Climate model predictions on rain and drought wrong, says study

Predictions that a warmer ­climate will lead to more rain for some but longer droughts for others might be wrong.

Predictions that a warmer ­climate will lead to more rain for some but longer droughts for others might be wrong, according to a study of 12 centuries worth of data.

The study, published today in science journal Nature, found there was no difference between 20th-century rainfall patterns and those in the pre-­industrial era. The findings are at odds with earlier studies suggesting climate­ change causes dry areas to become drier and wet areas to become wetter.

Fredrik Ljungqvist and colleagues at Stockholm University analysed previously published records of rain, drought, tree rings, marine sediment and ice cores, each spanning at least the past millennium across the northern hemisphere.

They found that the ninth to 11th and the 20th centuries were comparatively wet and the 12th to 19th centuries were drier, a finding that generally accords with earlier model simulations covering the years 850 to 2005.

However, their reconstruction “does not support the tendency in simulations of the 20th century for wet regions to get wetter and dry regions to get drier in a warmer climate”.

“Our reconstruction reveals that prominent seesaw patterns of alternating moisture regimes observed in instrumental data across the Mediterranean, western USA and China have operated consistently over the past 12 centuries,” the paper says.

The research also highlights the importance of using paleo­climate data to place recent and predicted rainfall-pattern changes in a millennium-long context, the report says.

Matthew Kirby from the ­department of geological sciences at California State Univer­sity said the Ljungqvist study “adds fuel to the fiery debate” about how warmer conditions will affect­ rainfall and drought.

He said the findings did not invalidate current predictive models. “But they do highlight a big challenge for climate modellers, and present major research opportunities both for modellers and for climate scientists who work with proxy data,” he said.

Professor Steven Sherwood, from the Climate Change ­Research Centre at UNSW, said the Ljungqvist study was “a nice study and a valuable proxy dataset, but it has many complexities in the way it was done”. “If this paper’s conclusion about model overprediction holds up to further­ scrutiny it will be extremely interesting,” he said. “I am not convinced that (it) will.”