1. That might be some reason to doubt Cambrian bioproductivity estimates

2. In the alternative, a huge part of the Northern hemisphere is unproductive most of the year, either Arctic or Desert. A warmer Arctic and a wetter desert (without equal trade-offs elsewhere) would give a giant boost to net bioproductivity.

3. The main protein involved in carbon capture is RuBisCO (‘The most abundant protein on earth’ – much of the whole biosphere is made of RuBisCO and Cellulose). The genetics of RuBisCO haven’t changed much in many millions of years, but it is indeed the subject of current investigations in genetic engineering. But if there were easy win modifications, you’d think nature would have found them. La Wik, “Some enzymes can carry out thousands of chemical reactions each second. However, RuBisCO is slow, being able to fix only 3-10 carbon dioxide molecules each second per molecule of enzyme.”

Also, CO2 enrichment has been used as a technique for a century, and, unlike with synthetic fixed-nitrogen, ‘green-revolution’-style horticultural efforts to create hybrids and new breeds that can make use of much higher levels of CO2 have not been … ahem … borne much fruit.

Like all catalytic enzymes, it has a certain reaction rate of chemical process, and a range of ideal concentration in chloroplasts. At optimal concentration, and given exogenous atmospheric temperature and pressure, the overall rate of photosynthesis is dependent on the concentration of the precursors. But plants will occasionally close their stomas to slow down photosynthesis even below this point because, by letting more CO2 in, they are letting too much moisture out. In a greenhouse you can keep them at maximum healthy hydration so they keep their stomas fully open all the time.

4. The other issue with RuBisCO is that its process reacts with ambient oxygen levels that are much, much higher than when photosynthesis evolved (indeed, are much higher because of a billion years of photosynthesis.) La Wik, “Thus, the inability of the enzyme to prevent the reaction with oxygen greatly reduces the photosynthetic capacity of many plants.” Some call this ‘oxygen poisoning’ or ‘oxygen toxicity’.

So you can squeeze some additional productivity of (a few) plants and other photosynthesizing critters like algae if you reduce the oxygen levels a bit and keep them low (but not too low). Obviously this this is much more capital intensive than merely raising CO2 levels. Even if you close off the outside air, raising CO2 levels by 300% only reduces O2 levels by 0.5% – which is negligible. You need electricity-hog membrane filters, and now you’ve created an environment unsafe for humans or, more importantly, pollinating insects.

The only people who do anything like that now (and on the smallest scales) are pharmaceutical companies and, of course, the highest ends of the Japanese ultra-premium fruit market.

Though, for people who grow in their basements without bees, you can pollinate on your own with a q-tip attached to an electric toothbrush or flosser, which is very boring, time-consuming, and labor-intensive. (One of the major problems with trying to grow food in space if you don’t bring bugs (and everything else they need) with you). But, in the near future, I can easily envision this monotonous task being automated and conducted by robots who don’t need oxygen.

It seems strange that bio-productivity has fallen so drastically since the Cambrian if CO2 levels are only 75% below an optimum level.

I used to grow tomatoes and occasionally some other plants (legal ones) in a small, enclosed greenhouse with a small gas-burning unit that produced heat, moisture, and CO2. Ideal CO2 levels for most plants was between double and quadruple ambient levels (now about 400ppm, so ideal between 800-1600ppm). After that, and even with ideal lighting and nutrients, you simply reach the biochemical limits of photosynthesis rates.

On the other hand, most animals including humans can go up to 10,000 ppm with no apparent effect.