[Vision2020] Stefan Rahmstorf on Realclimate.org 7-16-19: Can planting trees save our climate?

[Ted Moffett]

I've been impressed numerous times in reading Stefan Rahmstorf's climate
science published papers and other commentary by his ability to render what
is complicated professional science in terms more easily understood by a
layperson, as they say.  This is especially impressive given his extensive
scientific publications and imposing genius.

I recently heard a radio report on the claims discussed below regarding
forest planting mitigation of human sourced atmospheric CO2 emissions and
was skeptical.  Rahmstorf offers reasons for caution regarding an overly
optimistic hope for tree planting to solve the climate crisis.
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http://www.realclimate.org/index.php/archives/2019/07/can-planting-trees-save-our-climate/#comments


Can planting trees save our climate?
Filed under:

   - Climate Science
   <http://www.realclimate.org/index.php/archives/category/climate-science/>

 — stefan @ 16 July 2019

In recent weeks, a new study by researchers at ETH Zurich has hit the
headlines worldwide (Bastin et al. 2019
<https://science.sciencemag.org/content/365/6448/76>). It is about trees.
The researchers asked themselves the question: how much carbon could we
store if we planted trees everywhere in the world where the land is not
already used for agriculture or cities? Since the leaves of trees extract
carbon in the form of carbon dioxide – CO2 – from the air and then release
the oxygen – O2 – again, this is a great climate protection measure. The
researchers estimated 200 billion tons of carbon could be stored in this
way – provided we plant over a trillion trees.

The media impact of the new study was mainly based on the statement in the ETH
press release
<https://ethz.ch/en/news-and-events/eth-news/news/2019/07/how-trees-could-save-the-climate.html>that
planting trees could offset two thirds of the man-made CO2 increase in the
atmosphere to date. To be able to largely compensate for the consequences
of more than two centuries of industrial development with such a simple and
hardly controversial measure – that sounds like a dream! And it was
immediately welcomed by those who still dream of climate mitigation that
doesn’t hurt anyone.

Unfortunately, it’s also too good to be true. Because apples are compared
to oranges and important feedbacks in the Earth system are forgotten. With
a few basic facts about the CO2increase in our atmosphere this is easy to
understand. Mankind is currently blowing 11 billion tonnes of carbon
(gigatonnes C, abbreviated GtC) into the air every year in the form of CO2 –
and the trend is rising. These 11 GtC correspond to 40 gigatons of CO2,
because the CO2molecule is 3.7 times heavier than only the C atom. Since
1850, the total has been 640 GtC – of which 31 % is land use (mostly
deforestation), 67 % fossil energy and 2 % other sources. All these figures
are from the Global Carbon Project <https://www.globalcarbonproject.org/>,
an international research consortium dedicated to the monitoring of
greenhouse gases.

The result is that the amount of CO2 in our air has risen by half and is
thus higher than it has been for at least 3 million years (Willeit et al.
2019 <https://advances.sciencemag.org/content/5/4/eaav7337>). This is the
main reason for the ongoing global warming. The greenhouse effect of CO2 has
been known since the 19th century; it is physically understood and
completely undisputed in science.

But: this CO2 increase in the air is only equivalent to a total of just
under 300 GtC, although we emitted 640 GtC! This means that, fortunately,
only less than half of our emissions remained in the atmosphere, the rest
was absorbed by oceans and forests. Which incidentally proves that the
CO2 increase
in the atmosphere was caused entirely by humans. The additional CO2 does
not come from the ocean or anywhere else from nature. The opposite is true:
the natural Earth system absorbs part of our CO2 burden from the atmosphere.

Conversely, this also means that if we extract 200 GtC from the atmosphere,
the amount in the atmosphere does not decrease by 200 GtC, but by much
less, because oceans and forests also buffer this. This, too, has already
been examined in more detail in the scientific literature. Jones et al. 2016
<https://iopscience.iop.org/article/10.1088/1748-9326/11/9/095012> found
that the amount of carbon removed from the atmosphere amounts to only 60%
or less of the negative emissions, when these are implemented on the
background of a mitigation scenario (RCP2.6).

