Measuring Ocean Warming

The surface of the Earth has been warming for decades now (pretty much my whole life).  This isn’t news.

But the “surface” of the Earth—that tiny little slice where all humans live—is pretty complicated, notably including vast oceans of water and air which absorb and move around heat.  In fact, heating one place and time may result in cooling in another place or time.  And, of course, there are vastly complicated side effects, particularly on living things.  Relatively small changes in average temperature may have huge and catastrophic effects on biology—and that’s what’s likely to kill us.

While the headline numbers are about the atmosphere, the course an defects of increased atmospheric Carbon aren’t simple.  One big reason is that Earth is a watery planet, and the Oceans aren’t just lying there—they absorb, emit, and transport both heat and chemicals.

This winter an international group report efforts to create a solid observation based record of Ocean Heat Content (OHC), which they note accounts for about 93% of the heat imbalance, and is less variable and  “better suited for detecting and attributing human influences […] than more commonly used surface temperature records “ [1], p. 128.

The idea is to create a global record dating back to 1970, synthesizing many studies that recorded ocean temperatures with buoys.  This is a challenging task, and the extrapolations and interpolations require considerable care.  Cheng et al. report that several independent approaches are converging to a consistent picture.

The overall finding is that oceans are heating faster than earlier estimates.  One effect of this is that surface temperatures have not increased as fast as predicted, because much of the heat was being sucked down into the deep waters of the oceans. This slowdown has been widely noted as a “hiatus” in warming, but if this really is a shift of heat into the deep oceans then it certainly does not represent any kind of slowdown in overall global heating.

The bottom line remains that these empirical measurements seem to confirm that the theoretical models are in the ball part—or even underestimating global heating.  The models predict increasing warming, and if the oceans are warming raster than earlier estimates, then overall warming will probably increase faster.  This is likely to lead to very significant effects, including melting of the cryosphere, rising sea levels, and changes to habitats on land and sea. That’s not good news for those of us who live in these habitats.

These estimates are still far from perfect.  The researchers indicate that there is little data from continental shelves or the deepest water. Given that the ocean is all connected, these areas are certainly affected by the overall warming, but we don’t know precisely what is going on.  It would be wise to find out, no?

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1. Lijing Cheng, John Abraham, Zeke Hausfather, and Kevin E. Trenberth, How fast are the oceans warming? Science, 363 (6423):128, 2019. http://science.sciencemag.org/content/363/6423/128.abstract