Earth’s oceans have absorbed more than 90% of the warming caused by greenhouse gases, researchers estimate, with the stored heat showing up as warmer seawater. But a new analysis suggests scientists may have underestimated the size of the heat sink in the upper ocean—which could have implications for researchers trying to understand the pace and scale of past warming.

Seas pose a formidable challenge to climate scientists. On one hand, they are as big a player in the global climate system as the atmosphere. As a result, “global warming is ocean warming," oceanographer Gregory Johnson writes in a commentary on the new study, appearing today in Nature Climate Change. But vast swaths of the ocean are poorly measured, particularly in the Southern Hemisphere.

To fill that gap, the authors of the new study focused south of the equator in developing a new estimate of how much heat the ocean stores. In particular, scientists at the U.S. Department of Energy (DOE) and NASA looked at satellite measurements of sea-level height, which they can use as a proxy for heating. That’s because as oceans warm, water expands, causing sea levels to rise. The researchers found that climate models have generally simulated recent sea-level rise accurately, when considering hemispheric-scale changes. Then, using model simulations, they looked into the past, calculating how ocean heat content has changed since 1970. In particular, they tested whether observed changes in heat content were consistent with the change simulated by the models. The figures generally matched over more recent decades, but they uncovered a large discrepancy prior to 2004, when more comprehensive data began to pour in from instrumented Argo floats. The results suggest that previous estimates of the heat stored in the upper 700 meters of the ocean since 1970 have been too low. In the Southern Ocean in particular, they estimate past heat tallies were 48% to 152% too low. Globally, past estimates could be as much as 25% off.

“The thing that was surprising to me was the magnitude of this underestimation,” says lead author Paul Durack, an oceanographer with DOE’s Lawrence Livermore National Laboratory in California. The findings suggest that previous observed estimates may be underestimating the planet’s sensitivity to the buildup of carbon dioxide, he adds. (Climate sensitivity is a key issue in climate science.) The paper is valuable in its use of real-world data and simulations to “cross-check” one another, says oceanographer Gregory Johnson of the Pacific Marine Environmental Laboratory in Seattle, Washington.

Meanwhile, a second paper published today in the same journal tries to shed light on warming in another poorly sampled part of the ocean: the deep. The vast majority of existing ocean temperature records are collected by buoys and cover the sea’s top 2000 meters. The average depth of the ocean, however, is 4300 meters, with some basins extending down to 6000 meters. Using sea-level and buoy data, a team led by William Llovel, an oceanographer at the Jet Propulsion Laboratory in Pasadena, California, attempted to estimate warming below 2000 meters. Their results suggest that the deep ocean has barely warmed since 2005—but includes big error bars that leave plenty of uncertainty.

That uncertainty illustrates why better measurements of the ocean are required, Johnson says. And help may be coming: This year, researchers began testing new drifting research buoys, dubbed Deep Argo, that are capable of descending to 6000 meters to take temperatures and other measurements.

*Correction, 5 October, 7:30 p.m.: The headline and text have been corrected to make it clear that past observations, not models, may have underestimated ocean warming. The description of the first study's results has been altered in the interest of clarity.