Trees are carbon neutral, not carbon sinks. You grow it, and it's a carbon sink. But then it dies. If by fire it's releases the CO2 quickly, if not it rots slowly and reduces the CO2 slowly. Neither are a fix for our short term problems.

Growing algae or plankton on the ocean, where most of them die and land on the (mostly inert) deep ocean floor would help. Not perfect, but at least the CO2 is likely be consumed on the ocean floor slower than it's produced on the surface thanks to the cold, dark, high pressure environment on the ocean floor.

One research claims that the limit for growth in many places in the ocean is a lack of iron. So spreading iron would lead to a huge increase in life. Unfortunately the impact seems to last days.

It's true that they are not a permanent carbon sink unless fossilised, but forests do store huge amounts of carbon, both in the living biomass and in the soil under them.

There is a strong seasonal variation in atmospheric carbon (several ppm) as the northern hemisphere biomass uptakes carbon in the summer growth season.

The IPCC report estimates a possibility of 2GT/year from afforestation.

(And expect us to need 100-1000GT extracted by 2100 depending on how quickly we get rid of emissions.)

Right, so if you manage to start a healthy forest it is a carbon sink... as long as the biomass is increasing. After some decades it flattens out consuming land forever, but being mostly carbon neutral.

So to offset a non-zero amount of carbon emissions you have to steadily increase the number of acres of forest. Unfortunately this puts pressure on other things like farming, which causes them to be more energy intensive to get enough density.

From what I can tell the answers are things like nuclear, solar, and wind. Covering every parking lot with solar would be a good start.

On that topic, I found this ted talk on the feasibility of various renewable energy sources quite eye opening https://youtu.be/E0W1ZZYIV8o

Should perhaps add that the 2GT/y thing was in the context of tolerating a peak of 1.7 degrees and still being able to reach 1.5 by 2100 (since no scenario can stay below 1.5). It was not a bout a sustainable compensation for current emissions, if that was the impression.

Trees burned in a low oxygen environment (producing charcoal) are carbon negative. I don't know if this is feasible

True, but they are still a fuel waiting to catch fire.

Actually you could just bury them, really, really deep.

In the long run, this would just end up creating more coal. Some future civilization's robber-barons would dig it up and burn it.

Like the 1-million years long run

Trees take years, and a lot of labour to grow, cut, and bury.

A tree takes out ~1 tonne of carbon out of the atmosphere, over a 40 year lifespan.

Since the industrial revolution, we've emitted 400 billion tonnes of carbon into the atmosphere. By 2050, we'll emit another 150 billion tonnes.

If we want to sequester half of that, we'll need to grow, log, and bury 275 billion trees. Who is going to pay for this?

Bonus points: Digging up coal, so you can burn it, at the same time that you're growing trees, so you can bury them, has got to be one of the worst ideas in the energy-economy space.

I expect that covering the whole earth in forest wouldn't be enough.

Humanity has reduced forest cover and released fossil carbon. You're proposing to undo the one but it won't undo the other.