Carbon Sink Decline

This is a follow-up to yesterday’s post on how carbon sinks factor in when discussing goals for climate policy.

Obviously, CO2 levels in the atmosphere are increased only if human emissions are more than whatever is absorbed in carbon sinks at the time. Right now, around 55% of human emissions are absorbed and around 45% remain in the atmosphere.

These carbon sinks can decline in capacity in the future. After yesterday’s post, Glen Peters added some information to the discussion. Kees van der Leun said in response:

Thanks again. So the land and ocean sinks indeed dwindle when emissions go down.

This still doesn’t make much sense to me. To quote from one of the papers Glen Peters cited:

Reducing the amount of CO2 in the atmosphere reduces the natural carbon sinks due to the fact that vegetation productivity will decrease when CO2 decreases and that ocean CO2 uptake will also decrease with decreasing CO2.

That makes sense. CO2 levels have an influence. But why should the oceans or forests care or even know about emissions?

Searching with “carbon sink decline” gives several interesting results.

One of the results on the first page is this 2015 article by Joe Romm at Climate Progress. Romm is worried there about the decline of carbon sinks as discussed in a (at the time) recent study. However, the reason for such decline is not a lack of CO2 in the atmosphere, but feedback mechanism like permafrost melt and wildfires.

If carbon sinks only decline because of declining CO2 levels, that would be a great problem to have. The real problem is exactly the opposite. They decline because of rising CO2 levels.

Another result is this 2015 article at Carbon Brief, which reports on another study that explains that the Amazon rain forest carbon sink capacity has gone down by a third over one decade. Again, the cause for that is not a lack of CO2 in the atmosphere, but rather that trees grow faster with more CO2 available, which makes them more vulnerable.

To sum up, there is ample reason to believe that carbon sinks may decline as a consequence of higher levels of CO2. That of course makes the climate problem worse and increases the chance of runaway feedback loops.

In contrast the level of emissions at any given time doesn’t seem to be relevant at all. And as far as carbon sinks decline with lower levels of CO2, that’s a great problem to have, since it requires CO2 concentrations to go down in the first place.

Also, if emissions go down to 10% of present levels, we would end up with a net negative even if climate sinks declined, but only by 50% of present levels.

Anyway, as long as carbon sinks don’t decline to zero or negative (for example because of a permafrost feedback loop), any CO2 emissions equal or below the carbon sink absorption will not lead to more CO2 in the atmosphere. That’s true independent of what particular value the carbon sink number takes at any point in time.

As long as there is a working carbon sink, net-zero emission is whatever that value is. Right now, about 55% of present emissions.


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