SWEET:Super-Warm Early Eocene Temperatures and climate: understanding the response of the Earth to high CO2 through integrated modelling and data

The Earth's climate is currently changing rapidly, primarily due to emissions of greenhouse gases caused by human industrialisation. These emissions are projected to increase through this century, and under some scenarios atmospheric carbon dioxide (CO2) concentrations could reach more than 1000 parts per million (ppm) by the year 2100, compared with 280 ppm prior to industrialisation. In order to predict the sociological, environmental, and economic impacts of such scenarios, and thus to better prepare for them, the only tool at our disposal is climate modelling. In order to assess our confidence in predictions from climate models, they are routinely tested under conditions of known climate. However, this testing (and associated tuning of the models) is almost exclusively carried out under modern climate conditions, and relative to recently observed climate change, for which CO2 concentrations are less than 400 ppmv. As such, our state-of-the-art climate models have never been tested under the high CO2, super-warm climate conditions to which they are primarily applied, and upon which major policy decisions are made.

However, there exist time periods in Earth's deeper past (for example the Eocene, about 50 million years ago) when CO2 concentrations were similar to those expected by the end of this century; but climatological information from these time periods is currently sparse and is associated with large uncertainties, and the exact concentrations of CO2 are only poorly known. Recent changes in our understanding of how the geological record preserves climate signals, and developments in laboratory techniques, mean that for the first time there exists a new and exciting opportunity to remedy this situation and provide a much-needed evaluation of our very latest climate models in a super-warm world.

In SWEET, we will apply these emerging techniques, and develop new methodologies and tools, to produce a global dataset of Eocene temperatures. Coupled with new and high-fidelity reconstructions of Eocene CO2 concentrations, and state-of-the-art maps of the 'palaeogeograpy' (continental positions, mountain ranges, ocean depths etc.), we will use this dataset to test a state-of-the art climate model under high atmospheric CO2, Eocene conditions. The model, UKESM, is identical to that being used by the UK Met Office in the international 'CMIP6' project, which itself will be the primary input to the next Intergovernmental Panel on Climate Change (IPCC) assessment report. We will also use our data and additional model simulations (running at high spatial resolution) to investigate the relative importance of the various mechanisms which determine the response of the Earth system to high CO2 and to changes in palaeogeography.

A characteristic of SWEET is that we will take full account of uncertainties in the geological data and the modelling, and our model-data comparisons will be underpinned by a statistical framework which incorporates these uncertainties. We will also adopt a 'multi-proxy' approach by using several independent geological archives to reconstruct temperature. For one of these archives, namely the oxygen isotopic composition of the fossilised shells of microscopic marine creatures from the Eocene, we will apply a particularly innovative approach which will enable us to 'resurrect' previously discredited data, by using an extremely fine-scale 'ion probe' to investigate how these isotopic signatures of past climate change are recorded in individual fossils.

SWEET has strong links to UK Met Office, and to the international DeepMIP project, which is part of the 'Palaeoclimate Modelling Intercomparison Project', itself part of CMIP6. We expect our results to feed into the next IPCC assessment reports and therefore to ultimately inform policy.

Our exciting and innovative Impact Plan has three components, which are focussed on public outreach and engagement with policy-makers through the Intergovernmental Policy on Climate Change (IPCC).

Our public outreach will be centred on the SWEET project website, which we will promote via the participating universities' extensive media contacts (both traditional media and social media). Through the website we will produce regular blog postings from the postdoctoral researchers and the PhD student, charting progress through the project. The innovation here is that in addition to highlighting exciting new scientific results, the blogs will also focus on the process of actually carrying out science, including highs and lows, excitements and disappointments. As such, they will document the complete scientific process from a human perspective, and it is our hope that this will inspire undergraduates and schoolchildren to engage with science, in particular those who otherwise may have viewed science and scientists as unapproachable.

Another central aspect of our engagement with the general public will be via the first ever (to our knowledge) climate model simulations of the world of 'Game of Thrones' - a hugely popular book and television series (single episodes of which attract viewing figures in the 10's of millions). We previously had great success with a similar exercise as part of the Impact Plan of a previous NERC grant, in which we simulated the climate of J.R.R. Tolkien's Middle Earth (and which received a total of 100,000 Tweets/reTweets in the first 8 hours alone), and we anticipate that this new activity will have a similar, if not greater, global impact. The primary aim is to excite the general public about climate science and climate models, and to emphasise that climate models have the flexibility, because of their grounding in fundamental scientific principles, to be applied beyond just the modern Earth. As such, we will tackle the commonly held myth that climate models are just constructed for and tuned, to, the modern world. Note that we do not request any resource to carry out this activity.

Finally, we will aim to communicate our findings to policymakers via their inclusion in forthcoming reports of the IPCC (AR6, and AR7 if appropriate; note that five of the SWEET team were Contributing Authors to AR5). The work in SWEET represents a step-change in terms of evaluation of deep-time warm climates compared with what was presented in AR5. As such, we are confident that our work will be included in AR6. However we will facilitate this by sending copies of our papers to the appropriate IPCC authors, and by presenting our work at international conferences.