The rate of warming of the top 700 metres of the ocean in the Southern Hemisphere may have been underestimated, reports a paper published online in Nature Climate Change, whereas another independent study finds the deep ocean (below a depth of 2 kilometres) has not warmed and has had a negligible contribution to sea-level rise for the period 2005 to 2013. The two studies provide insights into where heat is being stored in the ocean and the implications for atmospheric warming and sea-level rise.

The ocean is a major heat reservoir for the climate system, storing over 90% of heat caused by human activities.

Paul Durack and colleagues combine climate model results with direct measurements and satellite sea-level data to study upper-ocean warming trends since the 1970s. They find that the reported observational warming rates have poor agreement with the model results and they attribute this to poor sampling in the Southern Hemisphere. Using data from the better sampled Northern Hemisphere, they suggest that estimates of the warming rates for the Southern Hemisphere should be increased by 48-152%.

To investigate deep-ocean warming in the context of sea-level rise and the global energy budget, William Llovel and colleagues use satellite sea level and ocean mass change data, in combination with direct measurements from Argo floats, which profile the water column from the surface to a depth of 2 kilometres. They find that the deep ocean did not warm over the period 2005-2013.

In an accompanying News and Views, Gregory Johnson and John Lyman discuss how the studies “highlight the importance of ocean temperature measurements to understanding and predicting our changing climate - including future increases in atmospheric temperature and sea level.”