I thought I would gather together all the random thoughts I've had on this topic over the years.
Once upon a time, I pulled this pretty little graphic off the web, found by Googling "photosynthesis reaction":
It balances! It's simple! But it's deceptively simple. To find out exactly how deceptive, click on the formula and go to to Wikipedia's article on photosynthesis. Turns out it's complicated!
But let's ignore that for a bit: the idea is that plants pull (1) carbon dioxide from the air, (2) water from the ground, and (3) photons from the sun, producing sugar and oxygen. Neat. This process has been going on for something like 3.5 billion years. It makes life "as we know it" possible.
I started wondering about it as a panacea/doomsday device when I read a fine book by Charles C. Mann, The Wizard and the Prophet. You'll notice that the formula above implies, falsely, that you need six molecules of carbon dioxide and six water molecules to (somehow) smoosh together more or less simultaneously, while being illuminated with those photons, and—ta da!—out comes sugar and oxygen. That's very unlikely, and so like many biochemical processes, it needs to proceed in steps, aided by enzymes. And Mann describes, fascinatingly, the key one:
Rubisco is the essential catalyst for photosynthesis, Like military recruiters who induct volunteers into the army and then return to their work, rubisco molecules take carbon dioxide from the air, insert it into the maelstrom of photosynthesis, then go back for more. The name "rubisco" was coined, jokingly, in 1979, to sound like a breakfast cereal; it is a sorta-kinda acronym for the compound's scientific name, ribulose-1,5-bisphosphate carboxylase/oxygenase. Rubisco's catalytic actions are the limiting step in photosynthesis, which means the rate at which rubisco funtions determines to rate of the entire process. Photosynthesis walks at the rate of rubisco.
Alas, rubisco is, by biological standards, a sluggard, a lazybones, a couch potato. It causes reactions to occur, but very slowly. Whereas typical enzymes catalyze thousands of reactions a second, rubisco deigns to involve itself with just two or three per second. It is onee of the pokiest enzymes known. When Warren Weaver bewailed the inefficiency of photosynthesis, he was unknowingly bewailing the torpor of rubisco. Years ago I talked to biologists about photosynthesis for a magazine article. Not one had a good word to say about rubisco. "Nearly the world's worst, most incompetent enzyme," said one researcher. "Not one of evolution's finer efforts," said another.
Not only is rubisco slow, it is inept. Carbon dioxide (CO2) consists of a carbon atom (C) flanked by two oxygen atoms (O), the whole in a straight line. Oxygen gas (O2) consists of two oxygen atoms. […] Rubisco is constantly searching, so to speak, for a linear molecule with two oxygen atoms on either end. But as much as two out of every five times, rubisco fails to pick up carbon dioxide, fumblingly grabs oxygen instead, and tries to shove the oxygen into a chemical reaction that can't use it. To get rid of the unneeded oxygen, plants have evolved an entire secondary process that pumps it out of the cell and re-primes the rubisco to try again for carbon dioxide.
Mann notes that some botanists considered ways to improve rubisco. Surely some sort of genetic tinkering would give us smarter, faster super-rubisco, contained in plants that would grow faster, need less fertilizer, produce more edibles.
But those efforts have come up empty, thanks to the the extremely unlikely evolutionary process that gave us the green plant life we know today. Read Mann (or Wikipedia) for the details, but it turns out the genetic code responsible for the rubisco molecule is the product of an unholy marriage between ancient protozoans and cyanobacteria. And as a result, it's impossible to use standard processes to improve it.
(If you read Mann's discussion, you'll be impressed at how (again) extremely unlikely this 3.5 billion-year-ago biochemical hack was. And, yet, without it, we wouldn't be here. Ponder, if you will, what this implies for the existence of extraterrestrial life.)
Okay, here's where I go into crackpot hand-waving territory. It's easy to do when your knowledge is superficial, and you don't have to actually do anything.
Those botanists were looking in the context of improved plant growth. Suppose we removed that constraint, and turned our chemical engineering wizardry on the problem divorced from the need to have it occur in plant cells. Perhaps with the help of AI, a smarter and faster catalyst could be designed to carry out the photosynthetic reaction, inside ingeniously-designed plumbing exposed to the air, supplied with water and sunlight! Have the plumbing release the generated oxygen into the atmosphere, and sequester the generated sugar! Or maybe jigger the reaction to produce some other easily-stored hydrocarbon!
And just imagine square miles of this plumbing, efficiently sucking carbon dioxide out of the atmosphere! All day long! Releasing sweet oxygen! And disposing of the "waste" hydrocarbons … um, somehow. Buried in deep, sealed pits, perhaps. ("Over there, next to the radioactive waste, Mel.")
Sorry for all the exclamation points. But, dude, you just solved global warming. Good job. Have a Nobel Prize or two.
There's a possible downside. This essentially gives us a global thermostat. Where to set it? We still need some CO2 in the atmosphere. How much?
I've made this observation before: if your family occasionally bickers about where to set the thermostat in your house, multiply that bickering by 5 billion or so, and give a lot of the participants dangerous weaponry. (See Neal Stephenson's latest novel, Termination Shock for a fictional treatment of this problem.)
As a bonus, however, this would get the climate nannies off our backs. They'd be disappointed that it would no longer be necessary to get to "net zero" carbon emissions. All their coercive schemes, up in (carbon-free) smoke. Yes, we'd have other things to fret about, like…
Well, another scenario: say it turns out that our "super-rubisco" process can be incorporated into plant cells. And say those supercharged plants escape into the wild. For some odd reason, they are not suitable for food. And (oops!) it turns out they are kind of the ultimate invasive species, rapidly crowding out conventional plants.
Darn it! We all eventually starve to death. But on a very green and cool planet.