The conscience can be calmed quickly.

If you book a flight these days, you can simply buy money to reforest forests as compensation.

The idea is that the carbon dioxide debt that one has shouldered with the flight is offset with living trees.

They should pull the climate-damaging gas out of the atmosphere again.

There is even the thesis that the young, regrowing trees bind more carbon than the old trees would have done because of their rapid wood growth.

It's a green sale of indulgences, but it has some pitfalls.

Pia Heinemann

Editor of Nature and Science

  • Follow I follow

Because the calculation is not that simple, as a new study in the journal "PNAS" shows.

For this, a research team led by Maria Mills from the University of Leicester used two different methods to measure how many greenhouse gases are emitted from deforested areas in the ten years after the deforestation of the soil and the plants - and how much is absorbed.

The amount of carbon absorbed and released by the microorganisms decomposing the deadwood was also taken into account.

The scientists carried out the measurements on eleven deforested areas in Malaysia over a period of seven years: some of them had been massively cleared, others only partially.

The result: All areas were a net source of carbon for at least a decade.

On 99 percent of the 455 measurement days, they emitted more carbon than they absorbed from the atmosphere.

Different time scales

The data collection and the results are "solid", Julia Pongratz assesses the study.

She is a climate researcher and professor of geography at the LMU Munich and investigates the role of afforestation and forests in climate protection.

However, she also emphasizes that the study covers only a very short period of time – speaking in terms of forest dimensions.

"On such short time scales, the effects of outgassing carbon dioxide are much more significant in comparison."

It is easy to understand that in the first few years after felling, the various decomposition processes on the ground release a lot of carbon dioxide.

Leaves rot, deadwood rots, and roots rot.

The further this degradation has progressed, the less carbon dioxide is released.

In addition, the new trees grow and bind more carbon dioxide.

"Only a full life cycle analysis would clarify how the CO2 balance is structured" - i.e. if the forest is examined over many decades until it has fully grown and the use of the products for which the wood is then used is also included.

Almut Arneth, who heads the Department of Ecosystem-Atmosphere Interactions at the Karlsruhe Institute of Technology, also assesses the study design and the results positively.

"The authors are absolutely right: the soil component is often overlooked when it comes to the influence of logging on the function of forests as a carbon sink." Soil respiration is often not taken into account.

How long their effect lasts after clearing is another question.

"This varies greatly between locations - it may be a decade or longer, but shorter in other forests and regions."

What happens later with the wood?

The study also highlights the role played by deadwood.

Whether or not this should be left in cleared forests and allowed to rot there is a matter of debate among forest scientists.

On the one hand, deadwood is important for biodiversity: it ensures that important minerals are returned to the soil and plays an important role in the water balance in ecosystems.

On the other hand, as this study also shows, it is “breathed in” – i.e. it releases carbon dioxide.

In addition, deadwood can be a flammable substance in forest fires, especially in drier and hotter regions.

Julia Pongratz says that removing and using dead branches and roots from deforested land could reduce the carbon footprint if they were processed into durable wood products.

"Burning it as bioenergy, on the other hand, would release the carbon dioxide back into the atmosphere even faster than slowly decaying in the forest."

Almut Arneth is generally critical of the discussion about the potential of reforestation.

"Climate change has required a rapid, massive reduction in fossil emissions for years, but this is still not happening," she says.

The debate about the additional – i.e. through afforestation – expected contribution of the forests tends to distract from this.

"I would like to restore forests, savannahs, grasslands, moors where possible - no question," says Arneth.

"This can certainly make a further contribution to reducing climate change - with huge added value for biodiversity and other ecosystem services.

But please start where it is necessary, and that is the fossil emissions.”

So the bad conscience about a flight can easily be calmed with the sale of indulgences from afforestation.

However, a look at the facts shows that planting trees does not help combat climate change in the short term.