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Comment by pmontra | Hacker News Reader

There is one thing that I don't understand. I plant 1000 trees, I sequester the CO2 I create by driving, heating my home and all the activities done by third parties to keep me alive (food, hospitals, etc). But that CO2 is still under the sky and most of it will be released when the trees die. It's not like we're pumping it back into oil fields or coal mines and never see it again. So this 1T trees initiative is a stop gap measure to buy us some time but it won't solve the problem. But it's better than doing nothing.

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If a tree is planted, grows, and then for example is burned down and paved over, that'd be true. But if a tree grows and eventually dies in a forest, it'll be replaced with new trees naturally, cancelling out the CO2 leaked from the tree dying. This C02 can also be stalled by harvesting the tree and using it in some way that stops C02 from being released for a longer time than natural.

I did some math on this, and if all the countries in the advanced world planted fast growing trees alongside all paved and unpaved roads, over 40 years, that could potentially be enough to sequester half of all carbon emitted by humans.

Given that carbon has a half life of between 20-200 years, and that we appear to be reaching peak emissions, and that a lot of estimates say our emissions should drop by 70% just given how often we replace power plants and how completive renewables are right now and are projected to be in the future -- it seems like planting trees really could be enough.

Could you share your math somewhere? I’m genuinely interested, and did your calculations take into account lowered cooling costs caused by tree shaded buildings?

It did not. But I think that's a HUGE consideration, as is the potential to reduce particulates in the air dramatically.

Here's a link to the discussion: https://news.ycombinator.com/item?id=20543605 -- on mobile so I can't figure out how to get the comment URL. If you're interested, just go there and search for my username -- onlyrealcuzzo. I'd love a second set of eyes on it. Not sure the math is right.

What does "carbon has a half life of between 20-200 years" mean in this comment?

Good question.

I've read in several places that 50% of carbon in the atmosphere is absorbed by the ocean within 20-200 years.

I'm guessing that's bad for the ocean, and maybe there's a limit, and the rate probably isn't constant. It's not a half-life at all.

That is horrible for the ocean. Acidification will be killing off whole ecosystems and lead to drastically lower carbon sequestration by our oceans. As well as other horrible problems for us humans, who are still quite dependent on functional living oceans.

I think the OP is saying that most trees you plant now will be dead in 20-200 years and returning their carbon to the atmosphere through natural decay. Trees end up being a temporary solution. A permanent solution basically requires us to bury the carbon back in the ground where we found it in the first place, or in some other form that doesn't decay.

One problem I can see right off the bat is at the trees might rec a lot of the pavement (from roots uplifting the pavement).

Walking around parts of Portland on the sidewalk is like being on a rollercoaster because of the tree roots.

Right, that's why you'd want to plant trees with specific types of roots that wouldn't cause this damage. But it's an important thing to consider.

Please provide numbers.

Numbers are important and should be discussed transparently. Lots of variation in guesstimates, fudging, simplifications, etc cloud the issue and give drastically different "good ways forward". So, please, correct my back-of-the-envelope estimate below:

E.g. from [1]: "Following the estimations of carbon stock including above ground biomass, the total stock in the studied chestnut forests could be ordered as follows: CF3 (105.8 t C ha-1) > CF1 (102.1 t C ha-1) > CF2 (76.3 t C ha-1). The pure chestnut forest CF3 characterized with the highest total carbon stock per hectare and only 20.6 % from it is accumulated in the aboveground tree biomass."

Chestnut is one of the best carbon sequestering forest options we have. Estimate (overly positively) that this capacity cycles over 100y. Fudge down the CO2 and CH4 release from decomposition. That gives ca 100 tC/ha over 100y, aka 1tC/(ha y) or 100tC/(km2 y).

Globally we have ca 35GtCO2 or 10GtC carbon emission per year. Current forests store about 15% of that, but we are currently loosing forests at a rapid rate, and we need to not only reach neutral but also sink the 100y or so of spewing we've already fucked up. So let's keep the number at 10GtC/y.

10GtC/y would then require 100E6km2 of NEW perfect assumption chestnut forest.

We have ca 500E6km2 of total land surface on earth. That means we need to set aside 20% of all land on earth for our NEW chestnut forest. But we don't have that much land that can sustain something like this. Forest grows less well outside of humid temperate to tropical regions. E.g. Antarctica, Sahara, the steppes, boreal forest regions, etc. E.g: All agricultural land, both fields and grazing, is 11% of total land area but over one third of all culturable land, the rest of which is already mostly forest. Even if we covered all growth-possible land area that is not already forest we simply don't have enough land. And we would be out of farmland.

Based on this back of envelope calculation tree planting is not a viable solution. It's a good step in the right direction, but nothing to get the crowds overly excited about.

