Sea change in salinity heralds shift in rainfall

SINGAPORE Thu Apr 26, 2012 2:57pm EDT

A man holding an umbrella watches large waves on the Marina beach as a cargo ship passes after Cyclone Thane hit the southern Indian state of Tamil Nadu December 30, 2011. REUTERS/Babu

A man holding an umbrella watches large waves on the Marina beach as a cargo ship passes after Cyclone Thane hit the southern Indian state of Tamil Nadu December 30, 2011.

Credit: Reuters/Babu

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SINGAPORE (Reuters) - Scientists have detected a clear change in salinity of the world's oceans and have found that the cycle that drives rainfall and evaporation has intensified more than thought because of global warming.

The finding published on Friday helps refine estimates of how different parts of the globe will be affected by increased rainfall or more intense droughts as the planet heats up, affecting crops, water supplies and flood defenses.

Scientists led by Paul Durack of the Lawrence Livermore National Laboratory reported clear changes in salinity patterns across the world's oceans between 1950 and 2000.

Oceans cover 71 percent of the planet's surface and store 97 percent of the world's water and are therefore the main source of moisture in the atmosphere through evaporation.

The global cycle of rainfall and evaporation of water from the land and surface of the ocean comprise the global water cycle, with some areas such as the tropics naturally wetter and others, such as large parts of Australia, the United States or northern Africa, drier.

Some ocean regions are saltier, meaning less rainfall and others are fresher, meaning high rainfall, making salinity measurements a good way to measure changes in rainfall patterns.

Durack and team, in a study published in the journal Science, found that the water cycle intensified 4 percent from 1950-2000, twice as much as projected by climate models.

"These changes suggest that arid regions have become drier and high rainfall regions have become wetter in response to observed global warming," Durack, a post-doctoral fellow, said in a statement.

Scientists have long understood the link between evaporation and rainfall and ocean surface salinity levels but have struggled to accurately quantify the relationship.

MAKING THE LINK

Durack and team combined salinity data from 1950-2000 and the relationship between salinity, rainfall and evaporation in climate models to find that for every degree Celsius of warming at the Earth's surface, the water cycle strengthens by 8 percent.

Temperature data shows the planet heated up by 0.5 deg C between 1950-2000. But climate models suggest the world is on track to warm by 3 deg C by the end of the century unless the current growth of greenhouse gas emissions is quickly halted.

A warming of that magnitude would mean the water cycle intensifying by up to 24 percent, with wet regions getting wetter and dry regions drier.

"This has big implications for dry regions, such as Australia, which are already dry," Durack said in an email to Reuters.

He said he believed his team's work was the first to formally quantify the link between the water cycle and salinity change.

"Once we developed the relationship between salinity and evaporation-rainfall change in models, we could then use this relationship to scale our observed salinity change estimate to provide an inferred evaporation-rainfall change estimate."

He said ocean salinity data might now receive more attention as a health check on the world's climate, boosted by measurements from 3,500 robotic devices called Argos deployed across the world's oceans and satellites.

Over the past decade, the Argo float system has revolutionized the way scientists understand how oceans operate by providing a large amount of data on temperature, salinity and other indicators.

"It's a Goldilocks' time for oceanography because we're had this abundant new source of data from the Argo system that we've been able to use to better understand the oceans and how they are changing," Durack said.

(Editing by Ed Davies)

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