cycles ∼1 Mya during the Mid-Pleistocene Transition (MPT). We document early MPT CO2 cycles that were smaller than during recent ice age cycles. Using model simulations, we attribute this to post-MPT increase in glacial-stage dustiness and its effect on Southern Ocean productivity. Detailed analysis reveals the importance of CO2 climate forcing as a powerful positive feedback that magnified MPT climate change originally triggered by a change in ice sheet dynamics. These findings offer insights into the close coupling of climate, oceans, and ice sheets within the Earth System.

Abrupt climate change is a ubiquitous feature of the Late Pleistocene epoch. In particular, the sequence of Dansgaard-Oeschger events (repeated transitions between warm interstadial and cold stadial conditions), as recorded by ice cores in Greenland, are thought to be linked to changes in the mode of overturning circulation in the Atlantic Ocean. Moreover, the observed correspondence between North Atlantic cold events and increased iceberg calving and dispersal from ice sheets surrounding the North Atlantic has inspired many ocean and climate modelling studies that make use of freshwater forcing scenarios to simulate abrupt change across the North Atlantic region and beyond. On the other hand, previous studies identified an apparent lag between North Atlantic cooling events and the appearance of ice-rafted debris over the last glacial cycle, leading to the hypothesis that iceberg discharge may be a consequence of stadial conditions rather than the cause. Here we further establish this relationship and demonstrate a systematic delay between pronounced surface cooling and the arrival of ice-rafted debris at a site southwest of Iceland over the past four glacial cycles, implying that in general icebergs arrived too late to have triggered cooling. Instead we suggest that--on the basis of our comparisons of ice-rafted debris and polar planktonic foraminifera--abrupt transitions to stadial conditions should be considered as a nonlinear response to more gradual cooling across the North Atlantic. Although the freshwater derived from melting icebergs may provide a positive feedback for enhancing and or prolonging stadial conditions, it does not trigger northern stadial events.