In November 2025, Cyclone Senyar generated extreme rainfall in parts of Sumatra, Indonesia, triggering thousands of landslides. Our new paper in the journal Current Biology demonstrates that these landslides might have a devastating impact on a critically endangered population of Tapanuli orangutan.

In November 2025, Cyclone Senyar brought extreme rainfall to large parts of Sumatra in Indonesia. I have written about this on previous occasions – the rainfall triggered vast numbers of landslides.

In my line of work, we often focus on the landslide impacts on the landscape, on human lives and on infrastructure. We rarely consider the impacts on th eanimal population. This is certainly a weakness that the Cyclone Senyar event brings to focus.

Part of the area devastated by the landslides is that slopes around the Batang Toru rover, an area of forest that is home to a rare species of orangutang. These great apes, Pongo tapanuliensis, live in a habitat known as the West Block of Tapanuli. There are only 800 individuals left in the wild, a situation that is highly precarious. The loss of even a small number of adults could tip the species towards extinction.

I was a part of a consortium of scientists that considered the landslide impacts of Cyclone Senyar on the habitat of these orangutangs. The results have just been published in the journal Current Biology (Meijaard et al. 2026) – the paper is open access and published under a creative commons license.

This image, from the paper, shows the landslide impacts of Cyclone Senyar:-

In the study area of 71,161 hectares, the mapping indicates that there were 50, 185 individual landslides, covering a surface area of 8,303 hectares. This is about 11% of the forested area. We then estimate the likely loss of the orangutang population, which is likely to be in the range of 18-120 individuals, with a central estimate of 58 individuals. This is likely to have been a devastating loss for this highly endangered population.

This level of habitat loss might also be placing a severe pressure on the remaining population, so further fatalities are very possible through, for example, reduced food availability.

The intensity of the rainfall was almost certainly supercharged by climate change. The impacts of Cyclone Senyar are being replicated widely – and of course we are now in the northern hemisphere tropical cyclone season again.

Our paper makes some policy recommendations for this population of orangutans. First, the government of Indonesia needs to permanently protect this area of forest against mining , palm oil and hydropower developments. Ideally, the protected area should be expanded. Second, Indonesia needs support for biodiversity-recovery, hazard forecasting and ecological restoration planning.

Reference

Meijaard, E. … Petley. D. … et al. 2026, Extreme rainfall further endangers the world’s rarest great ape. Current Biology. https://doi.org/10.1016/j.cub.2026.05.029

Text © 2026. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.

Planting more trees will decelerate climate change only if those trees are placed in optimal locations—primarily the tropics and subtropics—suggests new research published in Communications Earth and Environment. However, planting trees in locations like Alaska, Siberia, and large parts of the United States could actually lead to warming, said lead author and doctoral student at ETH Zurich Nora Fahrenbach.

Much of the current thinking in nature-based solutions, Fahrenbach said, is based on the idea that “more is better.”

As in, “we’ll plant a trillion trees, or we’ll plant more than a trillion trees, and we are going to get more cooling, right?” Fahrenbach said. “That’s something we show is just not the case.”

Fahrenbach researches reforestation potentials, or global maps that identify areas where trees could be planted to mitigate climate change. In this work, she and her colleagues compared three prominent reforestation potentials to determine the effect of tree placement on local and global temperatures.

One scenario involved reforesting about 926 million hectares focused mostly on the tropics and resulted in about 0.25°C of cooling by 2100. Another called for reforesting 894 million hectares, including large areas in northern temperate and polar latitudes, and resulted in 0.13°C of cooling by 2100.

The third scenario involved planting forests strategically over only 440 million hectares of mostly tropical and subtropical land (less than half of the area covered in the other scenarios) but also resulted in 0.13°C of cooling. Geography, the findings suggest, may matter more than quantity when it comes to the cooling benefits of reforestation efforts.

Let’s Get (Biogeo)physical

The researchers modeled all three scenarios using the same parameters: Trees were planted from 2015 to 2070 and then remained steady in their population until 2100.

All three models identified reforestation opportunities in regions such as the eastern United States, Amazonia, the Congo rainforest, and eastern China, as well as regions for which reforestation would not be as impactful, such as polar regions in the Northern Hemisphere. The researchers also found significant temperature changes across the Atlantic and Indian oceans as a result of atmospheric changes induced by reforestation, demonstrating an interconnected reality: Planting trees in one area doesn’t just change the local temperature but has effects across the world.

These local and nonlocal effects can be explained by a combination of biogeochemical and biogeophysical effects.

A biogeochemical effect relates to the movement of chemicals or chemical elements, such as trees absorbing carbon from the atmosphere.

A biogeophysical effect relates to the physical results of changing the land’s surface: Placing a tree in a snowy region, for instance, decreases the land’s albedo, meaning it causes the land surface to become darker and absorb more light, leading to more local heat. This rise in surface temperature also raises air temperature, creating cascading effects on wind patterns and oceanic currents.

Considering both processes together is essential for understanding whether a net cooling or net heating effect exists, but most policies focus only on biogeochemical effects, seeing trees solely for their ability to absorb carbon from the atmosphere, Fahrenbach said. They include prominent international policies such as the Paris Agreement and the United Nations’ Framework for REDD+.

“Really, we would also need to consider the biogeophysical effects,” Fahrenbach said. “That’s harder to do, right, considering those nonlocal effects, because just imagine, some country is going to plant a lot of trees, and that’s going to lead to warming somewhere else.”

A Call to Policymakers

Emilio Vilanova, a forest ecologist at the climate action nonprofit Verra, wrote by email, “The most important message for me is that this study emphasizes something that is often not well addressed in reforestation projects: Reforestation is not just about planting trees—it’s about designing where new forests go to maximize benefits and avoid unintended consequences.”

Vilanova also said the study puts the potential for reforestation efforts to address climate change in perspective. “Even very large reforestation efforts would only reduce global temperatures by about 0.13–0.25°C by the end of the century,” he said. “While meaningful, this finding also reinforces that reforestation is a helpful tool, not a stand-alone solution to climate change.”

Though the limited potential for change is sobering, the authors and Vilanova pointed out that this change does matter and that it matters how we think of our approach. They advocate for policies that adopt reforestation strategies based on location and that acknowledge both the local and nonlocal effects of reforestation.

“We really need to make sure that where we plant first, it has benefits locally, it has benefits globally,” Fahrenbach said.

—Andrew Meissen (@AndrewMeissen), Science Writer

22 April 2026: This article was updated to correct Nora Fahrenbach’s position at ETH Zurich.

This news article is included in our ENGAGE resource for educators seeking science news for their classroom lessons. Browse all ENGAGE articles, and share with your fellow educators how you integrated the article into an activity in the comments section below.

Citation: Meissen, A. (2026), Location, location, location: The “where” of reforestation may matter more than the extent, Eos, 107, https://doi.org/10.1029/2026EO260125. Published on 22 April 2026.

Text © 2026. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.