Human activities are the prime reason the world's oceans have slowly warmed over the past 40 years - a trend that will continue and will affect Western snowpack, California rainfall and other weather patterns, a new study concludes.

"Fifty years from now, it's going to be several degrees warmer," said Warren Washington, a climatologist at the National Center for Atmospheric Research in Boulder and a co-author of the report, published in today's Science.

"We're going to have major shifts in the precipitation zones," Washington said.

The study comes less than two months after National Aeronautics and Space Administration researchers reported finding a "smoking gun" signal of global warming - Earth absorbing more heat than it released.

The new study is one of the most comprehensive analyses of the temperatures in the oceans, which absorb much of the planet's heat.

Scientists collected detailed data on ocean temperatures across the globe, going back 40 years, and compared them with results generated by computer climate models.

Only when they included climate changes caused by people - the dumping of the greenhouse   gas carbon dioxide into the atmosphere through fossil-fuel burning and other activities, and sulfate pollution - did they find a match between the real data and the models.

"It was a distinctive signal, and for the models to capture it so well is quite remarkable," said co-author Tim Barnett of the Scripps Institution of Oceanography in La Jolla, Calif.

Barnett and his co-authors concluded that humans are changing ocean temperatures and that some computer climate models were so accurate in describing the current ocean conditions that they are probably just as good.