Does Tree Planting Reduce Global Warming?

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Jamie in Chile
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Does Tree Planting Reduce Global Warming?

Post by Jamie in Chile » Fri Jun 14, 2019 6:28 pm

It’s very complicated. If you don’t have time to read this article, the key point is:

to stop climate change, only plant trees in the tropics and nowhere else. Planting trees in the tropics helps with climate change. Planting trees in temperature regions is not a bad thing, and should have some good effects, but from a climate change standpoint, it's probably fairly close to neutral.

Note: This is mostly a copy and paste from another forum, so not an original article.

Planting trees has these effects (at least):

1. Locks up carbon out of the atmosphere as they grow, which reduces global warming.
2. Tree leaves absorb heat, which increases global warming. If the leaves were not there, some of this heat would be reflected back into the atmosphere by the ground and ultimately lost into space. To put this another way, trees reflect less on average than the average bare ground.
3. Forests can create cloudy and rainy weather. This can cause cooling that can reduce global warming.
4. Releases some chemicals produced by the trees into the atmosphere. There is a dispute between 2 scientists about whether this causing a cooling or heating effect to the atmosphere, but it's probable that the overall effect is smaller than points 1 and 2.
5. May reduce soil erosion, which is good for stopping global warming (there is carbon in the soil). However agriculture may be the real killer here, rather than lack of trees.

All of these effects will only exist for the lifetime of the tree or the wood. Forest fires send black soot (carbon) into the atmosphere to form CO2 and cause global warming. So, mass tree planting projects would need to plan to avoid fire or keep fires small as mass fires would mean the overall effect on global warming would be to increase it.

Effects 1,2,3 and 4 vary by various factors including what type of ground the trees are planted on, what type of tree, and the location on the Earth so we cannot make an absolute judgement about whether trees reduce global warming or not that would be correct for all cases. On average, the effect of planting trees is probably fairly neutral to climate change.

We can also consider some specific cases:

A Latitude of 20 or less (tropics).

Points 1 and 3 dominate and therefore the net effect is cooling. In such regions it makes sense to plant trees to reduce global warming.

B Latitude of between 20 and 50
Points 1 and 2 are in rough balance, points 3 and 4 are smaller, and so the overall effect on climate change cannot be said with certainty to be positive or negative but is probably fairly neutral and a small effect. Some evidence indicates that tree planting in temperature regions may cause cooling on a small timescale (e.g. 10 years) but warming on a longer timescale (e.g. 50 or 100 years) as explained in point iv) below.

C Latitude of above 50

Point 2 overcomes point 1, and therefore planting trees causes more global warming. Point 3 is small. Point 4 is more likely a cooling effect at these latitudes, because of the type of chemicals emitted by boreal forests, but probably not enough to overcome the dominating point 2. This is partly or mainly because of the large amount of snow at these latitudes (mainly Canada and Siberia) in the winter, which strongly reflect heat back into space. Planting trees blocks the light path to the snow and ice; the trees absorb the heat instead. Therefore we should not mass plant trees at these latitudes. The same logic may apply to high altitude areas of temperate regions. A good guideline is that in a place where snow lies all winter, do not plant trees.

Other thoughts
i) If you live in a temperature zone, and want to plant trees to fight climate change, it makes more sense to support a project in the tropics rather than a local one.

ii) There is a large amount of uncertainty. Therefore using tree planting to offset specific numbers is not fair, e.g. if you think your carbon emissions for the year were 9.7 tonnes and a website set if you pay x dollars it will plan y trees to offset that number. Any website offering that level of precision without mentioning about the huge margin for error should not be trusted. Likewise, marketing or media statements like "buying this electric car is equivalent to planting 13,456 trees" or "China has shut down x coal fired plants, equivalent to planting y trees" or any statement that links an amount of global warming/climate change/carbon in the atmosphere to a specific number of trees are at best exaggerating the level of precision and at worst total nonsense. Especially when we factor in other questions: would someone else have planted trees there anyway? How do we know how long before they are cut down or burnt down? If you want to offset a specific amount of emissions accurately, you need to do it another way. Not with trees.

iii) Trees don't last forever, so tree planting to offset carbon emissions doesn't really offset them, it just delays them. At some point 10 or 100 years later, the carbon will likely end up back in the atmosphere. Once that happens, the net effect will have been to cause warming by then (except in the tropics) because the carbon has been returned, and the differential albedo effect - i.e. greater absorption of heat by trees vs reflection by bare ground- has caused warming in the meantime.

iv) Point 1, tree growth sucking carbon out of the atmosphere occurs primarily in the early growth phase of a tree, so planting trees has a positive effect on climate change in the first few years in most cases. The albedo effect, point 2, builds up over time. So in many cases tree planting causes global cooling in the short term but warming in the long term (not sure if this point is valid in the tropics but this point is more for the temperate region).

v) Large scale forest fires in tropical regions are particularly disastrous from a climate change perspective.

Major source
https://www.pnas.org/content/104/16/6550.abstract

Other sources
https://www.nature.com/articles/d41586-019-00122-z
https://www.naturalclimate.solutions/the-science
https://www.theguardian.com/environment ... eandhealth
https://www.llnl.gov/news/models-show-g ... al-warming
https://www.bbc.com/news/science-environment-48043134 (soil)
https://forestsnews.cifor.org/24311/on- ... C62S2ddWSp
https://onlinelibrary.wiley.com/doi/abs ... /gcb.12483
https://agupubs.onlinelibrary.wiley.com ... 16GL071459 (this one also links to various other studies)

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