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 human-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.