Instead, e.g. azolla is around 15x more potent as sequestering growth stock than forests, and it grows floating on water. Genetically engineering azolla to grow fast in higher salinity would be a better way to combat climate change. We have plenty of ocean surface area. There will be downsides to such a solution, but compare with other solutions or the horrors of what will be if we don't act...

And I'm sure someone can cook up some gm-franken-algae that is even more potent.

My point, if any, is to make sure to actually run the numbers. Get different estimates then pay skilled scientists to get better estimates. Discuss the numbers, assumptions, simplifications. Don't get the general population stuck on low worth paths when they could start fighting for more valuable ones.

And I don't propose "stop planting trees". I argue that planting trees is a good thing, but only a minor item that is overshadowing more valuable options in the general debate. We need to get people to start facing reality to the point where they can start thinking about compromises. There is no "solution" that don't require drastic changes in how we use our land and live our lives.

Side note: nuclear is the only short term low land-usage footprint solution I can see, and drastic genetic manipulation biomass or vast nuclear powered direct capture are the only long term solutions I can guesstimate. Anyone that can show me interesting alternatives?

[1]: https://www.researchgate.net/publication/285516686_Carbon_st...

This is correct. At this point, we need to buy all the time we can, in addition to all other measures. In commercial forestry trees, are usually harvested when they’re 60 to 80 years old, but they remain net carbon sinks long after that if allowed to grow. Proper forest management practices minimize carbon re-emission from rotting plant matter.

> most of it will be released when the trees die

The idea is not to just plant trees but to increase the number of trees on the planet over the long term so that effectively the CO2 absorbed does not go back to the atmosphere.

This is aimed at reducing CO2 in the atmosphere and has to go hand in hand with reducing emissions.

The goal is to increase the biomass. There is no reason it stays constant. More soil, more forest, higher trees.

I tried to find literature, but came back empty handed, on whether it is better to let tree grows on a parcel, or cut grass regularly and let it compost, adding biomass to it.

>whether it is better to let tree grows on a parcel, or cut grass regularly and let it compost, adding biomass to it.

Of those two options, trees. Compost does not sequester carbon in and of itself. That's a wishful thinking myth that is based on a lack of understanding of soil. Compost is organic matter, in various stages of life, death and decay. We want soil to have more organic matter. So people assumed adding compost to dirt would accomplish this. It does not. The organic matter is decomposed into CO2 and returns to the atmosphere. We need to increase STABLE organic matter in the soil, and compost does not do that simply by adding it to dirt. The only thing that creates stable organic matter is microbes living in the rhizosphere in symbiosis with plants. In order to increase soil SOM, we need more root mass. The way compost can help is to plant a forest or meadow and keep soaking it in compost tea so that the roots get deeper and deeper, expanding the rhizosphere and the depth of the soil. Normal roots will only grow as deep as there are symbiotic microbes, and yet those microbes will only live where there are roots, so there's a catch-22. We can use the microbes in compost, dissolved into water as compost tea, to get microbes deeper than the roots, so the roots go deeper, and then keep doing that. There's people with grass that has 20 foot deep roots from doing this.

Thanks! I was looking for that kind of answers!

So does that mean that at any given place, there won't be soil for deeper than the roots go? What happens to the soul if all the roots die or are removed (assuming it is not washed away by water)? Does it decay into CO2?

Different plants have different behaviours. Some sequester carbon well in the ground, others have near zero long term sequestration value. Some of the best are algae and seaweed that are allowed to sink to the bottom when they die. Plants that grow in bogs behave similarly. Chestnut is one of the best long term carbon sequestering trees.

You're right. We need two things.

One is carbon-sinks. This can be done by permanent new trees. Or by replacing trees and use the old wood or plastics for buildings. Or make carbon based usables and permenantly bury their waste in deep landfils that don't let out co2. Or by new techniques for sequestering carbon from air. Planting trees and letting them die is only temporary.

Second and most important though is getting more heat out of the planet than in. Focusing on carbon is one thing. But reflecting energy to space also works, e.g. by white roofs, or mirrors, or heat conducting materials that send off energy in frequencies not absorbed by clouds and co2/water/methane.

Don’t plants convert CO2 into O2 and sugar, meaning the CO2 no longer exists? When the tree dies what mechanism causes CO2 to be released?

Fungi, bugs, bacteria, etc. break down the tree. Some CO2 gets released during this process and then CO2 gets released when the bugs die and decompose. CO2 also gets released if the dead forest burns.

Decomposition. Organisms that feed on the dead tree consume the sugars and O2 and make CO2.

If the tree is harvested for lumber or buried then the CO2 is not released back into the atmosphere

This comment does not scale linearly.

It is a stop gap, but it's at worst a 50 year stop gap and we can go a long way in reducing our gross emissions in that time.

trees sequester a lot more carbon into the ground than just into the physical tree itself

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