We can also compare the “negative emissions” from tree planting to our
other emissions. The 200 GtC would be less than one third of the 640 GtC
total emissions, not two thirds. And the authors of the new study say that
it would take fifty to one hundred years for the thousand billion trees to
store 200 GtC – an average of 2 to 4 GtC per year, compared to our current
emissions of 11 GtC per year. That’s about one-fifth to one-third – and
this proportion will decrease if emissions continue to grow. This sounds
quite different from the prospect of solving two-thirds of the climate
problem with trees. And precisely because reforestation takes a very long
time, it should be taboo today to cut down mature, species-rich forests,
which are large carbon reservoirs and a valuable treasure trove of
biological diversity.

There is another problem that the authors do not mention: a considerable
part of the lands eligible for planting are in the far north in Alaska,
Canada, Finland and Siberia. Although it is possible to store carbon there
with trees, albeit very slowly, this would be counterproductive for the
climate. For in snowy regions, forests are much darker than snow-covered
unwooded areas. While the latter reflect a lot of solar radiation back into
space, the forests absorb it and thus increase global warming instead of
reducing it (Bala et al. 2007
<https://www.pnas.org/content/104/16/6550>, Perugini
et al. 2017 <https://iopscience.iop.org/article/10.1088/1748-9326/aa6b3f>).
And increased regional warming of the Arctic permafrost areas in particular
would be a terrible mistake: permafrost contains more carbon than all trees
on earth together, around 1,400 GtC. We’d be fools to wake this sleeping
giant.

And there are other question marks. Using high-resolution satellite maps
and Google Earth, the researchers have analyzed where there is a suitable
place for forests where none is currently growing, leaving out farmland and
cities. With the help of machine learning technology, natural areas around
the world were evaluated to determine the climate and soil conditions under
which forests can thrive. The free and suitable land areas found in this
way amount to 1.8 billion hectares – as much as the combined area of China
and the USA.

But for many of these areas, there are probably good reasons why there is
currently no forest. Often they are simply grazing lands – the authors
respond that they have only assumed loose tree cover there, which could
even be beneficial for grazing animals. The Dutch or Irish pastures would
then resemble a savannah. Nevertheless, there are likely to be considerable
obstacles of very different kinds on many of these areas, which are not
apparent from the bird’s-eye view of the satellites. The authors of the
study also write that it is unclear how much of the areas found would
actually be available for planting.

Therefore, I’d still consider it optimistic to assume that half of the
calculated theoretical planting potential can be realized in practice. Then
we’re talking of 1-2 GtC of negative emissions per year. But that is
precisely what we will need urgently in the future. The current global
CO2emissions
can be reduced by 80-90 % through transforming our energy, heating and
transport systems – but there will remain a rest that will be hard get rid
of (e.g. from agriculture, industrial processes and long-haul flights) and
that we will have to offset in order to stabilize the global climate.

The study by the ETH researchers has another important result that has
hardly been reported. Without effective climate protection, progressive
warming will lead to a massive loss of existing forest cover, especially in
the tropics. At the same time, the models are not yet able to make reliable
statements on how forests can cope with new extremes, fire, thawing
permafrost, insects, fungi and diseases in a changing climate.

The massive planting of trees worldwide is therefore a project that we
should tackle quickly. We should not do that with monocultures but
carefully, close to nature and sustainably, in order to reap various
additional benefits of forests on local climate, biodiversity, water cycle
and even as a food source. But we must not fall for illusions about how
many billions of tons of CO2 this will take out of the atmosphere. And
certainly not for the illusion that this will buy us time before abandoning
fossil fuel use. On the contrary, we need a rapid end to fossil energy use
precisely because we want to preserve the world’s existing forests.

*Links*

Would a large-scale tree restoration effort stop climate change?
<https://blog.efi.int/would-a-large-scale-tree-restoration-effort-stop-climate-change/>Forest
expert Marcus Lindner from EFI points to the fires in Russia and the
success story in China.

How to erase 100 years of carbon emissions?
<https://www.nationalgeographic.com/environment/2019/07/how-to-erase-100-years-carbon-emissions-plant-trees/>
Plant
trees-lots of them. National Geographic shows the importance of indigenous
peoples as guardians of the forest.

Restoring forests as a means to many ends
<https://science.sciencemag.org/content/365/6448/24> The commentary in
Science on the Bastin study revolves around the question of how sustainable
reforestation can be designed with multiple benefits beyond mere carbon
storage.

Tree planting ‘has mind-blowing potential’ to tackle climate crisis
<https://www.theguardian.com/environment/2019/jul/04/planting-billions-trees-best-tackle-climate-crisis-scientists-canopy-emissions>
 Guardian
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