Climate researchers have spent decades trying to pin down the planet's equilibrium climate sensitivity. Also known by the initials ECS, that figure represents how much it would ultimately increase global average temperatures if the amount of carbon dioxide in the atmosphere doubles above the pre-industrial level.
Figuring out the ECS has huge implications for policy. If future warming is at the low end, humanity has more time to adapt and to shift energy production away from the fossil fuels that are loading up the atmosphere with extra carbon dioxide. If at the high end, efforts to adapt and shift energy production to low-carbon sources would need to be speeded up. The current assessment of the United Nations' Intergovernmental Panel on Climate Change (IPCC) states that ECS is likely to be in the range of 1.5°C to 4.5°C, extremely unlikely to be less than 1°C, and very unlikely to be greater than 6°C.
But a new study in the Journal of Climate suggests that the IPCC's estimates are much too high. In calculating their rival figures, authors Nicholas Lewis and Judith Curry take into account historical atmospheric and ocean temperature trends since the mid-19th century. Their estimates also draw on new findings since 1990 of how atmospheric ozone and aerosols are likely to affect global temperature trends. (They also address other researchers' concerns about an earlier ECS study that they published in 2015.)
"Our results imply that, for any future emissions scenario, future warming is likely to be substantially lower than the central computer model-simulated level projected by the IPCC, and highly unlikely to exceed that level," Lewis says in a press release from the Global Warming Policy Forum.
How much lower? Their median ECS estimate of 1.66°C (5–95% uncertainty range: 1.15–2.7°C) is derived using globally complete temperature data. The comparable estimate for 31 current generation computer climate simulation models cited by the IPCC is 3.1°C. In other words, the models are running almost two times hotter than the analysis of historical data suggests that future temperatures will be.
In addition, the high-end estimate of Lewis and Curry's uncertainty range is 1.8°C below the IPCC's high-end estimate.
Lewis and Curry's estimates are in line with the similarly low estimates reported by climatologists Thorsten Mauritsen of the Max Planck Institute for Meteorology and Robert Pincus of the University of Colorado in the July 2017 issue of Nature Climate Change. Using historical temperature data, those two researchers calculated an ECS of 1.5°C (0.9–3.6°C, 5th–95th percentile).
If these two studies turn out to be right, that will be good news for humanity.