NASA today announced the results of a new study by the National Center for Atmospheric Research. It shows that climate model projections that predict a greater rise in global temperature are more likely to be accurate than those predicting a lesser rise. NASA stated that the findings could provide a "breakthrough" in predicting the range of global warming expected in the future. The study was published this week in the journal Science.
The study looked at 16 leading climate models and observed how well each reproduces observed relative humidity in Earth's tropic and subtropic regions. They compared the models with data from NASA satellite instruments called the Atmospheric Infrared Sounder (AIRS) and the Clouds an Earth's Radiant Energy System (CERES), as well as a NASA data analysis named the Modern-Era Retrospective Analysis for Research and Applications (MERRA). The study's findings show that the climate models that more accurately show observed relative humidity also show the greatest amounts of warming as a result of greenhouse gasses in 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," said John Fasullo, research scientist at the National Center for Atmospheric Research. "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."
The study focused on dry subtropics, NASA stated, because seasonal drying and the associated decrease in clouds are similar to patterns projected by climate models. "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," said Fasullo.
NASA stated that because established physical laws that guide the atmosphere are difficult to translate into software, each climate model differ slightly in its predictions. In particular, those associated with clouds are too small because satellite failure, observational errors, and "other inconsistencies" make a consistent global cloud census difficult. Satellites such as the AIRS, though, are more reliable at measuring water vapor and estimating the global distribution of relative humidity.
"These results were hiding in plain sight," said AIRS Eric Fetzer, an AIRS project scientist who was not involved in the study. "We have known for 30 years that clouds complicate climate forecasts, but instead of looking directly at clouds, this study examines clear regions. Their conclusions indicate that better model physics for clear areas will lead to improved climate forecasts, but warming is likely to be at the high end of current forecasts."
(Image courtesy NASA/JPL-Caltech)