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13,670 nautical miles. 50-foot waves. One big collision.

Over the weekend a Saildrone -- a 23-foot long uncrewed marine robot -- withstood the tempestuous seas around Antarctica to complete the first-ever circumnavigation of the continent by a drone. National Oceanic and Atmospheric Administration (NOAA) scientists collaborated with autonomous vehicle specialists, Saildrone, to test whether the seafaring robot could survive the rough waters, and make successful scientific observations.

NOAA needs to gauge how much carbon dioxide -- the potent greenhouse gas now amassing in the atmosphere -- the southern seas are absorbing from the air, and it hopes Saildrones can help. Overall, the oceans soak up a huge amount of the CO2 that humanity emits into the atmosphere (some 30 percent), which has substantially curbed Earth's accelerating temperature rise. Now, understanding how much carbon the oceans will likely soak up in the future is critical to grasping how Earth's increasingly disrupted climate will transform society and the natural world.

"The ocean is doing a huge service for the globe," said Adrienne Sutton, a NOAA oceanographer working on the Saildrone missions. She noted that the carbon-absorbing oceans have kept the planet nearly 1 degree Fahrenheit (0.5 Celsius) cooler than if these global waters did not soak up greenhouse gases.

But as the seas warm and change, they might become less adept at absorbing the planet's mounting atmospheric carbon (for one, warmer oceans absorb less CO2). "That's a big problem," noted Sutton.

So oceanographers want to vastly expand their observations of the Southern Ocean, a notoriously harsh, remote, and little-observed region that scientists don't fully grasp. Some recent evidence suggests this ocean is hugely variable; in recent years it has absorbed significantly less carbon than during years prior. Will the Southern Ocean be a reliable carbon-absorber, known as a "carbon sink," in the years ahead? "That's an open question," said Sutton.

Understanding this vast, perplexing ocean requires more observations.

"We need to project its fate in the future," said Matthew Long, an oceanographer at the National Center for Atmospheric Research who had no role in the mission. 

Enter the Saildrone. As the drone travels the high seas it takes direct measurements of carbon dioxide. A fleet of sailing drones would dramatically improve oceanographers' grasp of the Southern Ocean's behavior. "The Saildrone can be part of a more complete observing system down there," said Sutton.

"They are an incredible research platform," added Long.

And they're quite sturdy. Saildrone said that it collided with a giant iceberg tall enough to knock off a wind-monitoring instrument affixed 20 feet high on the drone. Though, because the collision occurred at night, it's impossible to know just how big the iceberg was. Icebergs in the area are often miles long.

"It wasn't a small bump," said Sutton.

Still, the drone persevered, though many of its weather-monitoring instruments were damaged in the collision.

Looking ahead, if modern civilization hopes to dramatically slash its carbon emissions to net-zero (meaning reducing overall emissions by 100 percent through a combination of cutting emissions and pulling carbon out of the air), governments and policymakers must know almost exactly how much carbon the oceans are soaking up.

"The function of the ocean as a carbon sink is an integral component of our ability to meet net-zero," noted Long.

Though the public increasingly recognizes the mounting threats posed by climate change, the journey to net-zero emissions will be long. Atmospheric CO2 levels are not going down. Rather, they're increasing at rates that are unprecedented in both the historic and geologic record.

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