[counters]

> Wait... So, to undo it all we have to do is stop doing it? Doesn't this contradict the statement right before it?

It's not quite that simple.

The intuition that you're subtly relying on is the idea that the response or effect of one of these geoengineering treatments is linear. But unfortunately, that's not something you can assume about a dynamical system. In reality, the climate system can undergo certain types of hysteresis where "undoing" the forcing doesn't revert the initial perturbation, because you're suddenly on a different response curve. Probably the most famous example of this in climate dynamics is the way that the ice-albedo effect sets up a hysteresis in the trajectory towards a "snowball Earth" scenario. Apologies for the lack of links/references; Wiki has decent write-ups on this, and it's typically covered in the first chapter of a climate dynamics textbook.

The potential response to suddenly stopping a climate change mitigation strategy has a very well-popularized name: a "termination shock." In fact, Neal Stephenson used exactly this concept in his titular novel on the topic in 2021.

As a climate scientist, my mental model to better understand the risk of termination shocks and unintended consequences boils down to how fast the response of the climate system is. Marine brightening is "less risky" because the meteorological response to these interventions is extremely fast; a cloud-precipitation system will respond on the order of minutes to hours, and unless the intervention continues unabated, it will clean the air quickly, limiting the repsonse. Stratospheric aerosol injection is more complicated, but we have a very good analogue - very large scale volcanic eruptions like Mt Pinatubo. The response to these sorts of events is measured more on the timescale of 2-5 years, although knock-on effects (such as a shift towards more diffuse solar radiation reaching the surface, which has significant effects on terrestrial and oceanic biogeochemistry) could very much persist longer than that - and don't "snap back" nearly as quickly. A continuous, Pinatubo-like intervention would compound and introduce coupled atmosphere/ocean responses that could decade years or longer to fully play out. And that's _in addition_ to the near immediate (1-2 year) response in global average temperature, which would bounce back to most of the pre-intervention level very quickly.

These things are complex. There's a lot we don't know. But, there's also a lot we _do_ know. I would encourage anyone who does not have significant experience in climate dynamics to remain curious and avoid jumping to conclusions based on simple intuition; they're probably wrong.

[FloorEgg]

Thank you for this response. Of those that replied to me, yours seems the most balanced and scientific, and I learned the most from. I wish more often people engaged on HN like you have here.

Given your expertise in this, I'm curious what your take is on CO2 capture, not in terms of economic viability, but in terms of climate risk...

For example, if we were to discover a process that removed CO2 from atmosphere and converted it into a product profitably such that there was an economic incentive/positive feedback loop to remove CO2.

My intuition is that if we removed the CO2 too quickly or too much of it we may have unwanted consequences, but if the rate was managed and we slowed down and stopped at a certain equilibrium, would this be a theoretically ideal way to address the problem?

[counters]

Some things to consider.

First, what is "too quickly" with reference to CO2 removal from the atmosphere? At present, human civilization emits over 40 gigatons - or 40 trillion kilograms - of CO2 per year. And that increases the atmospheric burden by about 2.5 parts per million per year. So today, before you even start _reducing_ atmospheric CO2, you need to be sucking down at least 40 trillion kilograms of CO2. I struggle to imagine a scenario outside of total science fiction where that would be remotely possible.

Second, the equilibrium climate response to changes in greenhouse gas forcing take on the order of decades or centuries to realize. This is because the dynamics of the climate system are heavily buffered. For example, the ocean acts as a giant heat capacitor that slowly interchanges with the atmosphere. Were you to instantaneously halve the CO2 in the atmosphere, you'd likely see a pretty classic exponential decay in global average temperature (and other more nuanced responses); in the present climate, it's not clear we have already passed specific "tipping points" that would induce that hysteresis I described in the previous comment (in fact - one could read "climate tipping point" as a synonym for dynamical system hysteresis). Theoretically, one could try to "dial in" some particular equilibrium climate state, but it's not obvious over what timescale you'd have to intervene and what the cost of such an intervention would be.

The cool thing is none of this needs to be purely "theoretical." You could simulate all of this _today_ if you had a setup to run a global climate model. A "4X CO2" experiment where you branch from an equilibrium spin-up climate state and immediately apply a global quadrupling of CO2 has been a completely standard experiment as part of CMIP for over two decades. The opposite experiment is an established protocol for both the Carbon Dioxide Removal Intercomparison Project [1], which features an annual ramp down of CO2 at a 1% per year rate, and the Cloud Feedback Model Intercomparison Project [2], which features a more direct counterpart, with an abrupt decrease of atmospheric CO2 by 50%. There is a large body of literature discussing the results of these classes of experiments, but this is outside of my primary research focus and I can't turn you to particularly good papers off-hand. But they're easy enough to find.

[1]: https://www.geomar.de/en/cdrmip [2]: https://www.cfmip.org/experiments/cfmip3cmip6