The world's oceans are warming up and the rise is both significant and real, according to one of the most comprehensive studies into marine temperature data gathered over the past two decades.
Measuring the temperature of the oceans has not been easy, but the scientists behind the latest study believe there is now incontrovertible evidence to show that the top few hundred metres of the sea are warming – and that this temperature rise is consistent with man-made climate change.
The findings are important because ocean temperatures are seen as a more reliable and convincing signal of global warming than land-based measurements, which are prone to huge variability. This is due to the fluctuating influences of the weather and the spread of cities, which can artificially increase local terrestrial temperatures by the urban "heat island" effect.
Scientists involved in the study have looked at temperature recordings gathered by flotation devices that take measurements of the top 700m of the ocean. They conclude that this upper layer of ocean has warmed significantly between 1993 and 2008 – the period covered by the study – and that this is slightly faster than earlier estimates used in the last report of the Intergovernmental Panel on Climate Change (IPCC).
The research team, led by John Lyman of the US National Oceanic and Atmospheric Administration in Seattle, found that the temperature data gathered over the study period point to a "robust warming" of the upper ocean, despite uncertainties over some of the data.
Rising ocean temperatures are important because the sea is a huge "sink" for global heat and carbon dioxide – its capacity to store heat is about 1,000 times greater than the atmosphere. Warmer water is less able to absorb the extra carbon dioxide being pumped into the atmosphere as a result of burning fossil fuels, and as seawater warms it also expands, causing a rise in sea levels. The temperature measurements were gathered using devices originally developed by the military to estimate the speed of underwater sonar signals.
These expendable bathythermograph instruments (XBTs) have recently been replaced by more sophisticated Argos instruments that collect temperatures at different depths as they automatically sink and then resurface.
The use of two different kinds of instruments to measure the same physical phenomenon, and doubts about the accuracy of the early XBT devices, have raised questions about the reliability of the upper-ocean temperature record. This led to the reappraisal by Dr Lyman and his group, which is published in the journal Nature.
"They find a robust warming of the global upper ocean to be present in the data, in spite of considerable uncertainties arising from the observations themselves," said Kevin Trenberth of the US National Centre for Atmospheric Research in Boulder, Colorado.
Since 2003 there has been a slight levelling off of the rise in temperatures and the scientists said that they cannot explain this apparent stalling of ocean warming, because sea levels have continued to rise and the evidence suggests that this must be caused by both thermal expansion of the oceans, as well as the melting of land-based ice sheets and glaciers.
"It hasn't stopped heating up. Its not as if temperatures have stopped rising, it's just that they are not rising as fast as they were. We cannot explain this at present. It may be due to further problems hidden within the ocean data or the way they are processed," Dr Trenberth said.
Peter Challenor, of the National Oceanography Centre in Southampton, said the overall picture is clear – the oceans are warming up. "I'm convinced of that. Everything is consistent with it. The slope is statistically significant, whereas the levelling off in recent years isn't statistically significant," he said. "This study has removed many of the nagging doubts about the details. It shows the warming is real."