We cannot predict the future, but we can explore different climate policies using the same global climate and earth system models that run long-term projections to understand their changes to greenhouse gas emissions from decade-to-decade, or year-to-year and the implications for our environment.
Despite the vast majority of these global models being developed in the northern hemisphere, we are fortunate in Australia to have the Australian Community Climate and Earth System Simulator – ACCESS. ACCESS is tailored for Australian conditions and the Earth System Model (ESM) version, ACCESS-ESM, includes an essential additional component – the carbon cycle.
We can use ACCESS-ESM to investigate a range of future emissions scenarios and what they mean for Australia and the Southern Ocean. These scenarios include net-zero emissions which are required to stabilise the climate at a specific temperature level such as 1.5° or 2° C (above pre-industrial levels).
For example, CSIRO scientists working with the NESP Climate Systems Hub, Matthew Chamberlain and Tilo Ziehn, have been analysing results from the Zero Emissions Commitment Model Intercomparison Project (ZECMIP). These experiments are part of the World Climate Research Program’s (WCRP) Coupled Model Intercomparison Project (CMIP6) which underpins the latest IPCC assessment report.
The zero emissions commitment (ZEC) is defined as the change in global temperature after the cessation of carbon emissions. Previous estimates of ZEC for the present climate are neutral, that is, that the cooling effects of declining atmospheric CO₂ in zero-emission scenarios would be compensated by warming effects from slow processes in the climate.
However, the multi-model intercomparison of ZEC values of some CMIP6 models, including ACCESS-ESM, are positive in late cessation scenarios, which indicates continued global warming, even after emissions are ceased. In addition, the regional climate continues to evolve with some regions showing a cooling and some other regions showing a warming trend.
The role of the Southern Ocean
For Australia, scientists have found that the Southern Ocean is particularly sensitive to changes in temperature even after 50 years of the cessation of emissions. Hub researcher’s Chamberlain and Ziehn latest study (currently under review in Biogeosciences and available in Discussions for comments) investigated the climate mechanisms behind this ongoing global warming as seen in ACCESS-ESM by extending the ZECMIP framework. They found that the Southern Ocean is driving this global change. Their research based on ACCESS-ESM also shows that the later the timing of zero emissions, the greater the ZEC and ongoing warming.
The Southern Ocean contains regions of enhanced mixing, interacting with the deep ocean and is hence slower to respond to changes in the surface energy balance. Even in ZEC scenarios that cease emissions early and show a neutral ZEC globally, the Southern Ocean is still a region of surface local warming. This response of the Southern Ocean is also present in other models that participated in ZEC intercomparison.
Impact of global warming on marine ecosystems
Continuous warming in the oceans could have dire consequences for marine ecosystems as highlighted in a newly published paper, where hub researcher Tilo Ziehn was involved in another international study exploring the implications of a climate overshoot (temperatures significantly exceeding 2°C). The study combines the effects of changes in ocean temperature and oxygen content in their ability to support viable ecosystems. The analysis is based on multi-model simulations from two different CMIP6 experiments that simulate a global climate overshoot.
Results suggest that significantly overshooting temperature targets of the Paris Agreement will have long lasting (irreversible in our lifetime) impacts on marine ecosystems.
We need to better understand what the implications of those changes are for fisheries and resource management especially in the Southern Ocean. The Australian Earth System Model ACCESS-ESM can help us to explore different future scenarios and how this impacts our region.
Therefore, it is crucial that we continue to further improve the models we rely on for assessing potential future changes and their impacts. The NESP Climate Systems Hub is providing funding to continue the development of ACCESS-ESM, including improving the representation of key climate drivers like ENSO. However, further investment is needed to improve the representation of the carbon cycle, which is essential for exploring climate stabilisation, overshoots and even future climate interventions like carbon dioxide removal.
Find out more about ACCESS:
Even temporary global warming above 2℃ will affect life in the oceans for centuries in The Conversation.
Australian Community Climate and Earth System Simulator via CSIRO here.
ACCESS Earth System Model 1.5 (ESM1.5) here.
Dr Tilo Ziehn’s presentation to ACCESS Community Working Group in May 2023 is here.