Climate scientists use mathematical models to project the Earth’s future under a warming world, but a group of the latest models have included unexpectedly high values for a measure called “climate sensitivity”.
Climate sensitivity refers to the relationship between changes in carbon dioxide in the atmosphere and warming.
The high values are an unwelcome surprise. If they’re right, it means a hotter future than previously expected – warming of up to 7℃ for Australia by 2100 if emissions continue to rise unabated.
Read more: There are no time-travelling climatologists: why we use climate models
Our recent study analyses these climate models (named CMIP6 ), which were released at the end of last year, and what insights they give for Australia.
These models contain the latest improvements and innovations from some of the world’s leading climate modelling institutes, and will feed into the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report in 2021.
But the new climate sensitivity values raise the question of whether previous climate modelling has underestimated potential climate change and its effects, or whether the new models are overdoing things.
If the high estimate is right, this would require the world to make greater and more urgent emission cuts to meet any given warming target. These higher climate sensitivity values point to the urgent need to cut our greenhouse gas emissions. What is climate sensitivity?
Climate sensitivity is one of the most important factors for climate change, strongly influencing our planning for adaptation and mitigation of greenhouse gas emissions.
It’s a standardised measure of how much the climate responds when carbon dioxide concentrations in the atmosphere double. There are a few indices of climate sensitivity that the scientific community uses, and perhaps the most commonly used is “equilibrium climate sensitivity”.
We can estimate equilibrium climate sensitivity by raising carbon dioxide concentrations in models abruptly and then calculating the warming experienced after 150 years – when the atmosphere and ocean would return to a temperature balance.
In other words, giving the climate a “push” with more carbon emissions and waiting until it settles down into a new state.
The previous generation of models ( CMIP5 ) had equilibrium climate sensitivity values between 2.1℃ to 4.7℃ global temperature change. The values for the latest models (CMIP6) are from 1.8℃ to 5.6℃.This includes a cluster of models with sensitivity of 5℃ or more, a group of models within the previous range, and two models with very low values at around 2℃. What this means for our future Higher equilibrium climate sensitivity values mean a hotter future climate than previously expected, for any given scenario of future emissions . We’ll see Australian temperature increase in a low and high emissions scenario projections (temperature relative to 1995-2014, range of models shown as coloured bands, observations as a black line). According to these new models, Australian warming could crack more than 7℃ by 2100 under a scenario where greenhouse gas emissions continue to increase through the century.These higher temperature changes are not currently presented in the national climate projections […]
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