“Future warming likely to be on high side of climate projections,” concluded a new analysis by scientists at the National Center for Atmospheric Research (NCAR). And that “higher temperature rise would produce greater impacts on society in terms of sea level rise, heat waves, droughts, and other threats.”
Many in the media have been getting this story wrong — unintentionally lowballing the future warming we should expect this century if the NCAR analysis is correct. For instance, the Washington Post writes, “the world could be in for a devastating increase of about eight degrees Fahrenheit by 2100, resulting in drastically higher seas, disappearing coastlines and more severe droughts, floods and other destructive weather.”
Not quite. The news release makes clear that amount of warming would likely occur well before 2100. Since this confusion is quite common in climate coverage, I’ll quote at length from NCAR to set the record straight:
The most common benchmark for comparing model projections is equilibrium climate sensitivity (ECS), or the amount of warming that eventually occurs in a model when carbon dioxide is doubled over preindustrial values. At current rates of global emission, that doubling will occur well before 2100.
For more than 30 years, ECS in the leading models has averaged around 5 degrees Fahrenheit (3 degrees Celsius). This provides the best estimate of global temperature increase expected by the late 21st century compared to late 19th century values, assuming that society continues to emit significant amounts of carbon dioxide. However, the ECS within individual models is as low as 3 degrees F and as high as 8 degrees F.
At current rates of global emissions, that doubling (to 550 ppm) will occur around mid-century, and we might approach a quadrupling by 2100!
The “good news” is that inherent delays in the climate system mean we don’t hit the ECS immediately upon doubling. The “bad news” is that the ECS ignores key non-equilibrium feedbacks like the release of carbon currently locked in the frozen tundra (see “Carbon Feedback From Thawing Permafrost Will Likely Add 0.4°F – 1.5°F To Total Global Warming By 2100“).
The NCAR release continues:
“There is a striking relationship between how well climate models simulate relative humidity in key areas and how much warming they show in response to increasing carbon dioxide,” Fasullo says. “Given how fundamental these processes are to clouds and the overall global climate, our findings indicate that warming is likely to be on the high side of current projections.”
… Estimates based on observations show that the relative humidity in the dry zones averages between about 15 and 25 percent, whereas many of the models depicted humidities of 30 percent or higher for the same period. The models that better capture the actual dryness were among those with the highest ECS, projecting a global temperature rise for doubled carbon dioxide of more than 7 degrees F. The three models with the lowest ECS were also the least accurate in depicting relative humidity in these zones.
So the study didn’t find, as the Post and other media outlets assert, that “the world could be in for a devastating increase of about eight degrees Fahrenheit by 2100.” The study found the “global temperature rise for doubled carbon dioxide of more than 7 degrees F.”
The temperature rise we would see in 2100 would depend on how much beyond (or below) 550 ppm we are at that time plus the impact of the various feedbacks not incorporated into the ECS. If we hit 1000 ppm, warming would likely exceed 11 degrees F — possibly by a few degrees!
Of course, the first 7 F would devastate civilization, but, even so, 14 F would still be unimaginably worse — rendering large parts of the planet’s currently habited and arable land uninhabitable, superheated dustbowls and rendering large parts of the ocean, superheated, acidic dead zones.
Here is a figure from the release that helps to explain how the NCAR scientists — John Fasullo and Kevin Trenberth — used real-world observations to come to their conclusion:
Computer models that more accurately depict dry conditions in a key part of the subtropical atmosphere are also more likely to predict greater climate warming from increased greenhouse gases. In this graphic, each star indicates one of 16 leading global climate models. The left axis (“warming”) corresponds to equilibrium climate sensitivity (ECS) in degrees C, which is the amount of warming produced by each model when carbon dioxide concentrations in the atmosphere are doubled over preindustrial values. The bottom axis shows May-to-August relative humidity for a portion of the upper atmosphere between about 20,000 to 30,000 feet in height and between about 10° and 25° latitude south in the southern subtropics. (©UCAR. Image by Carlye Calvin, based on Fasullo and Trenberth, Science, 2012.)
Finally, NCAR notes that the findings ” could provide a breakthrough in the longstanding quest to narrow the range of global warming expected in coming decades and beyond.” Climate Central notes:
“It’s a very clever idea,” said Andrew Dessler, a climate scientist at Texas A&M University who studies clouds, among other things. “And it may well be right. They’re sensible people, and I have a lot of respect for them. But more work is needed to flesh out the details.”In a commentary also appearing in Science, Karen Shell of Oregon State University wrote that Trenberth and Fasullo’s approach “is an encouraging step that links observations to climate sensitivity” — that is, to the amount of heating a doubling of CO2 will cause.
But the case isn’t ironclad: relative humidity is clearly related to cloud formation, she noted, but there could be plenty of other factors that are nearly as important.
“In retrospect, this could turn out to have been a breakthrough,” Dressler said. “But we won’t know that for a while.”
The full study can be found here (subs. req’d).