Scientists are taking a new approach to modelling the Earth’s climate. Where once they relied on simple physical models to predict future temperatures for a given level of CO2 in the atmosphere, they now also look to what’s happening in the planet’s living systems – in particular, the Earth’s carbon cycle.
Climate change is caused by an imbalance in the natural carbon cycle, largely due to the burning of fossil fuels, land use changes and cement production. The increased release of carbon into the atmosphere from these activities can be amplified through climate feedbacks – the belching of CO2 and methane from melting permafrost, for example; or the effect of more acidic, warming oceans on the phytoplankton that help oceans mop up around one-third of humanity’s CO2 emissions.
Forests, soils, oceans, and the life within them are the carbon sinks or sources that will continue – or not – to soak up carbon, or release it to the atmosphere.
Clearly, the carbon cycle is the key to determining whether atmospheric CO2 can be held at ‘safe’ levels to avoid overshooting the 1.5–2°C warming threshold set through the Paris Agreement.
The new wave of climate models integrating the carbon cycle are known as Earth system models (ESMs). ESMs link together or ‘couple’ many component models which, combined, represent our state-of-the-art understanding of physical, chemical and biological processes occurring on land and ice, and in the oceans and atmosphere.
In Australia, a collaboration between the Bureau of Meteorology, CSIRO and universities has led to the development of an ESM known as ACCESS – the Australian Community Climate and Earth System Simulator – now used for climate change projections, and seasonal and weather forecasting.
ESMs such as ACCESS account not just for carbon flows, but also the availability and movement of other key elements needed for life, such as nitrogen and phosphorus.
Because of that added complexity, ESMs can simulate changes in climate with higher levels of confidence than a physical model; in other words, they can more realistically simulate how climate changes when CO2 is added to the atmosphere.
ACCESS will make an important contribution to the forthcoming Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), to be released in 2022.
That’s because the ACCESS development team participate in an international initiative called CMIP6: the latest Coupled Model Intercomparison Project. CMIP5 underpinned the IPCC’s Fifth Assessment Report, and CMIP6 will feed into the IPCC’s Sixth Assessment Report.
Through CMIP, researchers evaluate different climate models and ESMs using historical observations to characterise model responses and sensitivity.
These models also run a suite of different future emissions scenarios – ranging from business-as-usual, high-emissions futures, to low-emissions futures where climate impacts are minimised. This allows researchers to provide us with the most reliable projections available of climate impacts resulting from different economic and social pathways to the future.
Andrew Lenton, a CSIRO scientist working on ACCESS development, says the value of the CMIP model comparison is that it brings together an ‘ensemble of plausible futures’ that, between them, represent the range of possible responses to rising CO2.
“We know that some things will never happen, and some things will happen, but we don’t know how big they’ll be,” says Lenton, a carbon-cycle modeller with a focus on ocean acidification and the impacts of CO2-removal technologies.
“That’s one of the challenges around ESM modelling – to try to understand and project the integrated response of the earth system.
“ESMs can now help us answer questions like, what do changes in the Earth’s systems mean for land and ocean productivity? What do they mean for the ecosystem services, such as food and water, that the Earth provides for us? What are we confident about, and what are we not confident about? What is the cost of doing something, and what is the cost of not doing it?
“What’s unique to CSIRO’s research is the land and ocean carbon models we have developed. Australia response to climate will be different to other nations.”
As a climate scientist, Lenton is concerned about some of the more simplistic arguments thrown into the climate change debate around the carbon fertilisation effect – the idea that plants will thrive in a higher CO2 world and thus absorb more CO2 – as well as proposals put forward for certain negative emissions technologies. The latter are often seen as a ‘silver bullet’ for reducing CO2 levels in the atmosphere.
However, without potential impacts being investigated through ESMs, such technologies and schemes could prove ineffective or, at worst, even more damaging to the planet, warns Lenton.
Relying on the carbon fertilisation effect is also risky.
“CABLE [Community Atmosphere Biosphere Land Exchange], the Australian land-surface model coupled to ACCESS, has shown us that the limits to plant growth are not just carbon and nitrogen, but phosphorus as well,” Lenton points out.
“This is relevant for Australia, because we have phosphorus-poor soils.”
Tilo Ziehn is member of CSIRO’s land surface modelling team which developed CABLE. His current role is to make sure that CABLE works in ACCESS.
As he describes it, CABLE models the physics – including radiation, micrometeorology and surface fluxes – of soil and snow on Australia’s land surface, and also has a biogeochemical module that describes the movement of carbon, nitrogen and phosphorus.
“We know that plants may be more productive if CO2 levels rise, but on the other hand they also respond to the warming, changes in soil moisture, and the nutrient limitations that most likely offset the benefits of rising CO2,” he says.
“ACCESS with CABLE is one of the first models to include nitrogen and phosphorus limitation, which is important when we try to estimate future carbon uptake. Our simulations show that the plant uptake of carbon for future scenarios is very much reduced when you consider nutrient limitations.”
Like Andrew Lenton, Ziehn is concerned about the option of using negative emissions technologies to ‘reverse’ climate change without their impacts being tested in an ESM.
“The aim of the Paris accord is to keep global temperatures well below 2°C,” he says. “Many people seem to rely on the fact that negative emissions can get us there, but no one really knows if these ideas are practical and feasible, and what the consequences are.
“That’s why ESMs are so important – only they can simulate the consequences of those processes.”