I don't think planting trees is the biggest issue. Access to water is. More practical solution would be to fill some desert with sea water and grow plankton, kelp etc.

Deforestation is one of the reasons some deserts have spread so rapidly. trees behave as a shield to that encroachment

I recently saw a video about doing this - apparently the desert is actually a really bad option if we want to combat climate change - the core problem is that the desert currently reflects a lot of light from the sun. If we cover it with forest or whatever, then suddenly we adjust the albedo of the earth and absorb more heat, negating some of the impact. Combined with the energy need to pump (and desalinate for trees - your suggestion of kelp would presumably help there) water, etc... make it not really work.

Source: https://www.youtube.com/watch?v=lfo8XHGFAIQ

I recommend watching the video - it explains it better and goes into detail - it cites its own sources.

I assume trees are good at trapping the energy from sunlight + combining it with c02 and water to produce carbon, oxygen and more water.

So planting trees in deserts with somewhat decent access to underground water tables should help cool the planet and create a great renewable resource right?

I don't think planting trees is considered to be the unquestionably most practical solution. But it's a simple solution that people can do on their own, and it has a lot of other side benefits like habitat, shade, & proven health benefits in humans.

That’s the price of the seeds though. The cost of finding the land, digging holes, planting seeds and watering them to sustainable growth would be much higher.

Honestly if HN knows any charities focused on tree planting, let me know. Happy to donate 1k-ish if it puts 10k trees to our family’s name.

Is that just for the seedlings and not the actual labor of planting them?

I've seen quotes for parts of developing countries where planting trees would cost $100 USD per thousands trees.

See FAQ point #11: https://trees.org/faqs/

> 40 dollars to plant a thousand trees

Presumably these would be some of the cheapest trees to plant. When that space is used up but the first few billion trees, where will the next billion go? How much will that cost?

trees are only a co2 buffer, they don't make it dissappear. and trees require water, and the burn up (which puts the c02 back also)

Even if we had to replant the trees every decade, that would only be 5 billion dollars a year, forever. Trivially affordable at "world power" scale.

Lack of water is a much more real blocker, with space not far behind.