The Sea May Be Absorbing Way More Heat Than We Thought

Editor’s note, 11/14/18: The journal Nature is reviewing corrections to the study outlined in this story. They are related to the uncertainty in the authors’ measurements. We will update this story with new figures when they become available. The story appears below in its original form.

If you ever meet a climate scientist, give them a hug. Not only is the work important, it involves an absolute mess of variables—emissions, maybe sequestering those emissions, atmospheric patterns, maybe geoengineering that atmosphere. Data is often sparse or non-existent. So give them a hug.

The data problem is particularly acute in the oceans. A key part of figuring out how much the planet has warmed, and how drastically we need to cut emissions, is determining how the sea is changing. But that’s, uh, 325 million cubic miles of water. There’s just no way to sample all of it.

But scientists are making a dent. A network of 4,000 sensors, known as Argo, has been providing solid temperature data, though that data only covers the upper 2,000 meters of ocean. The average ocean depth is about twice that.

Today, researchers are sharing the results of a radically different method for determining how much heat our oceans have absorbed: They didn’t sample water, but air. And their findings are troubling. The oceans may have warmed 60 percent more than the figure that last month’s landmark IPCC report cited. That would mean that in order to keep warming to 2 degrees C above pre-Industrial Revolution levels—the goal from the Paris Climate Agreement—humanity would have to cut emissions 25 percent more than we thought. The new method needs more scrutiny from other climate researchers, but so far … yikes.

The researchers call their calculation “atmospheric potential oxygen,” or APO. It focuses on two gases: CO2 and O2. “When the ocean warms, it loses O2 and CO2, what we call potential oxygen,” says oceanographer and climate scientist Laure Resplandy, lead author on a new Nature paper describing the technique. “When it loses that gas, we can track it in the atmosphere.”

So far, so simple: As climate change causes the ocean to heat up, the water releases O2 and CO2 into the atmosphere, which a handful of land-based sensors then detect (they’ve been doing so for decades, giving the researchers lots of data to play with). Except the ocean isn’t the only source of these gases. Fossil fuels are, too—when oil and coal burn, they release CO2 and use up O2.

That muddies the APO signal, so the researchers had to control for fossil fuels by factoring in worldwide energy use. “We take all the fuel inventories,” says Resplandy, “and we say, OK, we have emitted that much and ended up consuming that much O2 and releasing that much CO2.”

Further complicating matters is that the planet acts as a carbon sink—trees pull CO2 out of the atmosphere, as do plant-like phytoplankton in the oceans. The APO calculations take standard estimates of this into account as well.

The researchers can then take their data and compare it to 80,000 data points from ocean sensors that have collected O2, CO2, and temperature readings at the same time. “Using those 80,000 points in the ocean we show that O2 and CO2 combined together are a good tracer of heat,” says Resplandy. The atmosphere should mirror that link. So they’ve essentially built a whole-ocean thermometer that works indirectly, by using gases to estimate how much heat the oceans are absorbing. Climate scientists might be able to use this as a supplement to directly sampling the water.

The findings so far aren’t encouraging. The researchers calculated that each year since 1991, the seas absorbed 13 zettajoules of heat energy, 150 times the energy we as humans produce annually. If a zettajoule sounds vaguely ominous and certainly big, think even bigger—a zettajoule has 21 zeros after it. Previous estimates put the number at 8 zettajoules.

“An ocean warmer than we thought means that the earth actually warmed more than we thought, and that means the earth is more sensitive to emissions than we thought,” says Resplandy. It’s also bad news for the denizens of the sea, which are struggling to adapt to hotter seas. Corals in particular.

How important is it for scientists to understand how much the oceans are warming, exactly? “Ocean warming is the critical thing in climate change,” says University of St. Thomas researcher John Abraham, who wasn’t involved in the new study. “Global warming is really ocean warming.”

A potential issue with this new method, though, is the foundational link between the gases and ocean temperatures, which is based on model calculations. No model, after all, is perfect. “Whether or not those model calculations are accurate enough is an area of concern,” says Abraham. “A change of X percent of an atmospheric gas gives you a Y percent change in ocean heat content. How well do we really know that quantitative relationship? If they're off by 10 or 20 percent, that's a big error.” It's worth noting, though, that Resplandy and her colleagues compared their data to those 80,000 data points from ocean readings, and determined that looking at CO2 and O2 combined is a good tracer of ocean heat.

Another major question here is the noise: accurately separating the CO2 and O2 quantities attributed to fossil fuels and other sources, and those from the oceans. “It is a delightfully innovative approach, using global atmospheric oxygen concentration ratios
to infer ocean heat content changes due to outgassing from warmer waters,” says NOAA ocean scientist Tim Boyer, who has studied ocean heat. He wonders, though, how accurately scientists can isolate the ocean’s production of these gases from other sources, and if the new method can calculate ocean heat change better than measurements of the water itself.

Other researchers will of course dive in to put the method to the test. If the authors are right, APO could become another tool to help scientists tease apart ocean warming, and by extension the monumentally complex problem that is climate change. Hugs alone, unfortunately, aren’t going to make their jobs easier.