Knowledge of our future climate rests almost entirely on the accuracy of complex numerical models of the Earth system. These models are rooted in our understanding of the current climate state and therefore require evaluation when simulating the substantially warmer climates expected in the future. One effective way of achieving this is to use the climate of the geological past, which represents actual examples of the Earth system operating in warm climate states. The early Eocene climatic optimum (~50 million years ago) is the warmest time interval of at least the last 65 million years, but previous attempts to harness this interval to test Earth system models have been limited by a lack of reliable information regarding temperature, atmospheric CO2 concentration (and other boundary conditions) and the inability of previous-generation models to adequately simulate key components of the climate system.

Recent step-changes in Earth system models, coupled with exciting and emerging techniques for paleo-temperature, CO2 and climate reconstruction, mean that we now have a unique window of opportunity for an integrated project, SWEET, that will:
*) Provide the first ever globally-comprehensive quantitative reconstruction of surface temperatures during the super-warm early Eocene climatic optimum (EECO, ~52 to 50 million years ago).
*) Reconstruct, at an unprecedentedly high temporal resolution, the CO2 concentration that drove EECO warmth.
*) Use this improved characterisation and understanding of the early Eocene climate system to evaluate a state-of-the-art CMIP6-class climate model.
*) Use an integrated model-data approach to understand the forcings which drove EECO warmth, and the mechanisms which mediated these forcings and resulted in a climate so unlike the modern.

Specifically, our outputs and objectives in SWEET are:
(O1) Reconstruction of global EECO warmth at an unparalleled spatial resolution by resurrecting records previously discredited due to diagenesis through a novel combination of in situ ion-probe oxygen isotope analysis and diagenetic modelling, and a targeted synthesis of new and existing surface temperature records from marine and terrestrial settings.
(O2) Characterisation of the drivers of EECO warmth through accurate and precise CO2 reconstructions by high-resolution boron isotope analysis of foraminifera recovered from recent drilling combined with innovative carbon-cycle modelling, and a suite of palaeogeographic reconstructions incorporating a range of possible tectonic scenarios.
(O3) Determination of EECO climate and climate sensitivity through the first ever IPCC AR6-class model (UKESM) simulations of a greenhouse climate.
(O4) First assessment of the ability of UKESM to simulate greenhouse super-warmth, in terms of absolute EECO temperatures and relative change to the modern and latest Paleocene, through model-data comparison utilising statistical methods which take appropriate and full account of uncertainties and correlations in the data.
(O5) New evaluation of the causes of EECO warmth, through an integration of new modelling and novel data, focussing on full characterisation and the relative importance of the relevant forcings, and internal mechanisms which respond to the forcings, such as atmospheric, biogeochemical, oceanic (via the Associated Studentship), chemistry, and land surface processes.

SWEET is organised in the form of 5 overarching Work Packages, with a total of 14 interacting sub-Work-Packages, 8 Milestones and 17 Deliverables, all oriented towards addressing the Aims and Objectives of SWEET highlighted above.