Tropical Reforestation Offers Biggest Climate Cooling Benefits, Study Finds

Reforestation’s Climate Impact Depends on Location, Not Just Scale

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 postdoctoral researcher at ETH Zurich Nora Fahrenbach.

Challenging the “More Is Better” Assumption

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.”

Comparing Reforestation Scenarios

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:

• Scenario 1: Reforesting about 926 million hectares focused mostly on the tropics resulted in about 0.25°C of cooling by 2100.
• Scenario 2: Reforesting 894 million hectares, including large areas in northern temperate and polar latitudes, resulted in 0.13°C of cooling by 2100.
• Scenario 3: 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) also resulted in 0.13°C of cooling.

The findings suggest that geography may matter more than quantity when it comes to the cooling benefits of reforestation efforts.

How Reforestation Affects Global Temperatures

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. Planting trees in one area doesn’t just change the local temperature but has effects across the world.

All three models identified reforestation opportunities in regions such as:

• The eastern United States
• Amazonia
• The Congo rainforest
• Eastern China

They also pinpointed regions where 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.

Biogeochemical vs. Biogeophysical Effects

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

• Biogeochemical effects: Relate to the movement of chemicals or chemical elements, such as trees absorbing carbon from the atmosphere.
• Biogeophysical effects: Relate to the physical results of changing the land’s surface. For example, placing a tree in a snowy region 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 will occur.