Future warming likely to be on high side of climate projections: analysis
difference is important to reconcile, as a higher temperature rise would produce greater impacts on society in terms of sea level rise, heat waves, droughts, and other threats.
Clouds are one of the main sticking points, say the NCAR authors. Although satellites observe many types of clouds, satellite failure, observing errors, and other inconsistencies make it challenging to build a comprehensive global cloud census that is consistent over many years.
Satellites, however, perform better in measuring water vapor, and estimates of the global distribution of relative humidity have become more reliable. Relative humidity is also incorporated in climate models to generate and dissipate clouds.
Fasullo and Trenberth checked the distribution of relative humidity in sixteen leading climate models to see how accurately they portray the present climate. In particular, they focused on the subtropics, where sinking air from the tropics produce very dry zones where most of the world’s major deserts are located. The researchers drew on observations from two NASA satellite instruments — the Atmospheric Infrared Sounder (AIRS) and Clouds and Earth’s Radiant Energy System (CERES) – and used a NASA data analysis, the Modern-Era Retrospective Analysis for Research and Applications (MERRA).
The seasonal drying in the subtropics and the associated decrease in clouds, especially during May through August, serve as a good analog for patterns projected by climate models.
“The dry subtropics are a critical element in our future climate,” Fasullo says. “If we can better represent these regions in models, we can improve our predictions and provide society with a better sense of the impacts to expect in a warming world.”
Accurate humidity yields higher future temperatures
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.
“Because we have more reliable observations for humidity than for clouds, we can use the humidity patterns that change seasonally to evaluate climate models,” says Trenberth. “When examining the impact of future increases in heat-trapping gases, we find that the simulations with the best fidelity come from models that produce more warming.”
The authors focused on climate models used for the 2007–8 assessment by the Intergovernmental Panel on Climate Change. The next-generation models being used for the upcoming 2013–14 IPCC assessment were found to behave in a similar fashion, as described in a preliminary analysis by the authors in a supplement to their paper.
“In addition to providing a path forward and focus for improving models, results strongly suggest that the more sensitive models perform better, and indeed the less sensitive models are not adequate in replicating vital aspects of today’s climate,” write the authors in the paper.
— Read more in John T. Fasullo and Kevin E. Trenberth, “A Less Cloudy Future: The Role of Subtropical Subsidence in Climate Sensitivity,” Science 338, no. 6108 (9 November 2012): 792-794 (DOI: 10.1126/science.1231083)
