Likewise, 2011 may be the hottest year on record during La Niña, a periodic cooling of the eastern tropical Pacific.

That's a bad sign, since La Niña years are generally relatively cool, said Steven Running, a professor of ecology at the University of Montana, who was not part of the study team.

So the new finding suggests that La Niña conditions that once produced strong global cooling now only slightly affect the overall temperature trend, Running said by email.

"What does it take now to have a cooling cycle?" he asked. "And what will happen in the next strong El Niño?"

El Niño is a warming of tropical waters in the central and eastern Pacific Ocean. During El Niño years,