One of the ways in which climate scientists evaluate the role of anthropogenic greenhouse emissions in the recent warming of Earth’s climate is to run climate models both with and without human activities. By comparing the results of each to the observed temperature trend, these “fingerprinting” studies can show how much of the temperature record can be explained by natural factors (such as solar activity and volcanic eruptions). This has commonly been applied to trends in atmospheric temperatures (as shown in the 2007 IPCC report), where it’s clear that the observed warming wouldn’t have happened without rising greenhouse gases.

Increasingly reliable records of ocean temperatures have now allowed some of these same researchers to confidently apply the technique to Earth’s seas. This is important because some 90 percent of all the energy trapped by human greenhouse emissions has ended up in the ocean, not the atmosphere. The trend with ocean heat content is clear—it’s rising. The question is whether that rise could be caused by natural variations.

Researchers averaged the results from a number of climate models, and compared that to global temperature records for the upper 700 meters of the ocean from 1960 to 1999. The temperature record is less complete for the deep ocean, and its massive volume and separation from the surface subdues its response to climatic changes. In addition to the global average, they also analyzed each of the major ocean basins (North and South Atlantic, North and South Pacific, North and South Indian) separately.

They found that the anthropogenic “fingerprint” was apparent in the observed temperature record at the 99 percent confidence level. That means the observed warming is beyond the variability seen in model simulations where greenhouse gases are kept constant, but is exactly what the models predict for a world in which humans change the composition of the atmosphere.

That result should hardly be a surprise at this point (though it is important). Perhaps it's more interesting to look at the differences between ocean basins. The Atlantic Ocean is warming considerably faster than the others—especially the North Atlantic, which is warming at about double the global average rate. This behavior, too, was simulated by the models.

This actually isn’t the first study to find an anthropogenic ”fingerprint” in ocean temperatures. However, previous work was limited to individual ocean basins and comparisons with just a couple of climate models. By utilizing multiple global data sets of ocean temperature (in which researchers have carefully accounted for the various measurement methods that have been used) and a larger number of the models that were used for the last IPCC report, this study has generated the strongest analysis to date.

Nature Climate Change, 2012. DOI: 10.1038/NCLIMATE1553 (About DOIs).