''It scientifically does not make sense to calculate trends over periods as short as 10 to 15 years,'' Dr Rintoul said. ''We could have just ignored it. We would never have picked any other 10-year period and tried to predict a trend. But because it has been a topic of widespread discussion, it does have some policy relevance. It was debated and decided that we needed to explain what we could say about changes over the recent past.''

The report noted that, calculated from the strong El Nino year of 1998, the rate of warming has been only 0.05 degrees per decade - compared with 0.12 degrees per decade since 1951. It concluded that the variation might involve a redistribution of heat within the ocean, and could be linked to volcanic activity or the solar cycle.

However there was ''medium confidence'' that it was just normal decade-to-decade variability.

A leap in understanding of the oceans played a key part in strengthening evidence for climate change, Dr Rintoul said. In the previous report in 2007 it was known that the oceans took up a huge amount of heat.

But since then scientists have identified biases in ocean data collected in the 1970s and 1980s. Once the data was cleaned up, a much clearer picture emerged. More than 93 per cent of the extra heat stored by the Earth in the past 50 years is found in the ocean, Dr Rintoul said.