Cooperative Institute for Research in Environmental Sciences

Greenhouse gases are indeed warming the planet, but not the way you think. Instead of warming Earth directly, it turns out they’ve triggered a middleman—the oceans—to do most of it.

In the last 20 years, temperatures over land have spiked by about half a degree Celsius on average—a dramatic increase for that time scale. Until recently, “Most people thought that increasing levels of the greenhouse gases carbon dioxide and methane above a location were directly responsible for the warming in that region,” said CIRES research scientist Gil Compo.

This makes intuitive sense, Compo said. When the sun’s rays hit Earth, the surface absorbs those rays and re-emits them as infrared radiation. Greenhouse gases, in turn, absorb some of the infrared radiation and re-emit it back to the planet, trapping in that heat.

But research by Compo and CIRES Fellow Prashant Sardeshmukh has shed light on another key player, the oceans.

Planet sauna

Over the last 50 years, the oceans have warmed by about 0.25 degrees Celsius. Back in 2005, Compo and Sardeshmukh began wondering how those hotter oceans have affected the rest of the planet.

To solve this riddle, they used data on sea-surface temperatures gathered since the 1960s from observation ships, buoys, and, by the late 1970s, satellites. They used that data run through seven different climate models to see the planetary effects of the warmer oceans. The model simulations didn’t directly “know” anything about the increase in greenhouse gases that had occurred, Compo said. All they knew about was the increase in sea-surface temperatures.

“The results were striking,” Compo said. “All of the land areas warmed in these specialized climate model runs.”

This suggested that the oceans were the ones driving Earth’s temperature increase. “Although a little bit of the warming is coming from the human-caused increases in carbon dioxide and methane directly above you, by and large, most of the warming over land is happening because the oceans have warmed,” Compo said.

Greenhouse gases not off the hook

That raised the million-dollar question: What’s warming the oceans?

Compo and Sardeshmukh’s study didn’t address that, but a related study by Martin Hoerling of NOAA’s Earth System Research Laboratory confirmed that most of the ocean warming was due to anthropogenic greenhouse gases. Other factors that may play a role in sea-surface temperatures include El Niño, volcanic emissions, and natural variability.

So while human increases in greenhouse gases aren’t directly warming the continents, they’ve unleashed a 500-pound gorilla—the oceans—which are.

Compo and Sardeshmukh started looking for mechanisms to explain how the seas could raise temperatures globally, and what they found is that the warmer waters have made the atmosphere above continents moister.

“Warmer oceans mean more evaporation,” Compo said. “That water vapor spreads uniformly throughout the upper troposphere, absorbing long-wave [infrared] radiation and emitting it back to the surface.” Water vapor actually traps in heat much more effectively than CO2, making it a potent greenhouse gas.

This process of global humidification and infrared-radiation surface warming, called hydrodynamic-radiative teleconnections, may also create a feedback cycle over the oceans: Humidity increases, trapping heat, which makes the oceans hotter; that, in turn, increases evaporation and humidity even more, generating a positive feedback loop. (Compo points out that this is not a feedback over land, though.)

The findings mean the impacts of carbon emissions are as important as ever. But they also have dramatic ramifications for climate models, many of which don’t currently take into account the complexity of sea-surface temperatures.

“We really have to model oceans correctly if we want to predict how warming will occur and how to best prepare for it,” Compo said.