Reconstructing past changes in global mean surface temperature (GMST) is one of the key contributions that palaeoclimate science can make in addressing societally relevant questions and is required to determine equilibrium climate sensitivity (ECS). Previous work has suggested that the temperature of the deep ocean (Td) can be used to determine GMST with a simple Td-GMST scaling factor of 1 prior to the Pliocene. However, this metric lacks a robust mechanistic basis, and indeed, such a relationship is intuitively difficult to envisage given that polar amplification is a ubiquitous feature of past warm climate states and deep water overwhelmingly forms at high latitudes. Here, we interrogate whether and crucially, why, this relationship exists using a suite of curated data compilations generated for key deep-time climate intervals as well as two independent sets of palaeoclimate model simulations. We show that models and data are in full agreement that a 1:1 relationship is a good approximation. Mechanistically, both sets of climate models suggest that i) increasingly seasonally biased deep water formation, and ii) a faster rate of land versus ocean surface warming are the two processes that act to counterbalance a possible polar amplification-derived bias on Td-derived GMST. Using this knowledge, we interrogate the quality of the existing deep ocean temperature datasets and provide a new Cenozoic record of GMST. Our estimates are substantially warmer than similar previous efforts for much of the Paleogene and are thus consistent with a substantially higher-than-modern ECS during deep-time high CO2 climate states.

Knowledge of how the Antarctic Ice Sheet (AIS) has varied in response to past climates can inform the prediction of future AIS behaviour. Water stable isotope records from Antarctic ice cores traditionally provide information on past temperature changes. However, these reconstructions neglect changes in atmospheric circulation, which can be induced by elevation changes. Here, we simulate an ensemble of idealised AIS elevation change scenarios using the isotope-enabled HadCM3 climate model during the Last Interglacial period (LIG). Our ensemble is used to investigate the isotope-elevation relationship. Changing AIS elevations linearly modify the response in surface air temperature, as precipitation and $\delta^{18}$O. Especially, we observe $\delta^{18}$O decrease with the AIS elevation, with higher slopes on the coast compared to the plateau, reflecting different processes. We note that the effect of sea-ice induced by AIS changes is small. These results help to isolate the effect of AIS changes on the LIG $\delta^{18}$O signals.

During the early to mid-Holocene vegetation expanded to cover much of the present-day Sahara. Although driven by a well-understood difference in the orbital configuration, general circulation models have generally failed to simulate the required rainfall increase. One possible explanation is the presence of systematic biases in the representations of atmospheric convection which might also impact future projections. We employ a Bayesian method to learn from an ensemble of present day and mid-Holocene simulations that vary parameters in the convection, boundary layer and cloud schemes. The model can reproduce the ‘Green Sahara’ rainfall if mixing between convective plumes and the environment is increased in the upper troposphere relative to lower down. This does not appreciably impact the present day simulation, meaning that the palaeoclimate reconstructions are able to narrow constraints on suitable parameter ranges. This suggests that other uncertain components of climate models could be targeted in this way.

Our limited understanding of millennial-scale variability in the context of the last glacial period can be explained by the lack of a reliable modelling framework to study abrupt climate changes under realistic glacial backgrounds. In this article, we describe a new set of long-run Last Glacial Maximum experiments where such climate shifts were triggered by different snapshots of ice-sheet meltwater derived from the early stages of the last deglaciation. Depending on the location and the magnitude of the forcing, we observe three distinct dynamical regimes and highlight a subtle window of opportunity where the climate can sustain oscillations between cold and warm modes. We identify the European-Arctic and Nordic Seas regions as being most sensitive to meltwater discharge in the context of switching to a cold mode, compared to freshwater fluxes from the Laurentide ice sheets. These cold climates follow a consistent pattern in temperature, sea ice and convection, and are largely independent from freshwater release as a result of effective AMOC collapse. Warm modes, on the other hand, show more complexity in their response to the regional pattern of the meltwater input, and within them, we observe significant differences linked to the reorganisation of deep water formation sites and the subpolar gyre. Broadly, the main characteristics of the oscillations, obtained under full-glacial conditions with realistically low meltwater discharge, are comparable to δ18O records of the last glacial period, although our simplified experiment design prevents detailed conclusions from being drawn on whether these represent actual Dansgaard-Oeschger events.

North Pacific atmospheric and oceanic circulations are key missing pieces in our understanding of the reorganisation of the global climate system since the Last Glacial Maximum (LGM). Here, using a basin-wide compilation of planktic foraminiferal δ18O, we show that the North Pacific subpolar gyre extended ~3 degrees further south during the LGM, consistent with sea surface temperature and productivity proxy data. Analysis of an ensemble of climate models indicates that the expansion of the subpolar gyre was associated with a substantial gyre strengthening. These gyre circulation changes were driven by a southward shift in the mid-latitude westerlies and increased wind-stress from the polar easterlies. Using single-forcing model runs, we show these atmospheric circulation changes are a non-linear response to the combined topographic and albedo effects of the Laurentide Ice Sheet. Our reconstruction suggests the gyre boundary (and thus westerly winds) began to migrate northward at ~17-16 ka, during Heinrich Stadial 1.