Corals reefs are suffering a severe underwater heat wave this year for the third time on record, including a mysterious warm patch in the Pacific known as "The Blob," scientists said on Thursday.

The bout of record high temperatures in parts of the oceans, stoked by climate change, is expected to kill more than 12,000 sq kms (4,600 sq miles) of reefs, or about 5 percent of the global total, they said.

The experts, including the U.S. National Oceanic and Atmospheric Administration (NOAA), said similar alerts about damage to reefs had been issued only in 2010 and 1998, both extremely warm years.