Jan 18, 2018

Emergent constraint on equilibrium climate sensitivity from global temperature variability

Peter M. Cox, Chris Huntingford, Mark S. Williamson
  • States that equilibrium climate sensitivity (ECS) remains one of the most important unknowns in climate change science
  • Defines ECS as the global mean warming that would occur if the atmospheric carbon dioxide (CO2) concentration were instantly doubled and the climate were then brought to equilibrium with that new level of CO2
  • States that the ‘likely’ range of ECS as stated by the Intergovernmental Panel on Climate Change (IPCC) has remained at 1.5–4.5 degrees Celsius for more than 25 years
  • Presents a new emergent constraint on ECS that yields a central estimate of 2.8 degrees Celsius with 66 per cent confidence limits (equivalent to the IPCC ‘likely’ range) of 2.2–3.4 degrees Celsius
  • Focuses on the variability of temperature about long-term historical warming, rather than on the warming trend itself
  • Uses an ensemble of climate models to define an emergent relationship between ECS and a theoretically informed metric of global temperature variability
  • States that this metric of variability can also be calculated from observational records of global warming, which enables tighter constraints to be placed on ECS, reducing the probability of ECS being less than 1.5 degrees Celsius to less than 3 per cent, and the probability of ECS exceeding 4.5 degrees Celsius to less than 1 per cent