How sensitive is climate to increased carbon dioxide in the long term?
Climate scientists involved in the EU-funded PLIO-ESS project have spent the last five years searching for a number. This is not any old number but rather one which will allow us to more accurately predict just how sensitive the global temperature is to carbon dioxide (CO2) emissions. Until now calculations for defining the CO2 emission stabilisation targets that could limit global warming have looked at how the earth’s climate system responds via fast feedbacks such as clouds and sea-ice albedo. This concept is known as short-term or Equilibrium Climate Sensitivity (ECS). Slow feedbacks ‘But more recently there has been a realisation that there are slower responding components of the Earth system — when you increase the amount of CO2, sea ice will respond relatively quickly, but there will be a delay until vegetation distribution reacts and ice sheets may only respond over millennia,’ says Prof Alan Haywood, project coordinator and professor of paleoclimate modelling at Leeds University, UK. This new definition of responsiveness, known as long-term or Earth System Sensitivity (ESS), emerged in the last decade and provides a different guide to the future of our climate. The PLIO-ESS team set out to provide a robust estimate of ESS using the last period in earth’s history when levels of CO2 were at modern levels three million years ago in the mid-Pliocene Warm period. They integrated records of mid-Pliocene vegetation and ice sheets into climate and earth system models. Using geological proxies for the two variables of vegetation distribution and ice cover, ‘we ran hundreds of new climate simulations in an attempt to densely sample this period of time,’ says Prof Haywood. With the help of the UK Met Office’s Hadley Centre for Climate Prediction and Research and climate modelling groups around the world, they used a wide variety of state-of-the-art models to calculate ESS, thus ensuring their findings were not dependent on one particular model. One unforeseen benefit of the project is the way it encouraged growth in climate modelling capacity for the Pliocene. In 2008 eight international research groups agreed to run Pliocene climate simulations, in 2016 that number increased to 16. Greater increase Five years on, the PLIO-ESS team now has enough evidence to state that ESS is definitely greater than ECS. ‘We expect that when the slower feedbacks start to come into play, they will inevitably increase the temperature above and beyond what would be predicted by ECS alone. So, the next question is how much greater?’ says Prof Haywood. Their findings indicate that ESS is up to twice as large as ECS, according to an article published in ‘Climate of the Past’ in 2013. ‘If you were expecting a 3°C rise from ECS, in the worst case scenario, it could be up to 6°C. But our best statistical answer is that ESS is 1.5 times greater than ECS so if you have a climate sensitivity of 3°C, our best answer is currently 4.5°C,’ says Prof Haywood. This result could fundamentally change the debate about how much CO2 can be put into the atmosphere before a climate red line is crossed. ‘So far there has been a strong focus on the earth’s response to the end of the twenty-first century,’ concludes Prof Haywood. ‘We now know that even if we stabilise the concentration of CO2 in the atmosphere, there will still be additional warming because of the longer term feedbacks.’
Keywords
PLIO-ESS, Earth Sensitivity System, climate stabilisation targets, CO2 emissions, climate modelling
