2.2: Two climate dynamics to be aware off

Removal effects are not instant

First, it is likely much slower to take effect than albedo modification, since it would take a lot of time and energy to build the infrastructure, which then must suck out the carbon little by little. Then the climate system itself needs some time to respond vis a vis feedback effects, e.g., the atmospheric water vapor concentration will take some time to respond to the decreased CO2 just as it takes time to do so when CO2 is increasing.

Natural sinks turning to natural sources

Fast-acting natural sinks can turn into fast-acting natural sources in a period of declining CO2.

graph of carbon decay over time
A paper by Ken Caldeira’s group emphasizes that a one-time removal of excess atmospheric CO2 is not enough, as more carbon will then be released from sinks like the oceans, so it would seem that there would need to be an ongoing removal project to enduringly drive down CO2 concentrations and the average surface temperature. This leads to about a factor of 2 overhead in total carbon dioxide that needs to be removed. Here is the relevant figure from Caldeira’s paper:

I’m not sure I understand the full implications, but the NAS report has some commentary in the introduction that may at least add some subtlety to this point, however:

I think it depends on the timescales being considered, as well. In any case, this is a complexity to be aware of but doesn’t strongly influence what we can say about the technologies below — we’re talking “orders of magnitude” here, not exact costs.

“The second misconception is that the natural sinks would reverse and become sources during a period of declining atmospheric CO2. Instead, the sinks are expected to persist for more than a century of declining CO2 because of the continued disequilibrium uptake by the long-lived carbon pools in the ocean and terrestrial biosphere. For example, to reduce atmospheric CO2 from 450 to 400 ppm, it would not be necessary to create net negative anthropogenic emissions equal to the net positive historical emissions that caused the concentration to increase from 400 to 450 ppm. The persistent disequilibrium uptake by the land and ocean carbon sinks would allow for achievement of this reduction even with net positive anthropogenic emissions during the 50 ppm decline.”