Since 1997 the amount of heat stored in the oceans has doubled the amount of heat stored since 1865. The so called “warming hiatus” was anything but for the oceans which have warmed as much in the past 20 years of accelerating greenhouse gas production as they did in the first 130 years of human industrial activity and forest destruction. The oceans have taken up over 90% of the excess heat resulting from skyrocketing greenhouse gas emissions leading to stronger storms over the oceans bringing heavier rains and larger floods to western Europe. Surprisingly, the deep oceans, over 2000 feet deep, are taking up about one third (35%) of the excess heat resulting from the global energy imbalance caused by increasing concentrations of atmospheric greenhouse gases.
Measuring the heat content of the oceans, especially the deep oceans, is a very difficult problem and there’s uncertainty in the data before modern ARGO floats were deployed. However, because over 80% of the heat uptake has happened since the end of World War II, uncertainty in the early years does not appear to affect the analysis much. What’s clear about the early years is that 4 major volcanic eruptions and a deep solar minimum led to a naturally cool period from 1880 to 1910 and the climate and the oceans have been warming since then. The oceanographers found the the observed data agreed with the combined results of climate models. Earlier studies that concluded the models were overestimating oceanic heat uptake were proven wrong. Those studies apparently underestimated the heat content increases in the sparsely sampled southern oceans.
A number of recent studies have shown that the Indian and Atlantic oceans have been warming rapidly but a key study from 2013, referenced in the article just published in Nature Climate Change, may explain why the deep oceans are taking up so much heat. Deep water formation around Antarctica is faltering. As the climate warms winds are tightening up around Antarctica. These tightening winds have changed the ocean currents, reducing downwelling which helps drive deep water formation. Moreover, relatively warm intermediate layer waters are melting the Antarctic glaciers from below at a depth of over 300 feet (100meters). The melting from below is creating a light fresh layer of water which floats on the surface, inhibiting deep water formation. Thus the deep waters around Antarctica are warming as cold deep water formation falters. Moreover, the surface fresh water layer slows winter heat loss from the warm intermediate waters, adding to the warming of intermediate layer waters in the southern oceans.
Jim Hansen’s draft paper studied a sudden, catastrophic rise in sea level at the end of the last interglacial period about 120,000 years ago. This rapid sea level rise was apparently caused by warming in the Antarctic region. That’s why Jim Hansen concluded that 2 degrees Celsius (3.6 degrees F) is not a safe level of warming. Temperatures at the end of the last interglacial were only about one half of a degree Celsius above what they are today. Rapid melting of Antarctic ice kept Antarctic surface waters and air temperatures cool then while the oceans heated up.
This new study by Lawrence Livermore and NOAA climate scientists may be showing that the climate and oceans are beginning to repeat the processes that caused rapid sea level rise at the end of the last interglacial period 120,000 years ago.