The oceans got hotter than we thought, but the heat stayed shallow | Ars Technica

Study shows we underestimated Southern Hemisphere warming.

A CTD (conductivity, temperature, depth profiler) is lowered into the water. This is the standard tool for oceanographers making measurements from a ship.
Andrew Meijers/BAS

Of the energy added to the climate system by rising concentrations of greenhouse gases, more than 90 percent has gone into the ocean. The monitoring of ocean temperatures has improved drastically over the last decade with the deployment of a vast fleet of Argo floats that drift around being our eyes and thermometers. Even so, they don’t yet cover depths greater than 2,000 meters, and their presence today doesn’t make up for their absence in decades past.

Fortunately, time travel with gadgets from the future isn’t the only way to improve our knowledge of what’s gone on in the deeps. Ocean warming also manifests itself in another way—as rising sea level. Seawater expands ever so slightly with increasing temperature. And given how absolutely massive the world ocean is, “ever so slightly” adds up. In fact, thermal expansion and melting ice have made roughly equal contributions to sea level rise so far.

Going deep

There’s been a lot of interest in recent years in quantifying the warming of the deep ocean, but not much is currently known about what’s going on below 2,000 meters. In a new study published in Nature Climate Change, a group led by William Llovel at NASA’s Jet Propulsion Laboratory combines sea level rise measurements with Argo data to look for the effect of warming in the deeps.

The researchers used precise satellite measurements of sea level rise between 2005 and 2013 and subtracted satellite measurements of glacial shrinkage to calculate the amount of sea level rise due to ocean expansion—around 0.8 millimeters per year, plus or minus 0.3 millimeters. The Argo temperature measurements, on the other hand, indicate that the upper 2,000 meters of the ocean warmed enough to raise sea levels 0.9 millimeters per year, plus or minus 0.15 millimeters.

That means that, within the uncertainty, there’s nothing left to be accounted for by temperature change below 2,000 meters. If those depths have warmed over the short time period since 2005, it can’t have been by very much. The warming—and there has been plenty—has taken place in the upper 2,000 meters.

Looking shallow

Another paper published in Nature Climate Change this week takes a similar tack to answer a different question: how much energy accrued in the upper 700 meters of the oceans between 1970 and 2004?—the latter date being when Argo started to come online. It’s not that we don’t have a lot of measurements, but the researchers (led by Lawrence Livermore National Laboratory’s Paul Durack) noted that most climate model simulations show more ocean warming in the Southern Hemisphere over that time than appears in our measurement-based estimates.

When a model disagrees with an observational dataset, it’s usually the model’s problem—but not always. Measurements were much more sparse in the Southern Hemisphere, so it wouldn’t be a shock to learn that something was missed. The fact that the discrepancy shrank as Argo floats began deployment at the end of this time period is also suspicious; previous studies had already suggested that Southern Hemisphere estimates were low, as well.

To investigate, the researchers turned, once again, to sea level. They compared the simulated sea level rise in the models to our satellite observations of sea level rise, which go back to 1992. There was no mismatch, not even in the Southern Hemisphere. The sea level rise resulting from the expansion of warming water implies that the models are right about the greater Southern Hemisphere ocean warming.

The researchers calculated the ratio of ocean expansion between the two hemispheres, which nearly matches the ratio of ocean volume (the Southern Hemisphere accounts for 60 percent of that). It follows that the ratio of heat energy absorption should be the same, yet the observational datasets showed the Southern Hemisphere accounting for only 35 to 49 percent of warming.

Increasing Southern Hemisphere heat uptake provides a simple prediction of what our measurements should have captured, had they been more complete. Depending on which dataset you use, that would increase the global heat uptake in the upper 700 meters by 24 to 58 percent for this time period, although it’s closer to 15 percent greater than the best estimate given in the latest IPCC report. That’s a lot of additional energy accounted for.

Sea level rise is complicated, and this broad correction will likely be refined in the future. Still, so long as it’s close to accurate, it’s quite important, as it implies that our greenhouse gas emissions have already added more energy to the climate system than our bookkeeping had revealed. For example, several recent studies have attempted to estimate our climate’s sensitivity to carbon dioxide emissions using simple equations representing observed emissions and observed warming—and come up with numbers lower than the consensus values. Increasing estimates of ocean warming will necessarily increase those calculated sensitivities somewhat.

Nature Climate Change, 2014. DOI: 10.1038/NCLIMATE2387, 10.1038/NCLIMATE2389 (About DOIs).

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