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The more severe climate models are likely the most accurate

A new study from the Carnegie Institution For Science has found that climate models that project more severe warming are the most accurate according to observation of the current climate conditions. The research suggests that climate models used by the Intergovernmental Panel on Climate Change are underestimating future warming.

Climate model simulations are used to forecast future warming for any given increase in the concentration of greenhouse gases in the atmosphere.

“There are dozens of prominent global climate models and they all project different amounts of global warming for a given change in greenhouse gas concentrations, primarily because there is not a consensus on how to best model some key aspects of the climate system,” explained study co-author Patrick Brown.

Climate models based on current emissions indicate that global temperatures will rise between 3.2 to 5.9 degrees Celsius over preindustrial levels by the end of the century.

The researchers set out to determine whether the upper or lower end of this temperature range is more likely to happen. The team started out their investigation based on the idea that climate models are the most useful when they precisely represent conditions of the recent past.

The experts compared model simulations and observations of patterns of energy flow from Earth to space. The study revealed that the models that best represent recent  energy exchanges between the planet and its surroundings project warming that is greater than average.

The findings of the study eliminated the lower end of the temperature range forecast by raw climate models. The results indicated that future average global warming will most likely be 0.5 degrees Celsius greater than what has been projected.

“Our results suggest that it doesn’t make sense to dismiss the most-severe global warming projections based on the fact that climate models are imperfect in their simulation of the current climate,” said Brown. “On the contrary, if anything, we are showing that model shortcomings can be used to dismiss the least-severe projections.”

The uncertainty in predicting future warming is mostly due to discrepancies in the ways that models simulate changes in clouds. The cooling effect caused by clouds as they reflect the sun’s energy away from the planet is shown by some models to increase and by other models to decrease.

“The models that are best able to recreate current conditions are the ones that simulate a reduction in cloud cooling in the future and thus these are the models that predict the greatest future warming,” said Brown.

“It makes sense that the models that do the best job at simulating today’s observations might be the models with the most reliable predictions,” added study co-author Ken Caldeira. “Our study indicates that if emissions follow a commonly used business-as-usual scenario, there is a 93 percent chance that global warming will exceed 4 degrees Celsius by the end of this century. Previous studies had put this likelihood at 62 percent.”

The research is published in the journal Nature.

By Chrissy Sexton, Staff Writer

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