A new earth system climate model of intermediate complexity has been developed and its climatology compared to observations. The UVic Earth System Climate Model consists of a three-dimensional ocean general circulation model coupled to a thermodynamic/dynamic sea-ice model, an energy-moisture balance atmospheric model with dynamical feedbacks, and a thermomechanical land-ice model. In order to keep the model computationally efficient a reduced complexity atmosphere model is used. Atmospheric heat and freshwater transports are parametrized through Fickian diffusion, and precipitation is assumed to occur when the relative humidity is greater than 85%. Moisture transport can also be accomplished through advection if desired. Precipitation over land is assumed to return instantaneously to the ocean via one of 33 observed river drainage basins. Ice and snow albedo feedbacks are included in the coupled model by locally increasing the prescribed latitudinal profile of the planetary albedo. The atmospheric model includes a parametrization of water vapour/ planetary longwave feedbacks, although the radiative forcing associated with changes in atmospheric CO 2 is prescribed as a modification of the planetary longwave radiative flux. A specified lapse rate is used to reduce the surface temperature over land where there is topography. The model uses prescribed present-day winds in its climatology, although a dynamical wind feedback is included which exploits a latitudinally-varying empirical relationship between atmospheric surface temperature and density. The ocean component of the coupled model is based on the Geophysical Fluid Dynamics Laboratory (GFDL) Modular Ocean Model 2.2, with a global resolution of 3.6°(zonal) by 1.8°(meridional) and 19 vertical levels, and includes an option for brine-rejection parametrization. The sea-ice component incorporates an elastic-viscous-plastic rheology to represent sea-ice dynamics and various options for the representation of sea-ice thermodynamics and thickness distribution. The systematic comparison of the coupled model with observations reveals good agreement, especially when moisture transport is accomplished through advection. Global warming simulations conducted using the model to explore the role of moisture advection reveal a climate sensitivity of 3.0°C for a doubling of CO 2 , in line with other more comprehensive coupled models. Moisture advection, together with the wind feedback, leads to a transient simulation in which the meridional overturning in the North Atlantic initially weakens, but is eventually re-established to its initial strength once the radiative forcing is held fixed, as found in many coupled atmosphere General Circulation Models (GCMs). This is in contrast to experiments in which moisture transport is accomplished through diffusion whereby the overturning is reestablished to a strength that is greater than its initial condition. When applied to the climate of the Last Glacial Maximum (LGM), the model obtains tropical cooling (30°N-30°S), relative to the pr...

Multimillennial simulations with a fully coupled climate–carbon cycle model are examined to assess the persistence of the climatic impacts of anthropogenic CO2 emissions. It is found that the time required to absorb anthropogenic CO2 strongly depends on the total amount of emissions; for emissions similar to known fossil fuel reserves, the time to absorb 50% of the CO2 is more than 2000 yr. The long-term climate response appears to be independent of the rate at which CO2 is emitted over the next few centuries. Results further suggest that the lifetime of the surface air temperature anomaly might be as much as 60% longer than the lifetime of anthropogenic CO2 and that two-thirds of the maximum temperature anomaly will persist for longer than 10 000 yr. This suggests that the consequences of anthropogenic CO2 emissions will persist for many millennia.

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The 2019/20 Black Summer bushfire disaster in southeast Australia was unprecedented: the extensive area of forest burnt, the radiative power of the fires, and the extraordinary number of fires that developed into extreme pyroconvective events were all unmatched in the historical record. Australia’s hottest and driest year on record, 2019, was characterised by exceptionally dry fuel loads that primed the landscape to burn when exposed to dangerous fire weather and ignition. The combination of climate variability and long-term climate trends generated the climate extremes experienced in 2019, and the compounding effects of two or more modes of climate variability in their fire-promoting phases (as occurred in 2019) has historically increased the chances of large forest fires occurring in southeast Australia. Palaeoclimate evidence also demonstrates that fire-promoting phases of tropical Pacific and Indian ocean variability are now unusually frequent compared with natural variability in pre-industrial times. Indicators of forest fire danger in southeast Australia have already emerged outside of the range of historical experience, suggesting that projections made more than a decade ago that increases in climate-driven fire risk would be detectable by 2020, have indeed eventuated. The multiple climate change contributors to fire risk in southeast Australia, as well as the observed non-linear escalation of fire extent and intensity, raise the likelihood that fire events may continue to rapidly intensify in the future. Improving local and national adaptation measures while also pursuing ambitious global climate change mitigation efforts would provide the best strategy for limiting further increases in fire risk in southeast Australia.

Atmospheric CO2 was ∼90 ppmv lower at the Last Glacial Maximum (LGM) compared to the late Holocene, but the mechanisms responsible for this change remain elusive. Here we employ a carbon isotope‐enabled Earth System Model to investigate the role of ocean circulation in setting the LGM oceanic δ13C distribution, thereby improving our understanding of glacial/interglacial atmospheric CO2 variations. We find that the mean ocean δ13C change can be explained by a 378 ± 88 Gt C(2σ) smaller LGM terrestrial carbon reservoir compared to the Holocene. Critically, in this model, differences in the oceanic δ13C spatial pattern can only be reconciled with a LGM ocean circulation state characterized by a weak (10–15 Sv) and relatively shallow (2000–2500 m) North Atlantic Deep Water cell, reduced Antarctic Bottom Water transport (≤10 Sv globally integrated), and relatively weak (6–8 Sv) and shallow (1000–1500 m) North Pacific Intermediate Water formation. This oceanic circulation state is corroborated by results from the isotope‐enabled Bern3D ocean model and further confirmed by high LGM ventilation ages in the deep ocean, particularly in the deep South Atlantic and South Pacific. This suggests a poorly ventilated glacial deep ocean which would have facilitated the sequestration of carbon lost from the terrestrial biosphere and atmosphere.

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The early part of the last deglaciation is characterised by a ~40 ppm atmospheric CO2 rise occurring in two abrupt phases. The underlying mechanisms driving these increases remain a subject of intense debate. Here, we successfully reproduce changes in CO2, δ13C and Δ14C as recorded by paleo-records during Heinrich stadial 1 (HS1). We show that HS1 CO2 increase can be explained by enhanced Southern Ocean upwelling of carbon-rich Pacific deep and intermediate waters, resulting from intensified Southern Ocean convection and Southern Hemisphere (SH) westerlies. While enhanced Antarctic Bottom Water formation leads to a millennial CO2 outgassing, intensified SH westerlies induce a multi-decadal atmospheric CO2 rise. A strengthening of SH westerlies in a global eddy-permitting ocean model further supports a multi-decadal CO2 outgassing from the Southern Ocean. Our results highlight the crucial role of SH westerlies in the global climate and carbon cycle system with important implications for future climate projections.

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