In a report to be published in Geophysical Research Letters, researchers from NASA and the U.S. Naval Research Laboratory explain why the hottest year on record in 1998 was followed by relatively cool years, when measured on a global scale.

They write that the relative stability in global temperatures observed over the last seven years was mainly due to a decline in incoming sunlight associated with the downward phase of the 11-year solar cycle, combined with a lack of strong El Niño events.

Those two influences have masked the warming caused by CO2 and other greenhouse gases, the researchers said.

Bob Henson of the National Center for Atmospheric Research in Colorado says that if El Niño continues to strengthen, “it's quite possible that the Copenhagen (climate change negotiations) meeting will take place during one of the warmest Decembers in the global record."

Photo: Sergey Tokarev - Fotolia