Abstract. Understanding natural and anthropogenic climate change processes involves using computational models that represent the main components of the Earth system: the atmosphere, ocean, sea ice, and land surface. These models have become increasingly computationally expensive as resolution is increased and more complex process representations are included. However, to gain robust insight into how climate may respond to a given forcing, and to meaningfully quantify the associated uncertainty, it is often required to use either or both ensemble approaches and very long integrations. For this reason, more computationally efficient models can be very valuable tools. Here we provide a comprehensive overview of the suite of climate models based around the HadCM3 coupled general circulation model. This model was developed at the UK Met Office and has been heavily used during the last 15 years for a range of future (and past) climate change studies, but has now been largely superseded for many scientific studies by more recently developed models. However, it continues to be extensively used by various institutions, including the BRIDGE (Bristol Research Initiative for the Dynamic Global Environment) research group at the University of Bristol, who have made modest adaptations to the base HadCM3 model over time. These adaptations mean that the original documentation is not entirely representative, and several other relatively undocumented configurations are in use. We therefore describe the key features of a number of configurations of the HadCM3 climate model family, which together make up HadCM3@Bristol version 1.0. In order to differentiate variants that have undergone development at BRIDGE, we have introduced the letter B into the model nomenclature. We include descriptions of the atmosphere-only model (HadAM3B), the coupled model with a low-resolution ocean (HadCM3BL), the high-resolution atmosphere-only model (HadAM3BH), and the regional model (HadRM3B). These also include three versions of the land surface scheme. By comparing withPublished by Copernicus Publications on behalf of the European Geosciences Union. observational datasets, we show that these models produce a good representation of many aspects of the climate system, including the land and sea surface temperatures, precipitation, ocean circulation, and vegetation. This evaluation, combined with the relatively fast computational speed (up to 1000 times faster than some CMIP6 models), motivates continued development and scientific use of the HadCM3B family of coupled climate models, predominantly for quantifying uncertainty and for long multi-millennial-scale simulations.

Abstract. The Eocene–Oligocene transition (EOT) was a climate shift from a largely ice-free greenhouse world to an icehouse climate, involving the first major glaciation of Antarctica and global cooling occurring ∼34 million years ago (Ma) and lasting ∼790 kyr. The change is marked by a global shift in deep-sea δ18O representing a combination of deep-ocean cooling and growth in land ice volume. At the same time, multiple independent proxies for ocean temperature indicate sea surface cooling, and major changes in global fauna and flora record a shift toward more cold-climate-adapted species. The two principal suggested explanations of this transition are a decline in atmospheric CO2 and changes to ocean gateways, while orbital forcing likely influenced the precise timing of the glaciation. Here we review and synthesise proxy evidence of palaeogeography, temperature, ice sheets, ocean circulation and CO2 change from the marine and terrestrial realms. Furthermore, we quantitatively compare proxy records of change to an ensemble of climate model simulations of temperature change across the EOT. The simulations compare three forcing mechanisms across the EOT: CO2 decrease, palaeogeographic changes and ice sheet growth. Our model ensemble results demonstrate the need for a global cooling mechanism beyond the imposition of an ice sheet or palaeogeographic changes. We find that CO2 forcing involving a large decrease in CO2 of ca. 40 % (∼325 ppm drop) provides the best fit to the available proxy evidence, with ice sheet and palaeogeographic changes playing a secondary role. While this large decrease is consistent with some CO2 proxy records (the extreme endmember of decrease), the positive feedback mechanisms on ice growth are so strong that a modest CO2 decrease beyond a critical threshold for ice sheet initiation is well capable of triggering rapid ice sheet growth. Thus, the amplitude of CO2 decrease signalled by our data–model comparison should be considered an upper estimate and perhaps artificially large, not least because the current generation of climate models do not include dynamic ice sheets and in some cases may be under-sensitive to CO2 forcing. The model ensemble also cannot exclude the possibility that palaeogeographic changes could have triggered a reduction in CO2.

In June 2021, western North America experienced a record-breaking heat wave outside the distribution of previously observed temperatures. While it is clear that the event was extreme, it is not obvious whether other areas in the world have also experienced events so far outside their natural variability. Using a novel assessment of heat extremes, we investigate how extreme this event was in the global context. Characterizing the relative intensity of an event as the number of standard deviations from the mean, the western North America heat wave is remarkable, coming in at over four standard deviations. Throughout the globe, where we have reliable data, only five other heat waves were found to be more extreme since 1960. We find that in both reanalyses and climate projections, the statistical distribution of extremes increases through time, in line with the distribution mean shift due to climate change. Regions that, by chance, have not had a recent extreme heat wave may be less prepared for potentially imminent events.

