Heating up: Ocean temperatures are a key component of measuring climate change.
Oceans in the southern hemisphere are warming faster than anticipated, with implications for rising sea levels and climate modelling.
A team of scientists in California has studied rising temperatures of the southern hemisphere over the decades between 1970 and 2004, and recommended lifting estimates of ocean heat content by between 48 and 152 per cent.
Lead author Paul Durack said it was the first time scientists have been able to quantify how big the gap is between earlier estimates and the reality of rising ocean temperatures.
Sea temperatures are a crucial yardstick for global warming as the ocean stores more than 90 per cent of human-induced excess heat.
Higher sea level temperatures are also closely linked with rising sea levels, because water expands as it warms.
Ocean warming down to two kilometres below the surface accounts for around a third of the annual rate of global mean sea-level rises.
The study by Lawrence Livermore National Laboratory and the California Institute of Technology, published in the journal Nature Climate Change on Monday, attributed the missed estimates to a history of poor sampling of temperatures in the southern hemisphere oceans, which make up 60 per cent of the world's oceans.
The region, which includes the Indian and South Pacific oceans as well as the South Atlantic and Southern oceans, has not been sampled nearly as frequently to date as oceans in the northern hemisphere.
Scientists have typically made conservative estimates about temperature rises when filling in the gaps.
At a global level, the research showed upper-ocean warming has been underestimated by 24 per cent to 58 per cent, Dr Durack said.
The research team measured temperatures directly from the ocean as well as using satellite data and climate modelling. They compared that data to sea-level rises and upper-ocean warming in the northern hemisphere to more accurately map warming trends.
The authors said the results were likely to alter methods behind some climate modelling.
A separate study, also published in Nature Climate Change, showed increasing warming in the upper two kilometres of the ocean from 2005 to 2014 (pictured).
Temperatures below two kilometres, however, barely budged over the period.
The study, also from the California Institute of Technology, used data from Argo, a 3600-strong fleet of robotic floats.
These have been used since 2004 and have dramatically increased the amount of information on ocean temperatures available to scientists.
