One morning in 2009 the researcher Bonnie Waring he would ride a bus up a mountain in Costa Rica. He would go to the La Selva Biological Station, where he would study the rainforest's response to drought.
Thousands of researchers around the world were grappling with the same questions, trying to understand the fate of tropical forests in a rapidly changing world.
Our society demands too much of these fragile ecosystems, which control the availability of fresh water for millions of people and are home to two-thirds of the planet's terrestrial biodiversity.
Today, after creating the problem, we are asking for an extra effort from these forests.
Plants absorb CO2 from the atmosphere, turning it into leaves, wood and roots.
It seems obvious to us now. We are capable of neglecting even miracles. However, this power has stimulated hope in scientists. That plants, especially fast-growing tropical trees, can act as a natural brake on climate change by capturing much of the CO2 emitted by burning fossil fuels.
Thus, governments, companies and non-profit organizations all over the world are committed to conserving or planting huge numbers of trees.
Can it work?
No. There aren't enough trees to offset human carbon emissions. And there never will be.
Waring recently conducted a study of the available scientific literature to assess how much forests could absorb CO2. If we maximized the amount of vegetation that the whole Earth could contain, we would be able to sequester CO2 enough to offset just ten years of greenhouse gas emissions at current rates.
At that point, we could no longer compensate for anything.
A land full of trees would only postpone the inevitable by four to ten years.
The fate of our species is inextricably linked to the survival of the forests and the biodiversity they contain.
Plants convert CO2 gas into simple sugars - the process known as photosynthesis. These sugars are then used to build the living bodies of plants. If the captured carbon ends up in the wood, it is locked away from the atmosphere for many decades. When plants die, their tissues decompose and become incorporated into the soil.
Some of the plant carbon can remain underground for decades or even centuries. Together, terrestrial plants and soil contain approx 2.500 gigatons of carbon, approximately three times more than what is retained in the atmosphere.
Since plants (particularly trees) are such excellent natural stores for carbon, it stands to reason that increasing the abundance of plants around the world could reduce CO2 concentrations in the atmosphere.
A titanic challenge
Plants need four basic ingredients to grow: light, CO2, water and nutrition (such as nitrogen and phosphorus, the same elements found in plant fertilizer).
Thousands of scientists around the world study how plant growth varies in relation to these four ingredients, to understand how vegetation will respond to climate change.
A truly demanding challenge, given that we humans are simultaneously altering so many aspects of the natural environment. We are warming the globe, changing rainfall patterns, cutting large tracts of forest into tiny fragments, introducing different species.
This makes it difficult to answer the question of how much carbon plants can absorb from the atmosphere.
Whatever that amount, however, researchers unanimously agree that terrestrial ecosystems will never be able to make it on their own.
Scientists estimate how much CO2 can be captured from trees and forests around the world.
In recognition of these limitations, scientists estimate that terrestrial ecosystems may contain enough additional vegetation to absorb between 40 and 100 gigatons of carbon from the atmosphere.
Once this additional growth is achieved (a process that would take a few decades anyway), there would be no additional capacity for natural carbon storage on earth.
It is very little. It is not enough.
Humans are currently pouring CO2 into the atmosphere at a rate of ten gigatons of carbon per year. Natural processes will struggle to keep up with the greenhouse gases generated by the global economy. We are committing suicide.
A single passenger on a return flight from Melbourne to New York City emits twice as much carbon (1600 kg C) as a half-meter-diameter oak tree (750 kg C) can capture.
The catastrophe and the hope
Despite all these physical constraints on plant growth, there are a growing number of large-scale efforts to increase plant cover. The so-called “nature-based” climate solution.
The vast majority of these efforts focus on protecting or expanding forests, as trees contain many times more biomass than shrubs or grasses and therefore represent greater carbon sequestration potential.
However, if terrestrial ecosystems are not seriously analyzed, such an activity can reduce biodiversity and even cause the opposite effect.
It seems like a paradox: can planting trees have a negative impact on the environment?
The answer is yes. To avoid environmental damage, we must avoid planting forests in places where they naturally do not belong. There is no point in deforesting the Amazon forest and then planting new trees elsewhere, perhaps arbitrarily in places with different characteristics.
Before undertaking any expansion of forest habitat, we must ensure that trees are planted in the right place because not all ecosystems on earth can or should support trees.
Planting trees in ecosystems normally dominated by other types of vegetation cannot result in long-term carbon sequestration.
The example of Scottish peat bogs
A particularly illustrative example comes from the Scottish bogs: large areas of land where low vegetation (mainly mosses and grasses) grows in constantly moist soil.
Because decomposition is very slow in acidic, waterlogged soils, dead plants accumulate for very long periods of time, creating peat. UK peatlands contain 20 times more carbon than is found in the nation's forests.
At the end of the 20th century some Scottish fens were drained for tree planting. The operation allowed the tree seedlings to establish themselves, but also accelerated the decay of the peat.
The ecologist Nina Friggens and his colleagues at the University of Exeter estimated that the decomposition of the dried peat released more carbon than the trees planted afterwards could absorb.
Forests are not “CO2 capture machines”
The same goes for the Arctic tundras, where native vegetation is covered in snow all winter, reflecting light and heat. Planting tall, dark-leafed trees in these areas can increase thermal energy absorption and lead to local warming.
But planting trees in forest habitats can also lead to negative environmental outcomes. From the perspective of both carbon sequestration and biodiversity, not all forests are the same: naturally established forests contain more species of plants and animals than human-planted forests. And they often contain even more carbon.
Policies to promote tree planting may unintentionally incentivize deforestation of well-established natural habitats.
Another example of “wrong” forests: Apparently Vida
REUTERS/Jose Cabezas
Another striking example: the program Looking like Vida of the Mexican government, which provides direct payments to landowners to plant trees.
The problem? Many rural landowners clear old, well-established forests to plant new seedlings. This decision, while economically sensible, actually involves the loss of tens of thousands of hectares of mature forest.
It does not work like this
Many well-intentioned organizations seek to plant trees that grow faster, as this theoretically means a higher rate of CO2 “drawing” from the atmosphere.
However, from a climate perspective, what matters is not how quickly a tree can grow, but how much carbon it contains when mature, and how long that carbon resides in the ecosystem.
As a forest ages, it reaches what ecologists call a “steady state.” A state in which the amount of carbon absorbed by trees each year is perfectly balanced by the CO2 released through the respiration of the plants themselves and by the trillions of underground decomposer microbes.
This phenomenon has led to the misperception that old forests are not useful for climate mitigation, because they no longer grow rapidly, and do not sequester further CO2.
The wrong “solution” to the problem is to prioritize tree planting, not the conservation of established forests.
Humans need healthy forests
Supporting natural ecosystems is an important tool in the arsenal of strategies we will need to combat climate change. In any case, however, as mentioned, terrestrial ecosystems will never be able to absorb the amount of carbon released by the combustion of fossil fuels.
Rather than be lulled into the false illusion that we can continue to deforest and torture the planet, as long as we plant new trees elsewhere, we must cut emissions at the source and seek additional strategies to remove accumulated carbon in the atmosphere.
Does this mean that current campaigns to protect and expand the forest are a bad idea? Absolutely not. The protection and expansion of natural habitat, especially forests, is absolutely vital to ensuring the health of our planet.
Forests are more than just carbon sinks. It is the complex green webs that tie together the fate of millions of known species, with millions of others still waiting to be discovered.
