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...

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An interlaboratory study of Mg/Ca and Sr/Ca ratios in three commercially available carbonate reference materials (BAM RS3, CMSI 1767, and ECRM 752‐1) was performed with the participation of 25 laboratories that determine foraminiferal Mg/Ca ratios worldwide. These reference materials containing Mg/Ca in the range of foraminiferal calcite (0.8 mmol/mol to 6 mmol/mol) were circulated with a dissolution protocol for analysis. Participants were asked to make replicate dissolutions of the powdered samples and to analyze them using the instruments and calibration standards routinely used in their laboratories. Statistical analysis was performed in accordance with the International Standardization Organization standard 5725, which is based on the analysis of variance (ANOVA) technique. Repeatability (RSDr%), an indicator of intralaboratory precision, for Mg/Ca determinations in solutions after centrifuging increased with decreasing Mg/Ca, ranging from 0.78% at Mg/Ca = 5.56 mmol/mol to 1.15% at Mg/Ca = 0.79 mmol/mol. Reproducibility (RSDR%), an indicator of the interlaboratory method precision, for Mg/Ca determinations in centrifuged solutions was noticeably worse than repeatability, ranging from 4.5% at Mg/Ca = 5.56 mmol/mol to 8.7% at Mg/Ca = 0.79 mmol/mol. Results of this study show that interlaboratory variability is dominated by inconsistencies among instrument calibrations and highlight the need to improve interlaboratory compatibility. Additionally, the study confirmed the suitability of these solid standards as reference materials for foraminiferal Mg/Ca (and Sr/Ca) determinations, provided that appropriate procedures are adopted to minimize and to monitor possible contamination from silicate mineral phases.

The western warm pools of the Atlantic and Pacific Oceans are a critical store of heat and power for the tropical climate system, such that accurately reconstructing past tropical sea surface temperatures is essential for understanding global climate history. Current low latitude Pliocene-to-recent climate reconstructions indicate that sea surface temperatures in the tropical warm pools have remained stable since the early Pliocene, despite 3-4 °C of global cooling. This is commonly thought to imply the operation of some sort of thermostatic regulation. An alternative possibility, that we explore here, is that this apparent stability is the result of the inability of certain geochemical proxy methods to accurately resolve sea surface temperatures in the Pliocene warm pool. We use both inorganic-and organic-proxies to reconstruct sea surface temperatures from the South China Sea, Caribbean and Western Equatorial Pacific. This new multi-proxy reconstruction indicates that in contrast to earlier findings, the western Pacific and western Atlantic warm pools during the Pliocene were ~2 °C warmer than today. Consequently, no thermostat mechanism limited the temperature of the warm pools of the Pliocene equatorial ocean. The Western Pacific Warm Pool, comprising the warmest surface waters (>28 °C) of the global oceans, is the main source area of heat and water vapour export to high latitudes 1 (Fig. 1). Similarly, the equatorial Atlantic warm pool, although substantially smaller (Fig. 1), represents another important source of moisture and heat to the Northern Hemisphere 2. Variations in the size and intensity of these warm pool regions, on intra-annual through to geological timescales, influence Walker and Hadley circulations and likely played a major role in the evolution of global climate

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Here we present the first boron isotope‐based pCO2sw (pCO2 of seawater) reconstruction from the eastern Arabian Sea using the planktic foraminifera species Globigerinoides ruber. Our results from sediment core AAS9/21 show that pCO2sw varied between ~160 and 300 µatm during the last 23 kyr. The ΔpCO2, the sea‐air pCO2 difference, is relatively small during the last glacial maximum and becomes more negative toward the Holocene, with the exception of a significant excess during the last deglaciation centered on the Bølling‐Ållerød. Throughout the record, ΔpCO2 is predominantly negative, probably as a result of enhanced biological productivity (and higher nutrient and carbon utilization) during the southwest monsoon. A reduction in ΔpCO2 during the last glacial maximum is therefore consistent with a reduction in the strength of this monsoon system.

[1] Recent work across the Mediterranean Sea has illustrated the salinity and overgrowth effects on planktonic foraminiferal Mg/Ca, which potentially confound the use of this as a temperature proxy for paleoceanographic reconstructions. To test and verify these effects, we present new Aegean Sea results which reveal Mg/Ca values that were unreasonably high to be explained by temperature or salinity variations alone, confirming that foraminiferal Mg/Ca is affected by diagenesis. We have specifically targeted Globigerinoides ruber (w, sensu stricto), from a series of modern core tops spanning a strong sea surface salinity gradient and a minor sea surface temperature range, along a north-south Aegean Sea transect. Scanning Electron Microscopy analyses show that G. ruber specimens were covered by microscale euhedral crystallites of inorganic precipitates. This secondary calcite phase seems to be responsible for the anomalously high Mg/Ca ratios and likely formed near the sediment/water interface from CaCO 3 supersaturated interstitial seawater. We also have clear evidence of diagenetic alteration in a north-south direction along the Aegean Sea, possibly depending on salinity and calcite saturation state gradients. These observations illustrate the necessity of alternative techniques (e.g., flow-through time resolved analysis or laser ablation inductively coupled plasma mass spectrometry) to potentially overcome these diagenetic issues and develop a more reliable and sensitive temperature proxy in similar subtropical settings characterized by high salinity, excessive evaporation, and restricted circulation.