“As a scientist I am optimistic”

An interview with Susan Trumbore on the tipping points of the fragile Amazon ecosystem and why it helps to eat less meat.

Professor Trumbore, in the discussion about the climate crisis, there is a lot of talk about tipping points. Activist groups also remind us almost daily of the danger of these tipping points. One example is the Amazon. In early August, eight Amazonian countries met in Brazil for a summit to discuss the future of the rainforest. What is a tipping point, particularly in the case of the Amazon rainforest?

A tipping point is a critical threshold in the Earth system. Once you cross it, you shift from one system to another. There is no way back. Usually, some kind of positive feedback kicks in at that critical threshold, which causes an accelerated transition. Yet, it doesn’t mean an immediate change and it depends on the kind of tipping point how long it takes.

In the Amazon ecological system you possibly shift from rain forest to savanna in the most extreme case, more realistically, though, to a degraded forest or novel kinds of vegetation that don't have the same function as the original forest. The rain forest is evergreen, has a lot more biomass and can store way more CO2. Even during the dry season, it's able to support transpiration because its roots extend deeper in the soil, which keeps the area cool and prevents vegetation from drying out. That continuous recycling of water back to the atmosphere supports rainfall downwind.

How does deforestation contribute to a possible shift of the entire ecosystem of the Amazon?

As we turn original forest into pastures and croplands, several effects kick in. Less water gets evaporated and the area heats up. Fires spread more easily and can kill more trees. Also, less water is returned to the atmosphere to sustain precipitation. That can lead to a tipping point where you don't have enough rain to support the forests anymore and the remaining vegetation would interact with fires and change rapidly to a more degraded forest that lacks the diversity and biomass of what it replaces. 

How certain are you about the tipping point in the Amazon?

We don't know enough to define a specific threshold for a tipping point for the vast and diverse area that is the Amazon. We also cannot be sure which functions – like biodiversity – would be lost forever and which – like evaporation – might recover in time. Theoretical models predict a tipping point at around 20 to 25 percent deforestation for the entire Amazon area. However, these models lack sufficient understanding to simulate all processes at work and how they interact, so their predictions come with high uncertainties. Today, we are at about 15 percent on average, but locally exceeding 20 percent.

That’s why you are running experiments in the Amazon?

Yes, we are working in Mato Grosso, which is one of the most deforested places in the Amazon. Here, about 23 percent of the forest has been lost already and this is where we are closest to a tipping point. It feels like driving through Kansas but with more forest: a patchwork of remnant forests in between soy fields.

We're measuring the carbon and water exchange to understand how forests are responding to droughts and what role deforestation plays in terms of the local climate. What we found is that the cropland, which is a lot hotter, still benefits to some degree from the cooling effect of intact forest next to it. But if we continue to deforest and create local heat islands, a tipping point for agriculture may be crossed beyond which it is getting too hot and crop yields go down.

And the remaining forest patches, especially at the edges, are becoming degraded as they are more vulnerable to disturbances like fire. We do not know if these fragmented forests will be able to maintain themselves in the next decades to centuries or if they will ‘tip’ into becoming degraded forests that never recover the biomass and diversity of the original forest. We work here because this may be the region closest to a tipping point.

How do you manage to talk about the urgency of action on the one hand and uncertainties on the other?

It’s hard. All of the years we've talked about climate change and nobody paid attention until it started affecting them personally. So I kind of feel like it doesn't matter what we say, unless we paint a picture of disaster. And even then I don't see that anybody listened until the disaster was upon us.  A good lesson here is that in the 1990s one still lacked the computational power to predict the extreme weather events that nevertheless surprise no one today.

Just because we don't know exactly, when the ecosystem will tip, it doesn't mean that we shouldn't be concerned or continue deforesting. Humans cut down forest on short timescales compared to the way vegetation responds to change. To me, the largest problem we are facing is the loss of biodiversity and biomass. Biodiversity tips with high certainty. What is gone, is gone. And in the Amazon, we may not even know what we lose because many species remain unclassified. We need to protect the original high biomass forests at all costs, because we rely on them now, and it can take hundreds to thousands of years to regrow what is lost.

