“The change in forests' carbon role is a threat to the climate”

The Amazon rain forest may have reached a tipping point. According to a recent study, it now emits more CO2 than it absorbs, which could speed the pace of climate change. Charlotte Grossiord, an expert in forest ecosystems and the head of EPFL’s Plant Ecology Research Laboratory, spoke with us about the mechanisms behind this shift and why we should be worried.
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The Amazon rain forest now emits more CO2 than it absorbs, meaning it has switched from being a carbon sink to a carbon source. This reversal – confirmed by a study recently published in Nature Climate Change – marks a major milestone in the climate change process. And that’s not good news.

Tropical forests play an important role in keeping the Earth’s climate in balance. They absorb 25–30% of the greenhouse gases emitted by human activity and serve as a brake on global warming. Unfortunately, the carbon-absorption capacity of the Amazon – which alone accounts for half of the world’s tropical forests – is now waning. This is due in part to the massive deforestation that has occurred in recent decades. But it’s mainly because of a degradation process that, according to the study, has contributed three times more to the loss of the Amazon’s aboveground biomass (73% is due to degradation and 27% to deforestation).

So what are the factors pushing the Amazon towards, or even past, this tipping point – and what does that mean for our planet? Grossiord gives us some answers.

This shift in the Amazon’s role could have major implications for climate change, but we haven’t heard much about it in the press. Is it really that important?

Charlotte Grossiord. ©EPFL/A.Herzog

Yes, it’s a critical issue. Over the past five to ten years, scientists have seen a clear decrease in the amount of carbon being absorbed by tropical forests like the Amazon. And if these forests are no longer able to remove as much carbon from the atmosphere as previously, that will accelerate the process of climate change.

When a chain of mechanisms is triggered that could affect the climate, people sometimes call that a tipping point or, in more scientific terms, a positive feedback loop. Is that what we’re seeing here? 

Yes, disappearing forests could be considered a positive feedback loop, along with glacial melting, for example. If more and more greenhouse gases are emitted into the atmosphere while forests are able to absorb less and less, the process of global warming will speed up, forests will absorb even less greenhouse gases, and so on. We’re entering into a big vicious circle. For the Amazon in particular, we can’t really say whether the tipping point is happening now, has already passed or is just around the corner. But we can say that it will have an accelerating effect on climate change. That doesn’t mean it’s too late to do something about it – on the contrary. We must act now to protect these ecosystems while we still can.

Especially considering that forests play an important role in the health of our entire planet, not just climate systems.

Tropical forests account for just 10% of the Earth’s surface but are home to around 50% of all known animal species, including 90% of primate species. They are thus essential when it comes to biodiversity. Not to mention all the other services they provide, such as for the water cycle. Forests create their own microclimates where 80% of precipitation occurs, and where the water is recycled back into the atmosphere through transpiration. Forests are also key in preserving soil quality. The list of benefits is long – and they all stand to disappear if widespread deforestation continues.

How do forests function as carbon sinks?

The trees of a forest remove carbon from the air, in the form of CO2, through photosynthesis. CO2 is like food for plants, which convert it into sugar and use it to grow. Trees subsequently release some of this carbon back into the air through respiration, but most of it is held inside them – in their leaves, trunks, branches and roots – and in the soil. When trees are chopped down, especially old trees, which generally make up most of a forest, all the carbon accumulated over the years or even centuries goes back into the air. That also happens when trees die as a result of fires and droughts, which are occurring more frequently. So it’s a combination of factors, most of which result from human activity, that are bringing us to the point where forests emit more greenhouse gases than they absorb.

Is it possible to quantify the lost absorption capacity relative to the global carbon cycle?

That’s hard to do because the capacity loss changes from one season to the next. A lot more carbon is released than absorbed during droughts, for example. For now, the Amazon and all the other tropical forests combined are still absorbing more carbon than they’re emitting. But as soon as that changes, the climate impact will be huge.

We hear a lot about deforestation, but the study shows that degradation, a slower process, is even more destructive. What is degradation?

Forests continually undergo natural regeneration whereby some plants die. But here we’re talking about something else. Degradation mainly results from the more extreme kinds of weather events we’ve been seeing over the past few years – longer droughts, higher temperatures, and increasingly frequent and destructive storms and fires. These events lead to higher plant mortality and consequently more plant decomposition, releasing a greater amount of captured carbon into the atmosphere. In addition, hot, dry weather conditions create fertile ground for large-scale forest fires. Massive deforestation doesn’t help, of course. Not only is it being carried out on an industrial scale where entire sections of the forest are being destroyed, but deforesters are also eating away at forests in an insidious process that’s harder to detect but very widespread. All that diminishes forests’ long-term capacity to serve as carbon sinks.

Could reforestation be a good solution?

An old forest will always be more effective in absorbing carbon than a young one, since old forests are more complex, have several levels of vegetation from the ground up to the canopy, and have more soil nutrients. So I’m not fully convinced by the idea of saving the planet by planting new trees. Most reforested areas consist of monoculture crops where a single species is planted and is still harvested, such as to make paper. It would be more helpful and effective to preserve existing forests rather than to artificially plant new ones, which growers sometimes do with invasive species that aren’t well-suited to that particular environment. And new trees are often planted in straight lines, which doesn’t account for the natural complexity of the many chemical and biological processes that constitute a forest ecosystem. Artificially planted forests don’t have the same potential for natural regeneration, which takes more time but is much more effective.

How about in Switzerland? Are we also facing a possible tipping point?

Yes, definitely. We’re seeing higher tree mortality rates here every summer as a result of increasingly frequent extreme weather events. It’s also true that the amount of forest land has increased, along with the process of natural regeneration – which is good news. But those benefits could be wiped out if overall tree mortality continues to rise. This is a broader problem that’s affecting central Europe as a whole. The drought and heat wave in the summer of 2018, for example, caused a lot of damage. Many trees simply didn’t produce any leaves the following spring.