Two sea otters take a break from feeding in a bay near Sitka Sound. Photo by Ed Schoenfeld.

There’s more scientific evidence that sea otters reduce the amount of greenhouse gasses in the atmosphere. That has meaning for Southeast Alaska, where the population is booming, and Southwest Alaska, where it’s dropped.

Researchers already linked sea otters to carbon sequestration.

It’s a simple case of cause and effect. Kelp absorbs and stores carbon from the atmosphere. Sea urchins eat kelp, lowering the amount of carbon the marine plants take in. Otters eat urchins, allowing healthy kelp forests to grow.

“Most of the carbon that’s in living tissues of organisms is tied up in plants,” says Chris Wilmers, an environmental studies professor at the University of California, Santa Cruz.

“So if plants are doing a lot better, then it stands to reason that maybe there’s more carbon in the system,” he says.

Wilmers led a project to calculate the amount of the greenhouse gas otters help remove. He spoke during a recent teleconference hosted by the Alaska Center for Climate Assessment and Policy. (Hear an interview with Wilmers on the program “Living on Earth.”)

He says scientists compared data collected over the past 40 years to more recent information. They studied sites from southern British Columbia to the end of the Aleutians. And they compared otter-rich areas to those with few of the marine mammals.

Researchers found areas where otters live had 5 to 10 percent less atmospheric carbon than those where they don’t. Wilmers says it’s about the same amount 5 million cars would emit in an average year.

Researchers also calculated its value on the European Carbon Exchange.

“We looked up what the price of carbon futures were when we were writing up the study. We multiplied it by the amount of carbon otters are indirectly sequestering. And it comes out to be roughly worth somewhere between $200 million and $400 million,” Wilmers says.

But there’s a down side. The greenhouse gases ocean plants remove from the atmosphere do not disappear.

“If the kelp are taking up more carbon and sinking that carbon into the deep ocean, then over long time-scales there will be more carbon in the ocean, so theoretically it could increase acidification,” Wilmers says.

Scientists know higher levels of acidity damage shellfish, including crab. It may also affect salmon behavior.

But Wilmers and his fellow researchers say the impacts are limited.

“The real point of the study is to show that an animal, in this case a predator, can have a big effect on the carbon cycle and its ecosystem. And if otters covered the planet, they could have a huge impact on climate change and ocean acidification,” he says.

“But the reality is they just occupy this little sliver of the planet. So where they do live, their impact is large.”

He says otters do not remove all sea urchins. But they eat enough to drop numbers significantly.

Otter populations are growing in Southeast Alaska, Katchemak Bay and some other locations.

While they eat urchins, they also consume crabs, clams and sea cucumbers. That’s impacting fishermen and divers, who harvest those valuable species.