A devastating series of extreme rainfall events in the rainforests of Sumatra and Borneo has led to the deaths of nearly 7% of the world's rarest orangutans, according to a study published this week in the journal ‘Current Biology’. The UK government, citing the study, has urged the international community to treat the protection of orangutan habitats as a climate emergency priority.
The research, led by Dr. Rina Sulistyowati at the University of Palangka Raya, estimated that between 2015 and 2023, extreme rains triggered landslides and flash floods that killed at least 1,200 Tapanuli and Bornean orangutans. With remaining populations estimated at fewer than 800 for the Tapanuli species and around 100,000 for the Bornean, these losses represent a catastrophic reduction in genetic diversity and a direct pathway to localised extinctions.
“We are watching the slow dissolution of an entire lineage,” said Dr. Sulistyowati in a press release. “These are not normal rains. They are fuelled by a warmer atmosphere holding 7% more moisture for every degree Celsius of warming.”
The mechanism is straightforward. As greenhouse gas concentrations rise, the atmosphere’s capacity to hold water vapour increases. This leads to intense, short-duration rainfall events that overwhelm the forest floor and trigger landslides on steep hillsides. Orangutans, slow-moving arboreal apes, are particularly vulnerable during periods of heavy rain when they seek refuge in tree canopies that can be swept away by mudslides.
The UK’s Foreign, Commonwealth and Development Office issued a statement calling for an “immediate scaling up of nature-based solutions and anti-deforestation measures.” The UK has committed £3 billion of its international climate finance to biodiversity projects, but critics argue this is insufficient given the scale of the crisis.
“The orangutans are a sentinel species,” said Dr. Helena Vance, Science and Climate Correspondent. “Their loss signals ecosystem collapse. When you remove the primary seed disperser from a rainforest, you disrupt the entire carbon cycle. These forests are not just homes for orangutans. They are massive carbon sinks. Losing them accelerates the feedback loop we are trying to break.”
The study used satellite imagery to track rainfall intensity and correlate it with orangutan mortality reported by field conservation teams. The data shows that the most severe events occurred in 2018 and 2022, when rainfall exceeded 200mm in a single day in regions where Tapanuli orangutans are concentrated.
Conservation groups, including the Borneo Orangutan Survival Foundation, have been working to create wildlife corridors and relocate orangutans from flood-prone areas. However, funding remains inconsistent. “We need a permanent funding mechanism tied to climate adaptation,” said Dr. Sulistyowati. “Not charity. Not emergency appeals. A structural response.”
The UK’s call for urgency comes ahead of the COP29 climate conference in Baku, where biodiversity loss is expected to be a contentious issue. The UK plans to propose a new metric for “climate-biodiversity risk” that would force countries to account for species extinction in their climate vulnerability assessments.
For now, the immediate priority is to secure the remaining orangutan populations. “Every individual matters at this point,” said Dr. Vance. “We are not just losing animals. We are losing nodes in a planetary life support system. The physics is clear. The policy must catch up.”
