Abrupt climate change events during the last glacial period and the Last Interglacial resulted from changes in the Atlantic Meridional Overturning Circulation (AMOC). Over the last 50 years, the AMOC has weakened and is projected to weaken further or even collapse this century due to freshwater influx from melting glaciers driven by climate warming. Despite numerous modelling studies investigating the impacts of an AMOC shutdown, little is known about its impact on Australasian hydroclimate, particularly under a climate warmer than the pre-industrial (PI). Using the ACCESS-ESM1.5 model, we assess the processes impacting seasonal hydroclimate in the Australasian region in response to an AMOC shutdown under PI and Last Interglacial (LIG) climatic conditions. While the broad hydroclimate response to an AMOC shutdown is similar in both experiments, notable regional differences emerge, highlighting the influence of background climate states. During austral summer (DJF), the AMOC shutdown leads to drier conditions over the Maritime Continent and increased precipitation over northern Australia under both PI and LIG conditions. However, the precipitation increase over Australia is weaker under PI than LIG. During austral winter (JJA), mid to high southern latitude regions of Australia and New Zealand experience drying in response to the AMOC shutdown under PI boundary conditions, while under LIG boundary conditions, only southeastern Australia and New Zealand exhibit drier conditions, with northwestern Australia displaying wetter conditions. These results underscore the complex and region-specific responses of Australasian hydroclimate to AMOC disruptions, highlighting the importance of considering background climate states when assessing such impacts.

Greenland ice cores reveal an abrupt cooling of up to 3.3°C 8.2 kyr ago (8.2 ka), lasting for roughly 160 years. The event was likely caused by a weakening of the Atlantic Meridional Overturning Circulation (AMOC) due to freshwater drainage into the North Atlantic. It was associated with a global-scale climate change but is recorded in very few high-resolution paleoclimatic time series from the Southern Hemisphere (SH). In this study, we investigate the 8.2 ka event in the SH, particularly the Australian climate response to a weakened AMOC. Five North Atlantic meltwater experiments are conducted with the Australian Earth System Model, ACCESS-ESM1.5, to evaluate the sensitivity of AMOC responses to freshwater perturbations under early Holocene conditions as well as their climate impact. Our results suggest a 100-year freshwater pulse reproduces a global climate change that best matches existing proxy records for the 8.2 ka event. Australian surface air temperatures show significant cooler conditions in the northern half of the continent but warmer anomalies in the south in response to a weakened AMOC. Australian hydroclimate displays a more complex response at 8.2 ka. Northern Australian precipitation is influenced by a southward shift in the mean position of the Intertropical Convergence Zone and a strengthened Indo-Australian summer monsoon, while the southern part of the continent is more sensitive to weakening of the winter westerly winds. These results highlight the importance of understanding the Australian climate response to a weakened AMOC under different background climate in order to better predict potential future impacts.

A document by Liam Cassidy. Click on the document to view its contents.

As more countries make net zero greenhouse gas emissions pledges, it is crucial to understand the effects on global climate after achieving net zero emissions. The climate has been found to continue to evolve even after the abrupt cessation of CO2 emissions, with some models simulating a small warming and others simulating a small cooling. In this study, we analyse if the temperature and precipitation changes post abrupt cessation of CO2 emissions are significant compared to natural climate variations. We find that the temperature changes are outside of natural variability for most models, whilst the precipitation changes are mostly non-significant. We also demonstrate that post-net zero temperature changes have implications for the remaining carbon budget. The possibility of further global warming post-net zero adds to the evidence supporting more rapid emissions reductions in the near-term.

Climate change projections for the Australian monsoon have been highly uncertain in previous generations of coupled climate models. The new Coupled Model Intercomparison Project Phase 6 (CMIP6) ensemble provides an opportunity to address the uncertainty in future projections for northern Australia. We find that the range in Australian monsoon projections from the available CMIP6 ensemble is substantially reduced compared to CMIP5, although models continue to disagree on the magnitude and direction of change. While previous CMIP5 studies identified warming in the western equatorial Pacific as important for Australian monsoon projections, here we show that the western Pacific is not strongly connected to northern Australian precipitation changes in the CMIP6 models. By comparing groups of models based on their future projections, we note that the model-to-model differences in Australian monsoon projections are congruent with the zonally averaged precipitation response in the Southern Hemisphere tropics within each model.

Recent climate change is characterised by rapid global warming, but the goal of the Paris Agreement is to achieve a stable climate where global temperatures remain well below 2°C above pre-industrial levels. Inferences about conditions at or below 2°C are usually made based on transient climate projections. To better understand climate change impacts on natural and human systems under the Paris Agreement, we must understand how a stable climate may differ from transient conditions at the same warming level. Here we examine differences between transient and quasi-equilibrium climates based on greenhouse gas-only model simulations at 1.5°C and 2°C global warming. We find substantial local differences between seasonal-average temperatures, with mid-latitude land regions in boreal summer considerably warmer in a transient climate than a quasi-equilibrium state at both 1.5°C and 2°C global warming. Our research demonstrates that the rate of global warming must be considered in regional projections.