In May 2009, Ari Friedlaender, an ecologist with Oregon State University’s Marine Mammal Institute, was cruising along the Western Antarctic Peninsula when he encountered something he’d never seen. In Wilhelmina Bay, the water was so thick with humpback whales that “we couldn’t count them fast enough,” he recalls.

In the end, he and his colleagues counted 306 whales feeding on an immense aggregation of krill. It was the highest density of humpbacks ever documented in the region.

The humpback population has been recovering ever since commercial hunting was banned in 1966. But the whales are also being helped by another factor: climate change.

In the past, there wouldn’t have been any humpbacks at all in Wilhelmina Bay in May, because the sea would have been covered with ice. The whales typically departed their feeding grounds along the Western Antarctic Peninsula by April, migrating thousands of miles north to spend the winter breeding in tropical waters.

But the sea ice is now advancing nearly two months later than it did in the 1970s and retreating a month earlier. Humpbacks can now stay in the Antarctic much later in the season and follow the krill moving inshore in large aggregations. Since that 2009 expedition, Friedlaender has been hearing the whales sing late in the season, a sign that they might be starting to breed right in Antarctic waters, without waiting to migrate north. That would be a fundamental change in their life history.

“We are just beginning to paint the picture of how quickly and well the humpback whales are able to use this habitat that was probably not available to them in the recent past,” Friedlaender says.

A World With Less Ice

It’s not just humpbacks, and not just the Antarctic: Around the planet, as whale populations recover from commercial hunting, they’re coming back to a different world. In the Arctic, north of the straits that connect it to the Pacific and the Atlantic Oceans, the seasonal open-water period has increased over the past thirty years by between one-and-a-half and three months.

For species such as polar bears, which depend on ice for their feeding behavior, that’s bad news. But humpbacks and other large whale species are benefitting from the change—at least for now.

View Images Using hooks and teamwork, Eskimos peel back a flap of skin and blubber (muktuk) from a bowhead whale caught in the Chukchi Sea and dragged onto the ice by the Eskimos. Photograph by Richard Olsenius, Nat Geo Image Collection

In the North Pacific off British Columbia, the humpback population has been growing steadily at a rate of about 7 percent per year and is now estimated at more than 21,000 animals. Two years ago the Canadian government downlisted the population from “threatened” to “species of special concern.”

In both the Pacific and the Atlantic, sub-Arctic species such as humpback and fin whales are spending more time in the Arctic waters around the Bering, Davis, and Fram Straits. Over the past five years researchers using underwater hydrophones to record whale calls have documented the increase in “summer” whales.

“Recovering populations of summer whales are taking advantage of a productive and more open Arctic,” says Sue Moore, a biological oceanographer with the Marine Ecosystems Division at the National Oceanic and Atmospheric Administration (NOAA). The retreat and the thinning of the sea ice, she says, has led to increased and earlier blooms of microscopic plant plankton. They in turn feed an increase in the tiny crustaceans—copepods and krill—that feed the whales.

Like their Antarctic relatives, the North Pacific humpbacks are staying late on the Arctic feeding grounds. “They might be up there still in November,” Moore says, “when people in Hawaii are starting to think that humpbacks should be coming down their way for mating.”

Good for the Natives Too

Bowhead whales, which spend their whole lives in and around the Arctic, are feeding better these days too. “It is a good time to be a bowhead,” Moore says.

The Alaska population, which migrates between the Bering Sea and the Chukchi and Beaufort Seas, had been reduced to just a few thousand animals when commercial whaling ended in 1910. It now stands at 17,000. A population that lives in the waters off eastern Canada and Greenland is increasing too.

View Images Several humpback whales interact at the water's surface in Cierva Cove. Climate change has made it easier for them to find plentiful food, but that could change as sea ice continues to decrease and carbon dioxide acidifies the ocean. Photograph by Carolyn Van Houten, National Geographic

“It is dramatic,” says Craig George, a senior wildlife biologist with the North Slope Borough's Department of Wildlife Management, who has monitored bowheads for the last 35 years in Alaska in partnership with the Inupiat hunters who still harvest bowheads. “The hunters say that back in the 1940s they would wait all day and see a couple of blows. Now it is hundreds of blows.” George has also documented a marked improvement in the body condition of young bowheads between 1989 and 2011.

Bowhead, humpback, and fin whales all use the same environment, but for now, not at the same time. Kristin Laidre of the University of Washington has studied the whales in Disko Bay in West Greenland. “You see bowheads leave, and within a week humpbacks move in,” she says. “It is amazing.”

As the sea ice retreats and whales change the timing of their migrations, however, they may eventually overlap and start competing for food. Things could get confusing for bowheads and humpbacks, which are both remarkable singers, says Kate Stafford of the University of Washington: “Because bowheads are spectacular mimics, it wouldn't surprise me if a bowhead started sounding like a humpback.”

Not Safe Yet

Humans are moving into ice-free Arctic waters too—and though we don’t hunt whales the way we used to, they’re still threatened by our noise, by collisions with ships, by oil spills, and by entanglement with fishing nets. “Growing large whale populations and increased use of the Arctic is going to lead to conflict,” says Laidre.

The effects of our carbon emissions are the biggest uncertainty in the whales’ future. As sea ice continues to decrease, and as carbon dioxide acidifies the ocean, what has so far been an increase in their food supply could flip into a sharp decrease.

Krill, for example, are vulnerable to acidification, and they are dependent on sea ice: Young krill spend the winter underneath it, feeding on algae that grow on the underside. Though Friedlaender sailed into an unusually dense aggregation of krill in Wilhelmina Bay, large decreases in krill biomass have already been reported in other parts of the Antarctic.

The whales will face pressure to adapt to rapid changes. “They are very good at it,” Moore says optimistically. “They are great oceanographers.”

One example of that, and of the way whales themselves affect their environment, comes from recent research on blue whales. In the Southern Ocean, blue whales were almost extirpated in the 1920s. Now the population is growing by 7.3 percent per year. In another half century it could return to its historic high.

More whales means more whale feces, and whale feces is extremely rich in iron, an essential plant micronutrient that the Southern Ocean is notoriously short of. As the resurgent blue whales defecate near the sea surface, says marine biologist Victor Smetacek of the Alfred Wegener Institute for Polar Research in Bremerhaven, Germany, the recycled iron triggers a phytoplankton bloom that feeds krill—which are then eaten by the whales.