Table of Contents Methods S1: Methodological details supporting main text analyses Tables S1: Values used to calculate dispersal distances in the simulations S2: Number of virtual species used for each global climate model (AOGCM) combination S3: Details of AOGCMs used in the simulations S4: Number of virtual species used for each AOGCM combination, shelf definition < 200 m S5: Post-hoc comparisons to assess statistical differences in proportional extinction between each of the greenhouse-icehouse transitions 200 m water depth S18: Definition of coastline orientation and small islands References References in support of the methods 2 S1. Methods S1.1. Simulation overview. We used a cellular automaton algorithm that linked a gridded geographic domain with a one-dimensional temperature landscape 1-3 to test the effect of paleogeography, sea level drop, and temperature change on extinction magnitude during three climate transitions: Late Ordovician, Eocene-Oligocene and Plio-Pleistocene. The geographic component of the model consisted of a global 1°x1° grid of shallow marine continental shelf for each of the three time periods. We generated virtual species that occupied grid cells in these continental margins as a function of their assigned temperature tolerances and dispersal abilities. The one-dimensional climate landscape was perturbed, and the geographic response of the virtual species recorded (Fig. 2). The framework builds on the model introduced by Qiao et al. 1 and Saupe et al. 3 , and is similar in concept to simulations explored by Rangel et al. 2 and Tomasovych et al. 4. S1.2. Paleogeography. We isolated the effect of continental configuration on expected extinction magnitude for the three target periods using paleogeographic reconstructions from 5 for the Late Ordovician (450 Ma), from Scotese 6 for the late Eocene (~37 Ma), and from Robertson Plc. for the Pliocene 7 (mid-Pliocene Warm Period, ~3.1 Ma). The palaeogeographic reconstructions needed to match those used in the AOGCMs (see S.1.4) and therefore derive from different sources (e.g., Blakey versus Scotese). For each paleogeographic reconstruction, we used the shallow marine areas around terrestrial continental margins (Fig. 1; Fig. S2), excluding Antartica for the Eocene and Pliocene. We considered both narrow and broad marine shelves: the former was generated by extending all terrestrial continental margins by one cell (1°), whereas the latter was generated by extending terrestrial continental margins by three cells (Fig. 1; Fig. S2). The broad margin in particular may be broader than most marine shelfs, given resolution of the climate model data (1° is approx. 100 km at the equator). S.1.2.1. Simplistic hypothetical climate gradient. Only paleogeography differed across the time periods of interest in these simulations. For the analyses in which the magnitude of climate change was held constant across all intervals, we generated hypothetical 'warm climate' and 'cold climate' temperature gradients by averaging interval-specific oceanatmosphere ge...

The glaciation of Antarctica at the Eocene-Oligocene transition (approx. 34 million years ago) was a major shift in the Earth's climate system, but the mechanisms that caused the glaciation, and its effects, remain highly debated. A number of recent studies have used coupled atmosphere-ocean climate models to assess the climatic effects of Antarctic glacial inception, with often contrasting results. Here, using the HadCM3L model, we show that the global atmosphere and ocean response to growth of the Antarctic ice sheet is sensitive to subtle variations in palaeogeography, using two reconstructions representing Eocene and Oligocene geological stages. The earlier stage (Eocene; Priabonian), which has a relatively constricted Tasman Seaway, shows a major increase in sea surface temperature over the Pacific sector of the Southern Ocean in response to the ice sheet. This response does not occur for the later stage (Oligocene; Rupelian), which has a more open Tasman Seaway. This difference in temperature response is attributed to reorganization of ocean currents between the stages. Following ice sheet expansion in the earlier stage, the large Ross Sea gyre circulation decreases in size. Stronger zonal flow through the Tasman Seaway allows salinities to increase in the Ross Sea, deep-water formation initiates and multiple feedbacks then occur amplifying the temperature response. This is potentially a model-dependent result, but it highlights the sensitive nature of model simulations to subtle variations in palaeogeography, and highlights the need for coupled ice sheet-climate simulations to properly represent and investigate feedback processes acting on these time scales.