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.

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

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.

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.

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...

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.

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.

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.

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?

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.