Emergent constraint on equilibrium climate sensitivity from global temperature variability

• Published in: Nature (London) Vol. 553; no. 7688; pp. 319 - 322
• Main Authors: Cox, Peter M., Huntingford, Chris, Williamson, Mark S.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 18.01.2018; Nature Publishing Group
• Subjects: 704/106; 704/106/694/1108; 704/106/694/2786; 704/172; Atmospheric models; Carbon dioxide; Carbon Dioxide - analysis; Carbon dioxide atmospheric concentrations; Carbon dioxide concentration; Climate change; Climate models; Climate science; Climate sensitivity; Climate variability; Confidence limits; Earth; Ecological balance; Environmental aspects; Evaluation; Forecasts and trends; Global temperature changes; Global temperatures; Global warming; Global Warming - history; Global Warming - statistics & numerical data; Greenhouse effect; Heat; Historic temperatures; History, 19th Century; History, 20th Century; History, 21st Century; Humanities and Social Sciences; Intergovernmental Panel on Climate Change; International aspects; letter; Models, Theoretical; multidisciplinary; Observation; Paris Agreement; Probability; Science; Sensitivity; Statistical analysis; Temperature; Temperature effects; Temperature variability; United Kingdom
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature25450

Equilibrium climate sensitivity—which remains the largest uncertainty in climate projections—is constrained to a ‘likely’ range of 2.2–3.4 K by taking into account the variability of global temperature about long-term historical warming. Narrowing down long-term global warming estimates Equilibrium climate sensitivity (ECS) is the long-term change in global mean surface temperature predicted to occur in response to an instantaneous doubling of atmospheric carbon dioxide concentrations. It is an inherently artificial metric, but is nonetheless an important tool when comparing climate models, and a key point of policy discussion. The seemingly intractable range of ECS estimates complicates policy making because the response of the real climate system to the lowest and highest predicted temperature change would translate into radically different policy options. Peter Cox and colleagues now constrain climate models by their ability to simulate observed variations in climate, and conclude that ECS has a central estimate of 2.8 degrees Celsius (°C), which sits towards the middle to lower end of current estimates, and a range of 2.2–3.4 °C. Importantly, their approach allows them to almost exclude ECS estimates above 4.5 °C or below 1.5 °C. Equilibrium climate sensitivity (ECS) remains one of the most important unknowns in climate change science. ECS is defined as the global mean warming that would occur if the atmospheric carbon dioxide (CO 2 ) concentration were instantly doubled and the climate were then brought to equilibrium with that new level of CO 2 . Despite its rather idealized definition, ECS has continuing relevance for international climate change agreements, which are often framed in terms of stabilization of global warming relative to the pre-industrial climate. However, 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 1 . The possibility of a value of ECS towards the upper end of this range reduces the feasibility of avoiding 2 degrees Celsius of global warming, as required by the Paris Agreement. Here we present 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. Our approach is to focus on the variability of temperature about long-term historical warming, rather than on the warming trend itself. We use an ensemble of climate models to define an emergent relationship 2 between ECS and a theoretically informed metric of global temperature variability. This metric of variability can also be calculated from observational records of global warming 3 , 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.