World’s oceans keep getting hotter

Assessments of the amount of heat being stored in the world’s oceans have been far too low in the past. Now a team of Californian scientists report that estimates of the ocean’s heat content should be boosted by up to 152%.

Scientists at the Lawrence Livermore National Laboratory and the California Institute of Technology found that oceans in the southern hemisphere are warming much faster than previously thought, with serious implications for rising sea levels and environmental change.

The team studied rising temperatures of the southern hemisphere between 1970 and 2004, and recommended lifting the extent of ocean heat content by 48-152%. Lead author Paul Durack said it was the first time scientists had been able to quantify how big the gap was between earlier estimates and the reality of rising ocean temperatures.

Durack said sea temperatures were a crucial yardstick for global warming because the ocean stored more than 90% of human-induced excess heat. Higher sea level temperatures were also closely linked with rising sea levels because water expanded as it warmed and ocean warming down to two kilometres below the surface accounted for a third of the annual rate of global mean sea-level rises.

In a report in the journal Nature Climate Change, the team said the latest findings corrected a history of poor temperature sampling in the southern hemisphere oceans. These include the Indian, South Pacific, South Atlantic and Southern oceans, which collectively comprise 60% of all the seas on earth.

The scientists measured temperatures directly from the oceans as well as using satellite data and climate modelling. They compared the data with sea-level rises and upper-ocean warming in the northern hemisphere to more accurately map heating trends. Their report says the results were likely to alter methods behind some climate modelling.

In a separate study also published in Nature Climate Change, another group of Californian scientists also found that the upper two kilometres of the oceans had experienced increasing warming between 2005 and 2013, whereas temperatures below two kilometres had hardly moved.

They used data from Argo, a fleet of 3,600 robotic floats set down in 2004 that have markedly boosted the amount of information collected on ocean temperatures. The scientists concluded that “as the dominant reservoir of heat uptake in the climate system, the world’s oceans provide a critical measure of global climate change”.

“Direct measurements of ocean warming above 2,000 metres depth explain about 32% of the observed annual rate of global mean sea-level rise,” the scientists said, noting that the deep ocean below 2,000 metres contributed little to global sea rises.