President Trump, his top officials, and Republican leaders in Congress propose to dial back action on climate change, arguing that the scientific consensus on human induced-climate change is unconvincing. That makes resolving scientific uncertainties all the more important. A mathematical analysis published today in the journal Nature Climate Change could explain one of the hottest disputes in climate science: just how sensitive Earth’s climate is to rising levels of CO2.
The metric targeted by University of Washington climatologist Kyle Armour in today's report—equilibrium climate sensitivity—is the warming at Earth’s surface caused by a doubling of atmospheric CO2. A doubling to 560 parts per million since the Industrial Revolution could occur by mid-century if global economies adopt the Trump Administration’s animosity towards climate action and fossil fuel consumption continues unabated.
Armour’s analysis affirms the range of possible climate sensitivity provided by climate models and the IPCC, which some recent studies argue is too high. His analysis also highlights a need for better satellite equipment to narrow the range—including missions that the Trump Administration placed on the chopping block last month.
Most of the extra heat that Earth has absorbed since the industrial revolution is soaked up by the oceans—more than 90 percent according to the Intergovernmental Panel on Climate Change (IPCC). Measurement of that heat has improved greatly in recent decades thanks largely to the growing array of Argo buoys. There are now 3,947 Argos freely floating around the globe.
Climatologists are using their data to make increasingly confident estimates of Earth’s present warming, and then comparing that to atmospheric CO2 concentrations to calculate the sensitivity of the climate. One highly-cited 2013 paper in Nature Geoscience pegs climate sensitivity at 1.9-2.0º C.
That is towards the low end of the 1.5 - 4.5º C range for climate sensitivity endorsed by the IPCC in 2014, and below the 2.2-4.7º C range predicted by leading climate models. This has led some researchers to conclude that climate models simulate a warmer future than is warranted.
Armour’s paper argues otherwise. He asserts that climate models are misunderstood, rather than oversensitive. The answer to the apparent discrepancy between observations and climate models, he says, is that climate sensitivity is a moving target.
It has been noted for several decades that climate models tend to predict that Earth will become more sensitive to CO2 as, for example, polar ice melts, exposing open ocean and land that absorb rather than reflect sunlight. Armour’s work tracks 21 leading models and quantifies the impact.
On average the models simulate a world 150 years hence that is 26 percent more sensitive to CO2 than under present conditions. Armour says this is most likely due to feedback mechanisms that have yet to take off fully, such as the increased absorption of sunlight at the poles as reflective ice sheets and sea ice melt away.
When Armour factored rising sensitivity into that 2013 observation-based Nature Geoscience report and recalculated climate sensitivity, he got a best estimate of 2.9º C—a value well within the IPCC’s consensus range and the range predicted by models. “It reconciles the models with the observations. There’s no evidence that the models are too sensitive,” he says.
Nicholas Lewis, an independent U.K.-based climate scientist and one of the 2013 report's coauthors, says Armour may be overstating the rise in climate sensitivity. By Lewis’ calculations the increase in climate sensitivity over time is more likely closer to 12 percent, rather than 26 percent.
And he argues that even that smaller bump could turn out to be a figment of the models. "There [is] no observational evidence that climate sensitivity increases with time in the real climate system,” writes Lewis in an email to Spectrum.
For Armour the biggest caveat in his research is the “huge range” of sensitivity shift predicted by the 21 models. While sensitivity never decreased during a model run, it remained flat with a few models and doubled with others.
Armour believes that models reach different outcomes largely by making "different bets" as to how warming will affect cloud cover in different regions. The problem is that today’s best observations can not distinguish which models are simulating cloud feedback ‘correctly’. "We must find ways to place observational constraints on this," says Armour.
Ocean readings have the required accuracy, but lack the resolution. Satellites provide the opposite—a global high-resolution view without the certainty to pick out the signal of warming-induced change amidst the system’s natural variability.
It is precisely the problem targeted by NASA's Climate Absolute Radiance and Refractivity Observatory (CLARREO), one of the four missions that the Trump Administration is asking Congress to cancel. CLARREO Pathfinder, the mission's first phase, would pack a finely calibrated spectrometer designed to cross-calibrate optical sensors on the entire fleet of U.S. and international Earth-observing satellites. Ultimately the mission promises to improve the longterm accuracy of space-based data fivefold to tenfold.
Backers of a complementary U.K.-based mission in development called TRUTHS estimate that their equipment could slash the time required to understand cloud response. Achieving a level of confidence that would require 25-40 years of data from current satellite technology, they say, could be achieved in 12 years with TRUTHS.
Armour says it makes no sense to scrap missions such as CLARREO as a response to uncertainty in climate science: “These are critical programs. They are literally our eyes in the sky.”
It is a point where he and Lewis tend to agree. Lewis says that “CLARREO's contribution of more accurate and comprehensive data is likely to speed up the reduction in uncertainty,” in estimates of climate sensitivity. That, he says, is a good investment: “I think it better to focus a higher proportion of funding on improving observational data and building up accurate long term records… So in principle I am all for projects like CLARREO and TRUTHS."