A marine heat wave known as 'the blob' is responsible for the largest die-off a single species in modern history. 

'The blob' is a large mass of unusually warm water off the coast of North America in the Pacific Ocean — and scientists have blamed it for killing half of Alaska's population of the common murre, a small seabird that resembles a flying penguin. 

The birds are short and stout with tuxedo-styled feathers, and nest in large colonies on islands or coastal cliffs, keeping close to the ocean where they hunt fish, squid, octopus and crustaceans.

The blob heat wave between 2014 and 2016 appears to have been the most intense marine heat wave in history, scientists say, raising ocean temperatures by as much as seven degrees Fahrenheit. 

This destabilized weather and coastal marine ecosystems from California to Alaska, diminishing the food supply for seabirds like common murres and causing a massive die-off. 

Their loss has a particular impact on Alaskan Native communities, as murres have traditionally been an important source of meat and eggs. Population decline now threatens traditional harvesting practices and cultural connections, experts say. 

Before the heat wave, roughly 8million common murres lived in Alaska — about a quarter of the world's population of the animal.

But now, scientists estimate that population shrunk to approximately 4million based on an analysis of survey data collected after the heat wave. What's more, the murres are showing no signs of recovery. 

In 2020, researchers studying common murre carcasses concluded the most likely cause of death was starvation due to the impact of the marine heat wave. At that time, they estimated the death toll to be between 62,000 and 1million. 

A new study has now revealed the full scale of this die-off, underscoring the catastrophic effect that oceanic warming has on animal populations.  

Study co-author Julia Parrish, a professor of aquatic and fishery sciences and biology, said: 'This study shows clear and surprisingly long-lasting impacts of a marine heat wave on a top marine predator species.'

She added: 'Importantly, the effect of the heat wave wasn't via thermal stress on the birds, but rather shifts in the food web leaving murres suddenly and fatally without enough food.'

Parrish was part of a team of researchers led by the US Fish and Wildlife Service that analyzed years of common murre colony survey data to estimate the total mortality of this die-off. 

They analyzed 13 colonies surveyed between 2008 and 2022, and found colony size in the Gulf of Alaska declined roughly 50 percent after 'the blob.'

Colonies along the eastern Bering Sea saw an even steeper decline of 75 percent.  

Together, this accounts for the deaths of approximately 4million common murres in Alaska due to this marine heat wave. The researchers stated they have found 'no evidence of recovery,' since the die-off occurred. 

They published their findings earlier this month in the journal Science.

Common murre populations have fluctuated before, experiencing smaller die-offs they were eventually able to recover from, the study authors stated. 

But this unprecedented decline may be too severe for the Alaskan population to rebound from, they noted. And as marine heat waves become more common due to climate change, this may not be the last time the species takes such a hit. 

A 2023 study led by many of the same authors showed a sea surface temperature increase of 1.8 degrees Fahrenheit that lasts over six months results in multiple seabird die-offs.  

That study also suggested seabird populations would take at least three years to recover from a marine heat wave.  

'Whether the warming comes from a heat wave, El Niño, Arctic sea ice loss or other forces, the message is clear: Warmer water means massive ecosystem change and widespread impacts on seabirds,' Parrish said in a statement. 

'The frequency and intensity of marine bird mortality events is ticking up in lockstep with ocean warming.'

It has been seven years since 'the blob' decimated Alaska's common murre population. The fact that they still have not shown signs of a rebound is concerning, according to Parrish. 

While it's difficult to pinpoint the reasons why this population has been unable to recover, it's likely the marine heatwave altered the resources and dynamics that common murres depend on, ultimately destabilizing the entire ecosystem.

For example, the marine species that make up their food supply may be struggling to rebound from the heat wave too. Or perhaps the loss of four million common murres affected their social interactions, which help them survive and thrive. 

'We may now be at a tipping point of ecosystem rearrangement where recovery back to pre-die-off abundance is not possible,' Parrish said.