Planting trees - necessary but not enough

We need to plant trees but that isn’t enough - we need to do more, and here’s why.

The bottom line

If we want any chance of limiting global warming to 1.5 C, as agreed in Paris in 2015, we will need to remove gigatons of CO2 from the atmosphere per year.

Nature-based methods such as reforestation/afforestation (planting trees where they originally grew and were cut down/planting trees where previously none were), changes in forest management, uptake and storage by agricultural soils, and blue carbon sinks will and should play a central role in our efforts to draw down excess CO2.

But these solutions are not enough to achieve the CO2 removal we will need. That’s why need to add other CDR methods to help out.

How much can we achieve with nature-based methods?

According to a report by the National Academies of Science, Engineering and Medicine, using exclusively nature-based methods, we can at best expect to remove half of the CO2 we need to. Realistically, the number will be much smaller, because the necessary changes to agricultural soil conservation practices, forestry management practices, and biowaste capture are unlikely to be adopted to the fullest extent possible.

This is how Prof. Tom Crowther, assistant professor of Global Ecosystem Ecology at ETH Zurich, co-chair of the advisory board for the United Nations Decade on Ecosystem Restoration, puts it:

The idea of planting trees everywhere is fundamentally misguided. We cannot simply plant a blanket of trees across the planet and hope to save the world."* Source

How many trees would we need to plant?

To remove half of the excess CO2 we have blown into the air since the beginning of the industrial revolution with trees alone, we would need to plant about 1 trillion trees on about 2 billion acres. If you have no idea how much land that is, you are not alone but here is a number that will make it more tangible: that is almost the entire area of the US or Europe. [link]

Clearly, that is not feasible. Even if it were, the consequences would be disastrous in other ways: organizations that fight global hunger are sounding the alarm about aggressive reforestation and afforestation programs that will divert arable land from agriculture, resulting in mass hunger and displacement of people around the world.

Here’s what Oxfam has to say on the subject:

While stopping deforestation and sustainably restoring and managing lands wherever possible is of course a good thing to do and brings enormous environmental and social benefits, it is mathematically impossible to plant enough trees to meet the combined net zero targets announced by governments and corporations, as there is simply not enough land to do this .
Land is a finite resource that is a vital lifeline for growing food. It is central to the lives and livelihoods of millions of small farmers and local communities around the world.”*
Oxfam, Tightening the Net: Net zero climate targets – implications for land and food equity* [link]

The most recent IPCC report mentions AFOLU (agriculture, forestry and other land use) CDR solutions as having risks.

AFOLU carbon sequestration and GHG emission reduction options have both co-benefits and risks in terms of biodiversity and ecosystem conservation, food and water security, wood supply, livelihoods and land tenure and land-use rights of Indigenous Peoples, local communities and small land owners. Many options have co-benefits but those that compete for land and land-based resources can pose risks.

Source: SPM-43

And here is another number to consider:

Direct air capture technology uses 99.7 per cent less land than planting trees, according to figures from Southern Green Gas, and can also be used across vast areas of non-arable land.
Source

It’s more complicated than “just plant trees”

So planting trees alone will not get us out of the mess we are in because we simply don’t have enough space to plant them. But, unfortunately, the problem does not end there, the way we plant trees these days often creates additional problems. To maximize the carbon dioxide removal by trees we need to change what we plant, where, and how.

Here are some examples.

Ecosystems vs monocultures

While planting trees has broad appeal, the reality is not as unequivocally positive as it may sound.

Large-scale tree-planting programs have high failure rates and result in limited carbon capture for many reasons [link]. Importantly, planting monocultures of fast-growing trees that can be used for timber and pulp production (which is how most reforestation/afforestation efforts work) leads to limited carbon dioxide removal and limited long-term carbon storage.

Instead, what we need to do is protect and restore diverse ecosystems that include trees and many other species. This is possible but is a much longer-term and more expensive undertaking than simply planting billions of seedlings. It would also mean that we immediately stop cutting down forests for agriculture, to build more housing or strip malls or other uses.

Reforestation: an excuse to cutting down forests

Reforestation can serve as an excuse for further cutting down forests. For one, large-scale reforestation of land that was previously used for agriculture can lead to other forests being cut down to replace the agricultural land lost to reforestation. In these cases, intact ecosystems are destroyed and replaced somewhere else with monocultures.

Afforestation has risks

Afforestation means planting trees where they naturally don’t grow. While planting trees everywhere sounds good at first, afforestation can also have negative consequences, e.g. it can destroy ecosystems adapted to these areas. The African savannah is a prime example.

Foresting savannas and other grasslands, however, removes specialized habitat for many animals, reduces the local biodiversity of grasses, and may introduce and even encourage the invasion of non-native species into the landscape.
Source

In addition, forests require a lot of water and afforesting dry areas can lead to water supply issues. For those who want to get into the thick of it, here is a publication about the effects of afforestation on water supply in China.

“Green-washing” or nature-based solutions create moral hazards, too

One of the arguments against using CDR approaches like DAC is that it gives polluters like the fossil fuel industry an excuse along the lines of “look, we suck all that carbon dioxide out of the air, so we can continue to use fossil fuels.” Interestingly, CDR opponents generally do not apply this argument to CDR solutions that are considered natural, such as planting trees.

But that is wrong. Tree planting, too, can be used to greenwash unsustainable industries “Look, we are planting all these trees over there, so we can continue to use fossil fuels.” If anything the risk is greater. And in fact, we’ve seen exactly this with questionable carbon offsets that rely on measures like not cutting down forests, or planting monocultures.

Trees aren’t all that permanent

Permanence is a big issue for carbon sequestration. If we put in a lot of effort and money to capture and remove carbon, ideally it should be permanent, that is, stay put for centuries or more.

In terms of permanence, trees aren’t terrible, but they aren’t great either. Trees naturally fall down, e.g. from winds, and then rot, which releases carbon dioxide back into the atmosphere. And climate change itself is making forests increasingly susceptible to fires, insects, and droughts.

Cutting down a forest releases carbon dioxide into the atmosphere in two main ways. The first is kind of obvious: all of those trees and plants are carbon. And once they’re cut down, they degrade. The second area is that when you cut down the large trees, the deep roots die. And the roots are an enormous store of organic carbon in the soil, which is turned into carbon dioxide.
Charles Harvey, professor of environmental engineering at MIT, source

We really don’t know how much CO2 trees sequester

An added problem is that we really don’t understand in any detail yet, how much CO2 is sequestered by plants over a period of time. Measuring that is a fairly new thing we want to do and so there isn’t a lot of established ways of doing it.

This is the area of the carbon cycle that we know the least about, because it’s really hard to measure. We don’t really understand all of the uptakes and releases from a forest. We don’t really understand how much is taken up into the soils and underground over time.
Charles Harvey, professor of environmental engineering at MIT, source

Nature-based solutions are not enough

In conclusion, nature-based solutions are great but they come with their own problems and limitations. Most importantly, nature-based solutions alone simply can’t be scaled to the size of the problem. We need to pursue a variety of approaches to carbon dioxide removal, including both nature-based solutions and emerging technological methods like direct air capture and the use of various types of minerals.

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