News & Views item

Science's H. Jesse Smith in the journal's 12 April 2013 issue reported on research by Balmaseda, Trenberth and Källén in Geophysical Research Letters DOI: 10.1002/grl.50382.

The addition of greenhouse gases to the atmosphere caused by fossil fuel burning and other anthropogenic activity has caused temperatures at the surface of Earth to increase significantly over the past century and a half, and rapidly during the interval from around 1975 until the early years of the 21st century. However, sea surface and surface-air temperatures have not risen over most of the past decade, encouraging some to question the continued reality of global warming, despite the fact that similar variability also can be seen in the instrumental records of the past century. The real question, then, is not whether climate warming has stopped, but where in the earth system the heat resides that would have caused the expected warming? Naturally, the first place to look is in the ocean, because that is where most of the heat taken up due to global warming is stored. Accordingly, Balmaseda et al. conducted a reanalysis of changes in the global ocean heat content from 1958 through 2009 and found that much of the warming has occurred below depths of 700 m, rather than in the surface ocean, and that much of that redistribution is due to changes in surface winds over that period. This helps to explain why air temperatures have not reflected this heating and shows that global warming is continuing, but out of our daily reach.

Abstract

The elusive nature of the post-2004 upper ocean warming has exposed uncertainties in the ocean's role in the Earth's energy budget and transient climate sensitivity. Here we present the time evolution of the global ocean heat content for 1958 through 2009 from a new observational-based reanalysis of the ocean. Volcanic eruptions and El Niño events are identified as sharp cooling events punctuating a long-term ocean warming trend, while heating continues during the recent upper-ocean-warming hiatus, but the heat is absorbed in the deeper ocean. In the last decade, about 30% of the warming has occurred below 700 m, contributing significantly to an acceleration of the warming trend. The warming below 700 m remains even when the Argo observing system is withdrawn although the trends are reduced. Sensitivity experiments illustrate that surface wind variability is largely responsible for the changing ocean heat vertical distribution.

For those who have accredited access the pdf file can be downloaded from http://onlinelibrary.wiley.com/doi/10.1002/grl.50382/pdf .