The 2014-2016 Pacific marine heatwave wiped out more than half – roughly 4 million – of Alaska’s common murre (Uria aalge) seabirds, representing the largest documented vertebrate die-off linked to warming oceans, according to a new study. “Although research on the impacts of global warming on marine birds has clearly suggested major shifts in species’ ranges and abundance, documented changes have been gradual (years to decades),” write the authors. “To our knowledge, this study is the first to show that climate impacts can be swift (1 year) and intense (eliminating half of the population).” Marine heatwaves – intense, prolonged, and expansive ocean warming events – are becoming increasingly common due to climate change. They are profoundly reshaping marine ecosystems globally. These heatwaves lead to mass mortality of key habitat-forming species like kelps and corals, triggering cascading trophic effects that alter ecosystem productivity, displace lower-trophic-level species, and amplify food shortages for top predators. Although many upper-trophic-level species suffer from reduced reproductive success, increased mortality, and even mass die-offs due to marine heatwaves, broader population impacts are poorly understood. Heather Renner and colleagues evaluated the impact of the largest recorded marine heatwave in the northeast Pacific, which took place from late 2014 to 2016, on Alaska’s population of common murre seabirds. Analyzing colony counts before that period (2008-2014), and after (2016-2022), Renner et al. documented a 52-78% population decline across 13 murre colonies, equating to the loss of 4 million birds within the span of less than 2 years. According to the authors, this represents the largest documented wildlife mortality event in the modern era. Moreover, the findings reveal no evidence of population recovery since, suggesting potential long-term ecosystem shifts and an overall diminished capacity to support historical seabird populations. “We suggest that the pronounced decline of an abundant and widespread upper trophic predator to less than half of its former population size in Alaska may signal a new threshold of response to global warming,” write Renner et al.