Extremes in weather more likely - scientists
Wet areas have become wetter and dry areas drier during the past 50 years due to global warming, a study of the saltiness of the world's oceans by a team including CSIRO researchers has shown.
The intensification of rainfall and evaporation patterns, which is occurring at twice the rate predicted by climate change models, could increase the incidence and severity of extreme weather events in future.
The team's leader, Paul Durack, said the finding was important because reductions in the availability of fresh water posed more of a risk to human societies and natural ecosystems than a rise in temperature alone.
"Changes to the global water cycle and the corresponding redistribution of rainfall will affect food availability, stability, access and utilisation," said Dr Durack, a former CSIRO researcher now at the Lawrence Livermore National Laboratory in California.
The fact that hotter air can hold more water underpinned predictions that recent warming of the globe's surface and lower atmosphere could have already strengthened the natural evaporation and precipitation cycle – increasing rainfall where it was higher than average and decreasing it where it was lower.
Initial attempts to study this "rich get richer" effect, however, were hindered by a shortage of good rainfall records on land and a lack of long-term satellite measurements. So Dr Durack and his Australian colleagues studied the oceans.
"The ocean matters to climate," said Richard Matear, a CSIRO researcher and member of the team. "It stores 97 per cent of the world's water and receives 80 per cent of all the surface rainfall." The team analysed about 1.7 million records of surface sea salinity collected worldwide between 1950 and 2000.
Their results are published in the journal Science. They found regions near the equator and the poles, where greater rainfall keeps surface waters less salty than average, had become even fresher during the past half century. Saltier areas, such as in the centre of oceans where evaporation dominated, had become even saltier.
Brian Soden, a meteorologist at the University of Miami in the US, said the study had important implications for extreme weather.
Warmer water moving faster from the surface into the atmosphere could fuel violent storms, and floods and droughts could become more intense.
Susan Wijffels, a CSIRO researcher and team member, said a network of 35000 Argo floats throughout the world's oceans would be vital for continued observation of salinity changes.