Washington:  It's believed that global warming is quite a recent phenomenon, but a new study has suggested that the world's oceans began warming more than 100 years ago, much earlier than previously believed.

The findings, published in the journal Nature Climate Change, could help scientists better understand the Earth's record of sea-level rise, which is partly due to the expansion of water that happens as it heats up, the researchers said.

"Temperature is one of the most fundamental descriptors of the physical state of the ocean," lead study author Dean Roemmich, an oceanographer at the University of California, San Diego, said.

"Beyond simply knowing that the oceans are warming, the results will help us answer a few climate questions," Roemmich was quoted as saying by LiveScience. The study is the first global comparison of temperature between the historic voyage of HMS Challenger between 1872 and 1876 and modern data obtained by ocean-probing robots.

During the four-year-long voyage, HMS Challenger sailed the world's oceans along a 69,000-nautical-mile track crossing the Atlantic, Indian and

Pacific oceans. Scientists among the 200-member crew on board had taken 300 ocean-temperature profiles, or measurements at several depths in each spot, with pressure-protected thermometers.

Roemmich and his colleagues compared that temperatures with data from the modern-day Argo project, which uses 3,500 free-drifting floats to measure the temperature and salinity, or salt content, of the world's oceans every 10 days.

It showed a 1.1-degree Fahrenheit (0.59-degree Celsius) rise in temperatures at the ocean's surface over the last 135 years, a result corroborated by a large body of sea-surface temperature data that goes back more than 100 years.

"That is a substantial amount of warming. Ocean warming has been previously linked to glacial melting and mass coral bleaching," Roemmich said.
Scientists have previously determined that nearly 90 percent of the excess heat added to Earth's climate system since the 1960s has been stored in the oceans. The researchers also looked at subsurface temperature differences between Challenger and Argo, taking into account several sources of error in the Challenger readings.

One issue with the Challenger data, Roemmich said, is that the scientists onboard didn't directly measure the depth of their thermometers; but measured only the length of the line extending the instruments into the water. Because of ocean currents, it's nearly impossible to get a line to be completely vertical in the water, resulting in an actual depth that is a little less than the full length of the line, he said.

"What you are then going to see is a temperature that is a little warmer than it would have been if the line has been perfectly vertical," he said, referring to the fact that temperatures are typically warmer at shallower depths.

Other Challenger errors include incorrect measurements of pressure effects on the thermometers and faulty thermometer readings, he added.

Accounting for these issues, Roemmich and his team found that, on average, global ocean temperatures increased by 0.59 degrees F (0.33 degrees C) in the upper ocean down to about 2,300 feet (700 meters).

This global temperature change is twice what scientists have observed for the past 50 years, suggesting the oceans have been warming for much longer than just a few decades.

"So that means that the ocean temperature is probably the most direct measure we have of the energy imbalance of the whole climate system," Roemmich added.