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.