PESSIMISTIC climate forecasts that predict higher levels of global warming are more likely to be accurate than those presenting a rosier picture, research suggests.

The glass-half-empty outlook arises from a study of how well climate computer simulations capture complex humidity processes.

Scientists compared the models’ results with real humidity effects observed in the tropics and subtropics.

Those that proved most accurate also happened to be the ones predicting the greatest temperature rises from greenhouse gas emissions.

“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 study leader Dr John Fasullo, from the National Centre for Atmospheric Research (NCAR) in the US.

“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 research is published in the latest issue of the journal Science. More than two dozen climate models are used by scientists grappling with global warming, all based on long-established physical laws.

On average, they predict world temperatures to rise by around 3C (5F) by the end of this century, compared with what they were at the start of the industrial age.

But individual models vary, with some predicting rises as low as 1.7C (3F) or as high as 4.4C (8F).

Dr Fasullo’s team checked humidity levels in 16 leading climate models to see how well they portrayed the present climate.

The researchers particularly focused on dry subtropical regions, where most of the world’s major deserts are located.

Observations show that relative humidity in the dry zones averages between about 15% and 25%. However, many of the climate models showed humidities of 30% or higher in these regions.

Models that better captured the actual dryness were among those forecasting the highest levels of global warming, showing temperature rises of more than 3.8C (7F).

The least accurate models were also those which predicted the smallest increases in global temperatures.

Dr Fasullo added: “The dry subtropics are a critical element in our future climate. 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.”