Deforestation also means a socioeconomic threat to Brazil, which depends on hydro power. It can affect rainfall patterns regionally and thus river discharge.

Totally, yes. Locally, rainfall that is not evaporated because of deforestation can cause flooding. 

How do you assess the impact of the Amazon Summit on the future of the rainforest?

Lula was president of Brazil between 2003 and 2011, a time when large regions of forest were protected and deforestation rates declined even as soy agriculture increased – intensified use of previously deforested lands allowed for economic growth without cutting more forest.

Brazil has had the forest code since 1965, a law that regulates deforestation. With Bolsonaro the laws were not strictly enforced and deforestation increased. Since Lula became president again in 2023, deforestation has started to fall again, but it is important to recognize that what worked previously is highly vulnerable to global prices for agricultural products. It is very complex. The summit ended in an agreement that encourages countries to enforce existing law but also sends a signal to the involved countries that they don’t have to face the socioeconomic challenges on their own.

What does the forest code say?

Private land owners can only deforest 20 percent of their land, with 80 percent maintained under native vegetation. Interestingly, only 10 percent of the land is owned privately, much more has been grabbed illegally on undesignated public forests  – these areas account for 30 percent of current deforestation. It was part of Lula’s previous achievements that large parts of the land, around 50 percent, is protected today as indigenous reserves or sustainable reserves.

What about illegal deforestation related to, for example, timber mafia, drug smuggling, or illegal mining?

A lot is about governance and willingness to enforce the existing law. Farmers supported the idea of zero illegal deforestation. They do value intact rainforest around and they can still legally buy land and deforest 20 percent of it under the forest code. However, this means they are branded ‘bad people’ internationally for deforesting even if they obey the law – this is how farmers can become disenchanted and it likely helped elect Bolsonaro. As Bolsonaro said, Brazil must have the same right to economic growth and if other countries – like Europe and the US deforested to develop, why can’t Brazil do the same? We need to remember that there are multiple stakeholders in this discussion and should be willing to compensate farmers for providing forest protection. Such payments help to prevent deforestation in the first place by making the forest as valuable as what would replace it.

I guess Lula has a more holistic understanding of sustainable growth and considers the social and environmental dimensions in addition to economy.

Sustainability means not just to keep existing forests but to make better use of the lands that are already deforested. Mega-agriculture works in Brazil only through the continuous use of fertilizers and is vulnerable to global prices. One of the great research needs is to find better ways to retain and rebuild nutrients in Amazon soil systems to ensure greater resilience and sustainability.

In addition to local citizens and representatives of indigenous groups, representatives from Germany, Norway and France were also invited to the summit. Why was this?

Norway and Germany have been the principal funders of the Amazon fund. France and Spain have discussed joining. The Amazon fund is a mechanism to take money from high-income nations to preserve the forest and prevent deforestation. Deforested land used to grow soy is worth more than intact forest. Without external funds, there's not much incentive to preserve that forest other than that it's illegal to cut it down.

It's a matter of global responsibility, so I am sure, these few countries are not the only players?

Yes, for example, private companies have gotten into the contribution business after the UN climate conference COP26. Especially the ones that want to become carbon neutral by some time.

On the other hand, offsetting programs promise to reduce our carbon footprint by planting new trees.

There is a lot of carbon stored in one big tree and it can take hundreds of years to be replaced. It makes more sense to preserve the high biomass rainforests rather than planting new trees. However, if you want to preserve it, you have to pay what it's worth. From these offsetting programs, I think you can get about 10 US-Dollars per hectare. To effectively prevent deforestation, it needs to be ten times that.

Is there something that we as individuals can do?

The demand that's driving deforestation is coming from a growing demand for animal products worldwide, which drives the need to grow animal feed. Most of the soy that is grown in the area, where we are researching, goes to feed pigs that get eaten in Germany. So, eat less meat.

In Germany alone, about 60 percent of the agricultural area is used to grow animal feed.


Are you an optimist as a scientist?

I am. Most of my experience was during the presidentship of Lula, when the government actively was looking for science to help design policies. That's very refreshing as an American. We as humans are deciding what the planet looks like. And the knowledge that we're generating could be useful to make wise decisions and plan out a future that's not just sustainable but nice to live in.

Interview: Tobias Beuchert

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