Here's a consistent trend—just before the annual United Nations Climate Conference convenes, climate researchers take the opportunity to publish their more alarming findings. On November 26, the 18th Confererence of the Parties (COP-18) to the U.N. Framework Convention on Climate Change will meet in lovely Doha, Qatar. Of course, this is not skullduggery, just the prudent pursuit of publicity.
As example of this phenomenon, the journal Science is publishing today an article, "A Less Cloudy Future: The Role of Subtropical Subsidence in Climate Sensitivity," (sub required) which aims to refine predictions of climate sensitivity. Climate sensitivity is generally defined as how much a doubling of carbon dioxide in the atmosphere would boost average global surface temperature. The new study looks at how well 16 prominent climate models simulate the observed relative humidity of the earth's subtropical regions. They focus on this because current climate models do not handle the effects of clouds very well and the goal is to associate seasonal changes in cloudiness over the subtropics with changes in humidity. As the press release for study explains:
Climate model projections showing a greater rise in global temperature are likely to prove more accurate than those showing a lesser rise, according to a new analysis by scientists at the National Center for Atmospheric Research (NCAR)….
NCAR scientists John Fasullo and Kevin Trenberth, who co-authored the study, reached their conclusions by analyzing how well sophisticated climate models reproduce observed relative humidity in the tropics and subtropics.
The climate models that most accurately captured these complex moisture processes and associated clouds, which have a major influence on global climate, were also the ones that showed the greatest amounts of warming as society emits more greenhouse gas into the atmosphere.
"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."
How much warming are the researchers talking about? The consensus estimate of climate sensitivity has hovered around 5 degrees Fahrenheit for the past 30 years. The new study reports:
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 [equilibrium climate sensitivity], projecting a global temperature rise for doubled carbon dioxide of more than 7 degrees F [emphasis added]. The three models with the lowest ECS were also the least accurate in depicting relative humidity in these zones.
Keep in mind that it is generally agreed that the earth's average temperature has increased by about 1.5 degrees Fahrenheit over the past century as atmospheric carbon dioxide increased from the pre-industrial 280 parts per million (ppm) to 395 ppm now.
If carbon dioxide emissions continue at their current levels or increase, atmospheric carbon dioxide will double before the end of this century. If this study is right, it's going to get a lot toastier around planet earth than many climate models are projecting. But some researchers disagree.
In September, University of Alabama in Huntsville climatologist John Christy testified [PDF] before the House Energy and Power Subcommittee where he compared how well the leading climate models matched actual trends in global average temperature. See chart below:
Note the the actual data cited by Christy are the circles and squares at the bottom and include two different satellite data series and data series from the National Climatic Data Center and NASA. In Christy's analysis nearly all of the climate models are projecting much higher global average temperatures than the thermometers and satellites are finding. Yes, indeed the science is settled.