What would happen if the Earth’s temperature went up a couple of degrees, or if carbon dioxide (CO2) emissions doubled? All signs point to terrible effects, from widespread heatwaves to negative impacts on biodiversity. Now, scientists’ predictions have unexpectedly gotten even worse.

Some recent models have even predicted temperature increases in excess of 5 degrees celcius if CO2 emissions, which are setting records, double—an increase of just 3 degrees is expected to be devastating for humanity. In a study of these recent models, researchers found a surprising reason for the change: clouds.

Climate modelers rely on a centralized set of guidelines that enables comparison among their models. The models from this generation will be used to inform this year’s Intergovernmental Panel on Climate Change (IPCC) report and shape policy decisions to mitigate climate change. The world is already worryingly close to the temperature limits set in the Paris climate agreement, so the predictions of the latest climate models come as dire news.

“Most of the differences among models in climate sensitivity come from the clouds,” said Paulo Ceppi, a co-author of the study. “And that's basically because it's really hard to simulate clouds accurately in climate models.”

Ceppi and his co-authors compared models from the two most recent generations of guidelines based on climate sensitivity, a metric that describes the amount of surface warming a doubling in CO2 emissions would produce. Their study was published last month in the journal Geophysical Research Letters.

While the previous generation of climate models predicted on average a climate sensitivity of 3.3 Kelvin, the authors found that the most recent models average a climate sensitivity of 3.9 K. Additionally, the ranges of climate sensitivity produced by these models grew, from between 2.1 and 4.7 K to between 1.8 and 5.6 K.

Mark Zelinka, the first author of the study, said in an email that the only dramatic change between the two generations of models was how climate warming due to cloud cover had been incorporated. He explained that clouds reflect some sunlight that hits them, and as the planet warms from CO2 emissions, cloud cover will decrease. This in turn causes a positive feedback loop and further warms the climate.

Ceppi added that clouds also have their own greenhouse effects and can act as a blanket in the same way as greenhouse gases. The strength of this property depends on how high a cloud is, adding another layer of complexity that has only recently been incorporated into models.

Just because clouds are being modeled more realistically does not make the model as a whole better, though. In a preprint under review for the journal Earth System Dynamics, researchers at the University of Exeter looked at whether high estimates of climate sensitivity made sense in the context of historical observations.

First author Femke Nijsse said she was spurred to look into sensitivity because the last IPCC report considered a likely range to be lower than what several new models have predicted.

“We were quite surprised that this new generation of models showed quite a few models with a very high sensitivity,” she said.

Nijsse and her team found that at least six current models were inconsistent with historical climate data, likely because of the new cloud modelling.

Ceppi agreed that some of the models predicting a higher sensitivity than before may be unrealistic; this presents a “bit of tension,” since these new models were designed to better represent the climate processes that occur in clouds.

“On the one hand, they should be better, but on the other hand, at least some of these models seem to produce sensitivities that are too high compared with observed temperature changes,” he said. “That's a bit of a puzzle there, that's something we need to resolve.”

While there is some disagreement among the different climate models, Zelinka stressed that the next IPCC report, scheduled for release in 2022, will take into account a number of measurements concerning climate sensitivity. Even so, differences of a Kelvin or two still lead to similar general conclusions, he said.