The traditional formula on climate change that says that regions that have a moist climate will experience additional rainfall is no longer valid, scientists say.

The old formula that dry gets drier and wet gets wetter (DDWW) is less universally valid than previously assumed, stressed a team of climate researchers from the Swiss Federal Institute of Technology (ETHZ) in Zurich.

“Our results emphasise how we should not overly rely on simplifying principles to asses past developments in dryness and humidity. This can be misleading, as it cannot do justice to the complexity of the underlying systems,” said Peter Greve from ETHZ and the lead author of the study.

Traditional analyses using metrics could describe climate characteristics above the ocean, but not over land.

In their new study, ETHZ researchers led by co-author Sonia Seneviratne, a professor for land-climate dynamics, took into account the specific climatic properties of land surfaces where the amount of available water is limited compared with the ocean.

For the study, they compared data from between 1948 and 1968 and 1984 to 2004 which allowed them to extract trends in terms of a region’s humidity and dryness.

They found that there was no obvious trend towards a drier or wetter climate across three-quarters of the land and only half of this surface area follows the DDWW principle.

Some regions which should have become wetter according to the simple DDWW formula have actually become drier in the past — this includes parts of the Amazon, Central America, tropical Africa and Asia, found the team.

However, the DDWW principle does still apply to the oceans, concluded the study that appeared in the journal Nature Geoscience.