An assessment of Earth's climate sensitivity using multiple lines of evidence. https://doi.org/10.1029/2019RG000678

“Earth's global “climate sensitivity” is a fundamental quantitative measure of the susceptibility of Earth's climate to human influence. A landmark report in 1979 concluded that it probably lies between 1.5‐4.5°C per doubling of atmospheric carbon dioxide, assuming that other influences on climate remain unchanged. In the 40 years since, it has appeared difficult to reduce this uncertainty range. In this report we thoroughly assess all lines of evidence including some new developments. We find that a large volume of consistent evidence now points to a more confident view of a climate sensitivity near the middle or upper part of this range. In particular, it now appears extremely unlikely that the climate sensitivity could be low enough to avoid substantial climate change (well in excess of 2°C warming) under a high‐emissions future scenario. We remain unable to rule out that the sensitivity could be above 4.5°C per doubling of carbon dioxide levels, although this is not likely.”

And from Bloomberg:

“A major new study of the relationship between carbon dioxide and global warming lowers the odds on worst-case climate change scenarios while also ruling out the most optimistic estimates nations have been counting on as they attempt to implement the Paris Agreement.

A group of 25 leading scientists now conclude that catastrophic warming is almost inevitable if emissions continue at their current rate, even if there’s less reason to anticipate a totally uninhabitable Earth in coming centuries. The research, published Wednesday in the journal Reviews of Geophysics, narrows the answer to a question that’s as old as climate science itself: How much would the planet warm if humanity doubled the amount of CO₂ in the atmosphere?

That number, known as “equilibrium climate sensitivity,” is typically expressed as a range. The scientists behind this new study have narrowed the climate-sensitivity window to between 2.6° Celsius and 3.9°C..

That’s smaller than the current range accepted by  the United Nations-backed Intergovernmental Panel on Climate Change, which has for almost a decade used a spread between 1.5°C to 4.5°C—a reading of climate sensitivity that has changed little since the first major U.S. climate science assessment in 1979. Improving these estimates is “sort of the holy grail of climate science,” says Zeke Hausfather, director of climate and energy at the Breakthrough Institute and one of the study’s authors.”

Short comment:

How many CO2 doublings from preindustrial levels of 285ppm we are going to see?

We are now at 415ppm (0.7 doublings), a likely range is 0.9 to 1.2 doublings by 2100. (RCP8.5 and SSP5-8.5 involve 1.6 to 2.0 doublings, but are not now plausible.)

The worse case 4c + scenarios are now looking unlikely. But so is sub 2.5c. This still argues for action to limt warming to - say - 3c. But it likely means adapatation/mitigation along with innovation could be higher priorities (not that this area is really on govt agendas at all). 

Links:

Paper here.

Bloomberg article here.