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Comment by biotinker | Hacker News Reader

Of the amount of CO2 released each year by humans (~30gt), only about half persists in the atmosphere. The other half is re-absorbed by the biosphere.

This is based on estimated total biosphere emissions, estimated human emissions, and annual CO2 PPM increases. I did some math on that and wrote it up here if anyone wants to dig deeper: https://biotinker.dev/posts/climate1.html

In the case of the zero-emission miracle, atmospheric CO2 levels would start dropping at about the same rate they are currently climbing. As you mention, there is climate lag and feedbacks, but the "return to normal" would start fairly quickly.

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I'm sorry but I think you're missing a lot in your assumptions.

For example, there are only rough estimates on how much methane is trapped in the permafrost. The first estimate I saw by Natalia Shakhova et al was 1400gt over a decade ago (so about 45 years of human emissions at the current rate). I found this recent MIT article which mentions 1500gt [1]. Let's not forget methane has a 25x more heat trapping capacity than CO2. Do the math on that. Also the permafrost is just one of many feedbacks.

Another point you're not considering is that the capacity of the biosphere to absorb carbon is being reduced. For example, the Amazon used to be considered a carbon sink but it is becoming a carbon source [2].

[1] https://climate.mit.edu/explainers/permafrost

[2] https://www.nature.com/articles/d41586-021-01871-6

You're right about methane; that's much more difficult to find data on, though, especially with regard to how much would be released and at what point. But if those tipping points get hit, then nothing CO2 related will save us. Feedback loops here dying down I'll classify under the "hope they're included in the miracle that gave us zero carbon emissions" :)

I'm not convinced on your second point; I'll need to see more data. I read the paper you linked, it doesn't give hard numbers, so it's difficult to compare the net loss of carbon flux in the amazon, to gain of carbon flux in other areas that are getting net-reforested, such as the eastern United States. Examples of local areas flipping from sink to source, and others becoming larger sinks, don't paint a clear picture of the directionality of the whole system.

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