A catastrophic weather event in northern Sumatra has dealt a devastating blow to one of the most endangered primate populations on Earth. A British-led study published today in the journal *Current Biology* reports that four days of unprecedented extreme rainfall in November 2023 caused the deaths of an estimated 7% of the world's remaining Tapanuli orangutans. The finding serves as a stark illustration of how climate change is amplifying the vulnerability of already critically endangered species.
The Tapanuli orangutan (*Pongo tapanuliensis*), identified as a distinct species only in 2017, numbers fewer than 800 individuals. Its entire range is confined to a single forest block of about 1,000 square kilometres in the Batang Toru ecosystem. Between November 20 and 23, 2023, this region received more than 300 mm of rain in 96 hours, equivalent to the average monthly rainfall for that area. The deluge triggered landslides and flash floods that swept away trees, destroyed nests and directly killed at least 42 orangutans, according to the study's lead author Dr. Matthew Nowak of the Sumatran Orangutan Conservation Programme.
“We typically think of orangutan threats as habitat loss or poaching,” Dr. Nowak said. “But this event shows us that extreme weather, made more intense by climate change, can kill a significant proportion of a population in a matter of days.” The team documented carcasses of all ages, including several females with dependent infants. Genetic analysis confirmed that the dead represented distinct family lineages, reducing the species’ genetic diversity as well as its numbers.
The study used satellite rainfall data and ground surveys to establish the link between the rainfall and mortality. Climate models suggest that the likelihood of such extreme precipitation events in Sumatra has increased by about 30% due to 1.2°C of global warming. “What was once a once-in-a-century event is now on track to become a once-in-a-decade event,” Dr. Nowak added. “The orangutans simply cannot adapt that fast.”
The Tapanuli orangutan is already threatened by habitat fragmentation from a hydroelectric dam and road construction. Now, climate change adds a new dimension of risk. “We are entering an era where conservation planning must account for stochastic climate shocks,” said Dr. Helena Vance, Science and Climate Correspondent. “Preserving large, contiguous forests with altitude gradients may help, but the atmospheric physics is unforgiving: a warmer atmosphere holds more moisture, meaning heavier rainfall is inevitable.”
The paper calls for urgent climate mitigation to slow warming, alongside on-the-ground measures such as creating emergency food caches and monitoring high-risk areas after storms. However, the authors caution that such measures are stopgaps. “Without global emissions reductions, the very existence of the Tapanuli orangutan is in the hands of the weather,” Dr. Nowak warned.
As I write this, the Sumatran rainforest is experiencing another unseasonal downpour. The rain beats against the canopy, and I cannot help but think of the lives it may claim. The orangutans cling on, but the margin for error is shrinking. Their future depends on decisions made far from their forest home.
