evidence: The failure of climate models to predict this hiatus has long perplexed scientists and bred some public mistrust in climate models. Climate-change skeptics claim the hiatus is proof that global warming doesn’t exist, and that climate models overestimate greenhouse gases’ warming effects. Marotzke ardently disagrees. He shared with the audience a study published in Nature earlier this year in which he and co-author Piers Forster of the University of Leeds analyzed 114 model simulations of 15-year global mean temperature trends since the beginning of the 20th century. If their analysis showed that models consistently overestimated or underestimated the amount of warming compared to real-world observations, then the models must have a systematic bias. Fortunately the simulations performed fairly well, producing a range of predictions for each 15-year period in which actual observed temperature trends for those periods fell. Even if the observed trends at times fell close to range edges, they were not biased to one side or the other. Although the models didn’t accurately predict the current warming hiatus, which is not unusual, they also failed to predict other accelerated warming or hiatus events. In fact, the models underestimated warming in some periods compared to the observations. "The claim that models systematically overestimate warming from increasing greenhouse gas concentrations is unfounded," said Marotzke.