Seth Borenstein, Washington
JULY was the hottest month for the world's oceans in almost 130 years of record keeping.
The average water temperature worldwide was 17 degrees Celsius, according to the National Climatic Data Centre, the branch of the US Government that keeps world weather records.
June was only slightly cooler, while August could set another record, scientists say.
The previous record was set in July 1998 during a powerful El Nino in the Pacific. The coolest recorded ocean temperature was 15 degrees in December 1909.
Meteorologists say there is a combination of forces at work: a natural El Nino weather pattern just getting started on top of worsening man-made global warming, and a dash of random weather variations.
Already the resulting ocean heat is harming threatened coral reefs. It also could hasten the melting of Arctic sea ice and help hurricanes to strengthen.
The Gulf of Mexico, where warm water fuels hurricanes, has temperatures dancing around 32 degrees. Most of the water in the northern hemisphere has been considerably warmer than normal.
The Mediterranean is about 3 degrees warmer than normal. Higher temperatures rule in the Pacific and Indian oceans.
The phenomenon is most noticeable near the Arctic, where water temperatures are as much as 5.5 degrees above average.
The tongues of warm water could help melt sea ice from below and even cause thawing of ice sheets on Greenland, said Waleed Abdalati, director of the Earth Science and Observation Centre at the University of Colorado.
Breaking heat records in water is more ominous as a sign of global warming than breaking temperature marks on land, because water takes longer to heat up and does not cool as easily as land.
''This warm water we're seeing doesn't just disappear next year; it'll be around for a long time,'' said climate scientist Andrew Weaver of the University of Victoria in British Columbia.
It takes five times more energy to warm water than land.
The warmer water ''affects weather on the land'', Dr Weaver said.
''This is another yet really important indicator of the change that's occurring.''
Georgia Institute of Technology atmospheric science professor Judith Curry said water was warming in more places than usual, which has not been seen in more than 50 years. Add to that an unusual weather pattern this northern summer where the warmest temperatures seem to be just over oceans, while slightly cooler air was concentrated over land, said Deke Arndt, head of climate monitoring at the climate data centre.
The pattern is so unusual that he suggested meteorologists may want to study this pattern to see what is behind it.
The effects of that warm water already were being seen in coral reefs, said C. Mark Eakin, coordinator of the National Oceanic and Atmospheric Administration's coral reef watch.
Long-term excessive heat bleaches colourful coral reefs white and sometimes kills them.
Bleaching has started to crop up in the Florida Keys, Puerto Rico and the Virgin Islands. Typically, bleaching occurs after prolonged high water temperatures.
That usually meant September or even October in the Caribbean Sea, said Dr Eakin. He found bleaching in Guam on Wednesday.
It is too early to know whether the coral will recover or die.
Experts were ''bracing themselves for another bad year'', he said.
The problems caused by the El Nino pattern are likely to get worse, the scientists say.
An El Nino occurs when part of the central Pacific warms up, which in turn changes weather patterns worldwide for many months. El Nino and its cooling flipside, La Nina, happen every few years.