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taw comments on Undiscriminating Skepticism - Less Wrong
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There seems to be plenty of motivated arguing on both sides. But even though climate science is complicated the basic mechanism for CO2 raising temperatures is really simple and well supported by basic science. No one is disputing CO2's absorption spectrum (that I know of). It's possible that CO2 might not have any such effect on aggregate in a complicated system, but that would be quite remarkable and I don't think any mechanism has been proposed (other than that global warming is miraculously balancing out a coming ice age).
Not particularly remarkable. Homeostatic systems are the norm in the world, not the exception; and there are plenty of negative feedback mechanisms for CO2, starting from the most trivial one of more CO2 -> more photosynthesis -> (hopefully) more biomass not biodegraded back into carbon circulation.
I think it's widely accepted such mechanism will bring CO2 levels back to their original equilibrium once anthropogenic emissions end, unfortunately over thousands of years. But - similar mechanisms for methane and CFCs are far faster and we might be already past peak atmospheric methane/CFC.
The upper bound for photosynthesis is constrained by plant populations and the area they cover, not atmospheric CO2 -- adding more CO2 to the air doesn't necessarily increase photosynthetic activity. Human metabolism doesn't increase in step with the number of calories you consume; there's a limit to the base rate at which those biological processes can operate, independent of how much of their base inputs are lying around. Biology is more complicated than that.
RuBisCO activity is usually the limiting step in photosynthesis, and it depends on CO2 concentrations (or CO2 to O2 ratios). Adding more CO2 to the air will increase photosynthetic activity, there's no doubt about it.
RuBisCO is the rate-limiting factor for plants, yes. But there's more CO2 in the air naturally than they can adjust upward to compensate for, even before we factor in human-generated sources. The RuBisCO reaction is not maximally-efficient, which is why attempts to increase the rate of enzymatic activity are at the forefront of genetic engineering research into carbon sequestration. Additionally, the two relevant parameters (carbon dioxide fixing and oxygen incorporation) may already have struck a maximally-efficient tradeoff balance in many species of plants; self-modifying to favor increased CO2 fixation is not a trivial step; the gains here can be translated to losses over there, elsewhere in the biosystem. The organism is not its parts.
Anyway, if tomorrow we come up with plants that have a higher efficiency rate of carbon dioxide fixing, and they start pulling more CO2 from the air per unit time, that won't fundamentally change that the population of plants and the room for them to grow is the determining factor in how much photosynthesis gets conducted -- the RuBisCO reaction occurs in plants and protists such as algae when we're talking about the macroscale, and basically nothing else.
Posit an artificial photosynthetic cell that can pack greater efficiency than the best of plants into the same surface area, and things are different. But we don't have any such thing as yet.