A United States climate study says global warming will cause Australian deserts to get bigger and expand to the south.

The study's lead researcher William Lau, of the University of Maryland, said changes in atmospheric circulation was contributing to global dryness, in terms of a reduction of relative humidity in the sub tropics.

Dr Lau said he believed the changes were directly related to prolonged droughts.

"We provide a physical basis for inferring that greenhouse warming is likely to have contributed to the observed prolonged droughts worldwide in recent decades," he said.

The research was published this week in the US Proceedings of the National Academy of Science.

Dr Lau's study looked at the Hadley cell, which moves warm air away from the tropics, and found a strengthening of the cell manifested in a "deep tropics squeeze" leading to increased frequency of dry events in particular latitudes.

"What we found is that any region in the sub tropics that has a desert region, they have a semi-arid region, those climatologically are the zones of sinking air motion," he said.

"What we've found is sinking air motion is actually expanding, so all those areas at the edge of the deserts, are being expanded, that's why Australia is affected in that way.

"In addition, regions within the tropics ... they actually lose moisture, because moisture is being transported into the heavy precipitation region, at the expense of the neighbouring region which actually gets moisture divergence, so Australia it has its own desert, it has its own sub-tropical dry zone and that area is part of that global drying pattern that we see in the model."