July was the warmest the world's oceans have been in almost 130 years of record-keeping.

The average surface water temperature worldwide was 17 C, according to the National Climatic Data Center, the branch of the U.S. government that keeps world weather records. That was 0.6 C higher than the 20th-century average, and beat the previous high set in 1998 by a couple hundredths of a degree.

The coolest recorded global surface ocean temperature was 15.1 C in December 1909.

Meteorologists said there's a combination of forces at work this year: A natural El Nino system just getting started on top of worsening man-made global warming, and a dash of random weather variations. The resulting ocean warmth is already harming threatened coral reefs. It could also hasten the melting of Arctic sea ice and help hurricanes strengthen.

The Gulf of Mexico, where warm water fuels hurricanes, has temperatures dancing around 32. Most of the water in the Northern Hemisphere has been considerably warmer than normal. The Mediterranean is about three degrees warmer than normal. Higher temperatures rule in the Pacific and Indian oceans.

The heat 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 Center at the University of Colorado.

Water temperature slow to change

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 warm up and does not cool off 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," 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 is warming in more places than usual, something that has not been seen in more than 50 years.

Add to that an unusual weather pattern this summer where the warmest temperatures seem to be just over oceans, while slightly cooler air is concentrated over land, said Deke Arndt, head of climate monitoring at the climate data centre.

The pattern is so unusual Arndt suggested meteorologists may want to study that pattern to see what's behind it.

Temperature has impact on coral reefs

The effects of that warm water are already being seen in coral reefs, said C. Mark Eakin, co-ordinator 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 — much earlier than usual. Typically, bleaching occurs after weeks or months of prolonged high water temperatures. That usually means September or October in the Caribbean, said Eakin. He found bleaching in Guam Wednesday. It's too early to know if the coral will recover or die. Experts are "bracing 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 flip side, La Nina, happen every few years.

During an El Nino, temperatures on water and land tend to rise in many places, leading to an increase in the overall global average temperature. An El Nino has other effects, too, including dampening Atlantic hurricane formation.

Warm water is a required fuel for hurricanes. What's happening in the oceans "will add extra juice to the hurricanes," Curry said.

Hurricane activity has been quiet for much of the summer, but that may change soon, she said. Hurricane Bill quickly became a major storm and the National Hurricane Center warned that warm waters are along the path of the hurricane for the next few days.

Hurricanes need specific air conditions however, so warmer water alone does not necessarily mean more or bigger storms, said James Franklin, chief hurricane specialist at the National Hurricane Center in Miami.