Cool compromise

How can global warming be mitigated? One way is to plant trees, as they remove CO2 from the atmosphere. However, trees reduce another cooling effect: the reflection of sunlight on the land surface. Model calculations show what will most likely benefit the climate.

Do something for the climate, plant a tree! What sounds like a good idea at first is not always the best solution. “Forests reflect significantly less sunlight than most other terrestrial ecosystems, for example. Instead, they absorb more light, which contributes to global warming,” says Dr. Alexander Graf from the Institute of Bio- and Geosciences (IBG-3). Researchers call this effect – the reflection of sunlight – albedo.

Albedo (reflection of sunlight)- two extrem cases

Trees also have another, long-term positive influence on the climate: “They absorb CO2, which has a cooling effect on the climate,” says Graf. Before trees can absorb CO2, however, they have to grow first. “Therefore, if trees are planted, the warming effect due to the reduced albedo makes itself felt much faster than the cooling effect from the increasing CO2 storage,” explains the Jülich researcher.

Together with scientists from 14 countries, Graf investigated the extent of the influence of the two effects and the role that different types of vegetation play. They used data from the global FLUXNET network, which provided information from 176 regional environmental monitoring networks on greenhouse gas, water and energy balances, including the reflection of sunlight. This also includes locations managed by IBG-3.

The international team used the data to calculate various scenarios – including what would happen if each site was reorganized to absorb the maximum possible amount of CO2. “This would mean afforestation for some locations, while for others it would only mean a change in agricultural or forestry methods and the selection of plant species,” says Graf. In the long term, this would lead to the maximum possible cooling effect on global warming. Still, no matter how many trees were theoretically planted, the researchers arrived at the same result for all locations: in the first 20 years, there will be moderate global net warming before the strong cooling effect sets in worldwide.

“The question is whether we can afford additional slight warming over the next two decades. We may have already passed climate tipping points during this period,” says Graf. An alternative could be to moderately increase both CO2 absorption and albedo – at least at locations where this is possible. This could mean planting dark arable land with catch crops during the fallow period, which increases the albedo and binds more CO2, or, in other places, to plant an initially loose stand of trees on a light-coloured meadow instead of dense forests on dark, bare ground.

“This balanced scenario immediately results in a cooling effect on global warming, although, in the long term, this is less pronounced than the effect in the case of ‘CO2 optimization’, which sets in later. Nevertheless, it could be a good compromise,” summarizes Alexander Graf.

Text: Christian Hohlfeld/Barbara Schunk | graphics: SeitenPlan I image: Forschungszentrum Jülich / Sascha Kreklau

Original publication:

Alexander Graf et al. Joint optimization of land carbon uptake and albedo can help achieve moderate instantaneous and long-term cooling effects. Communications Earth & Environment volume 4, Article number: 298 (2023). DOI: 10.1038/s43247-023-00958-4

Behind the Paper: Reflect sunlight or use it to store carbon?

One of the trade-offs of reforestation for climate change mitigation is the low albedo of forests - especially in snowy climates. We found there is more than just trees or snow to the conflict between biological carbon sinks and albedo - but also that there is hope to reconcile both. Read more

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Dr. Alexander Graf

Researcher micrometeorology and land-atmosphere interactions

• Institute of Bio- and Geosciences (IBG)
• Agrosphere (IBG-3)

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+49 2461/61-8676

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