The common murre, a large black-and-white seabird native to northern waters, has become far less common in Alaska over the past decade due to the impacts of climate change.

A study published Thursday in Science reveals that a record-breaking marine heat wave in the northeast Pacific from 2014 to 2016 triggered a catastrophic population collapse, wiping out four million birds—about half the species in the region.

Strikingly, they have shown little signs of rebounding, suggesting long-term shifts in the food web that have locked the ecosystem into a troubling new equilibrium.

"There's a lot of talk about declines of species that are tied to changes in temperature, but in this case, it was not a long term result," lead author Heather Renner of the Alaska Maritime National Wildlife Refuge told AFP.

"To our knowledge, this is the largest mortality event of any wildlife species reported during the modern era," she and her colleagues emphasized in their paper.

The finding triggers "alarm bells," Renner said in an interview, as human-caused climate change makes heat waves more frequent, intense, and longer-lasting.

Emaciated Carcasses

With their dapper, tuxedoed look, common murres are sometimes called the "penguins of the north."

Their slender wings power them across vast distances in search of food and make them expert divers. But even these hardy seabirds were no match for an unprecedented environmental catastrophe.

The largest marine heat wave ever recorded began in the late fall of 2014, spanning a massive swath of the northeast Pacific Ocean from California to Alaska.

It persisted for over two years, leaving devastation in its wake. During this time, some 62,000 emaciated murres washed ashore along the North American Pacific coastline—dead or dying from starvation.

Experts point to two key reasons for the bird deaths: elevated ocean temperatures reduced both the quality and quantity of phytoplankton, impacting fish like herring, sardines, and anchovies—the mainstay of the murre diet.

At the same time, warmer waters increased the energy demands of larger fish, such as salmon and Pacific cod, which compete with murres for the same prey.

"We knew then it was a big deal, but unfortunately, we couldn't really quantify the effects," explained Renner.