Probably the most vexing problem in climate change science is determining how hot the planet would become if the amount of carbon dioxide in the atmosphere doubles from it pre-industrial level of 280 parts per million. Known as equilibrium climate sensitivity (ECS), the Intergovernment Panel on Climate Change's Fifth Assessment Report put the likely range of ECS as being between 1.5 and 4.5 degrees Celsius.
The current level of carbon dioxide in the atmosphere is around 403 parts per million. Carbon dioxide in the atmosphere is now increasing annually at about 3 parts per million. If that rate of increase keeps up, the amount of carbon dioxide in the atmosphere will double by the end of the century.
As bad as an increase of 4.5 degrees Celsius would be - and it would be really bad considering that the difference between now and an ice age is 4 to 7 degrees Celsius - a big concern has been that researchers have not been able to rule out that future temperatures might exceed the 4.5 degrees Celsius. In a new study published in Nature, University fo Exeter mathematician Peter Cox and his colleagues claim that they have constrained ECS to a lower range of values. From the abstract:
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. … [Their procedure] 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.
To greatly simplify, Cox and his colleagues compared how climate models handled year-to-year variations in surface temperatures with the historical temperature record. In doing so, they found that the climate models that yielded higher ECS values failed to replicate what has been happening to actual temperature trends.
In his perspetive article in Nature, Leeds University climatologist Piers Forster explains, "Their analysis revealed that only climate models that produce relatively small values of ECS match the variability seen in the historical temperature record." Forster adds, "If the upper limit of ECS can truly be constrained to a lower value than is currently expected, then the risk of very high surface-temperature changes occurring in the future will decrease. This, in turn, would improve the chances of keeping the temperature increase well below 2?°C above pre-industrial levels, the target of the Paris Agreement under the United Nations Framework Convention on Climate Change."
That would be good news indeed.