Heat wave wiped out millions of seabirds, study reveals

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

A study published on 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, the population has shown little sign of recovery, suggesting long-term shifts in the food web that may 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," said Heather Renner, lead author and biologist at the Alaska Maritime National Wildlife Refuge.

"To our knowledge, this is the largest mortality event of any wildlife species reported during the modern era," Renner 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 appearance, 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 late 2014, spanning a massive swath of the northeast Pacific Ocean from California to Alaska.

It persisted for more than 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 cite two key reasons for the mass die-off: elevated ocean temperatures reduced both the quality and quantity of phytoplankton, which impacted fish like herring, sardines and anchovies – the primary food source for murres. Simultaneously, 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," Renner explained.

For years following the event, breeding colonies failed to produce chicks, complicating efforts to assess the full impact.

Earlier estimates placed the death toll at around a million birds, but a more robust analysis – drawing on data from 13 murre colonies – revealed the true toll was four times higher.

"It is just so much worse than we thought it was," Renner said of the new findings.