Climate scientists on Wednesday suggested that they may be able to rule out some of the most dire scenarios of what would happen if greenhouse gas levels in the atmosphere were to double.
Unfortunately, the same scientists say the best-case scenarios are also probably unrealistic.
How a doubling of atmospheric greenhouse gases would affect the climate is of tremendous importance, as humans are running out of time to avoid that outcome.
With current atmospheric concentrations at 405 parts per million, as opposed to about 280 parts per million before the dawn of the industrial era, the planet is already about halfway there.
In the new study in the journal Nature, Peter Cox and Mark Williams of the University of Exeter and Chris Huntingford of the United Kingdom's Centre for Ecology and Hydrology attempt to recalculate the "equilibrium climate sensitivity", a highly influential metric that describes how much the planet will warm if carbon dioxide doubles and the Earth's climate then adjusts to the new state of the atmosphere.
For decades, climate studies have found a very broad range of possibilities for this key figure - somewhere between 1.5 and 4.5 degrees Celsius, with 3 degrees Celsius (or 5.4 degrees Fahrenheit) sitting smack in the middle of the range.
But the inability to further narrow this estimate has been a big problem, Cox said.
"The problem is the low end sort of implies maybe you don't need to do much about climate change except adapt, and the high end implies it's kind of too late," he said.
Recently, however, different teams of scientists have hit upon an approach to try to narrow the results - trying to find an "emerging constraint" on what the possible future outcome could be by comparing high-powered climate change models, which predict the future, with currently observed elements of present climate.
"You find something in the models which is well correlated to future response and can potentially be observed in the real world," Ben Sanderson, a climate expert at the National Center for Atmospheric Research, said of the approach. Sanderson was not involved in the current study.
To that end, Cox and his colleagues focused on a simple measurement: How much the climate varies on an annual basis, irrespective of greenhouse gas emissions.
"What we did is look at how the variation from year to year, having taken out the long-term trend, was related to the sensitivity of the system, across all models," Cox said.
The result allowed the scientists to narrow the probable climate sensitivity range to between 2.2 and 3.4 degrees Celsius, with a central figure of 2.8 degrees C (5.04 degrees Fahrenheit), somewhat lower than the previous central estimate.
They said this amounts to narrowing the uncertainty around the figure by 60 percent.
"We think the likely range is 2.2 to 3.4, so that kind of very much downplays high climate sensitivities above 4, and low climate sensitivities below 2," Cox said.
The study asserted that there is less than a 3 percent possibility that the climate sensitivity is lower than 1.5 degrees, and less than a 1 percent possibility that it is higher than 4.5 degrees.
If correct, this would be very good news - suggesting that we may not have to reckon with truly hellish temperature increases by the end of this century.
"I should thank Cox and colleagues for helping me to sleep a little easier in my bed at night," Piers Forster, a climate scientist at the University of Leeds in the United Kingdom, wrote in a commentary on the new study, also published in the journal Nature.