How would reforestation in the tropics affect the global climate?


  • A study examined the effects of reforestation of all the tropics on the global climate.
  • This simulation was created using the UK Met Office’s climate change model.
  • If the tropics were reforested, the model predicts that the oceans, soil, and vegetation would absorb less carbon dioxide.

What if every piece of farmland in the tropics, from Brazil to Congo, India and Indonesia, were abandoned overnight and turned back to forests? This is the question we investigated in our new research. Trees and forests have become increasingly important in climate emergency plans, but our work shows that once you take into account how the soil, oceans, and other parts of the world Earth system would react, planting trees is not as powerful a solution as it could be. seem first.

Of course, abandoning agriculture in the tropics cannot be a solution to climate change. This was a hypothetical and idealized experiment, but one that helps us explore how the global carbon cycle might respond to forest restoration and large-scale tree planting. And targeting the tropics shows maximum impact because trees grow there quickly.

To investigate the issue, we used the UK Met Office’s climate change model – a computer simulation of the Earth as a system in which the oceans, land, and climate interact and influence each other. We simulated two futures. First, a scenario where the world takes serious action to limit warming to less than 2 ℃. The second scenario was the same, except that all agriculture across the tropics was stopped and the original vegetation, mostly forests, would recover.

The difference between the two scenarios shows that the new tropical trees would store an additional 124 billion tonnes of carbon by the year 2100, or about 13 years off the current rate of fossil fuel emissions. All this extra carbon would have been taken from the atmosphere by photosynthesis. However, the carbon in the atmosphere – which counts for climate change – would decrease by only 18 billion tonnes, or the equivalent of just two years of emissions. What explains the huge difference?

The reason is that other parts of the earth system neutralize the effect of new tree growth. If the tropics were reforested, our model predicts that the oceans, soils, and vegetation would absorb less carbon dioxide. By the end of the century, tropical trees would absorb an additional 124 billion tonnes of carbon (or 124 gigatons, Gt), but tropical soils would absorb 83 Gt less carbon, because the renewal of dead plants is much slower in forests than in forests. with grasses and crops dying every year, which means lower carbon inputs to forest soils and lower carbon storage.

Lower levels of carbon dioxide in the atmosphere would also affect vegetation and soils elsewhere in the world. Less carbon dioxide in the atmosphere would mean slower plant growth. And just like in the tropics, less carbon would be absorbed by soils.

There would be another major change in the Earth system. The oceans slow climate change by removing carbon dioxide from the atmosphere: as CO₂ levels rise, some of the extra carbon dioxide dissolves in seawater. And in our model, the oceans would absorb less carbon since the new tropical trees would lower the atmospheric CO.

As a reminder: we started with new trees in the tropics by removing 124 Gt of carbon from the atmosphere. Avoiding deforestation adds an additional 10 Gt. But once you subtract the carbon that is no longer stored in tropical forest soils (83 Gt), in soils and vegetation elsewhere (18 Gt), and in the oceans (15 Gt), it won’t there is not much left.

How the full tropical reforestation scenario adds up.

Image: Simon Lewis, Alexander Koch, Chris Brierley

Remarkably, the reforestation of the tropics – after taking into account the reactions of the soil, vegetation and the ocean – results in only 18 Gt of carbon taken from the atmosphere. This is an 86% reduction from our initial excess carbon in tropical trees. This 18 Gt is equivalent to only ten parts per million reduction in atmospheric carbon dioxide.

The Earth system is working against us

What we found in our hypothetical scenario is the reverse of what is happening today. When carbon dioxide is released from the combustion of fossil fuels, just under half of these emissions remain in the atmosphere and contribute to climate change. The rest is absorbed by the oceans, soil and vegetation. This has been a great free grant from nature.

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Image: Integrated energy systems in cities (Source: Net Zero Carbon Cities: An Integrated Approach, 2021, World Economic Forum)

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But here’s the catch: Just as the amount of carbon absorbed from land and oceans increases as we inject more carbon dioxide into the atmosphere, the reverse happens when we remove it from the atmosphere. The Earth system begins to work against us when we plant trees or use other methods to remove CO₂ from the atmosphere.

These results give food for thought. Even something as drastic as reforestation of the entire tropics – far beyond any plausible political outcome in the real world – would have less influence on the climate than you might think. But these results also highlight that the best way to avoid dangerous global warming is not to release fossil carbon into the atmosphere in the first place.

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