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Plants can make global warming worse

Some of the codswallop on what to do about global warming — okay, ALL of the codswallop — is driving me nuts. Nuts, I tell you. If a little knowledge is a dangerous thing, next-to-no knowledge is radioactive.

The latest comes via the Times: Plants buy Earth more time as CO2 makes them grow. The first breathless sentences are “Trees and plants are growing bigger and faster in response to the billions of tons of carbon dioxide released into the atmosphere by humans, scientists have found. The increased growth has been discovered in a variety of flora, ranging from tropical rainforests to British sugar beet crops. It means they are soaking up at least some of the CO2 that would otherwise be accelerating the rate of climate change. It also suggests the potential for higher crop yields. “

Um, Mr. Leake, Environment Editor? You skipped class that day in Basic Chemistry, didn’t you? And then you did it again in Basic Bio.
The super-simplified version of the photosynthetic reaction is

                energy --><br />CO2 + H20  -----> carbohydrate + O2<br />        <---  rubisco  --->    

Carbon dioxide and water, with the addition of light energy and of enzymes to catalyze the hundreds of reactions, become carbohydrates and oxygen. Rubisco (aka RBCl, ribulose bisphosphate carboxylase/oxygenase) is a critical enzyme in the process. More on that in a second. Enzyme-dependent reactions all produce more product if more of the limiting factors are added, up to a point. Carbon dioxide is a limiting factor, so adding more results in more carbohydrate under current conditions. Duh.

That is not something “scientists have found” yesterday. They’ve known this for some one hundred years. All that’s happened recently is that some very careful experiments have shown how much extra carbon is deposited from the extra CO2 in the air. Perhaps, given that this process happens in every photosynthetic reaction in every plant, it is not so amazing to find it “in a variety of flora”? The amazing thing would be finding a sugar beet that did not show increased growth.

But now comes the really fun part. Notice the arrows next to rubisco pointing in both directions, and that “carboxylase / oxygenase” in the name. The reaction can go in either direction, and that depends on temperature. At “ordinary” temperatures about four reactions fix carbon for every one that liberates it instead. So there’s net carbon fixation. The hotter it gets, the more the reaction goes backward. The plant releases more carbon into the air than it fixes into sugars.

The temperatures that tip the balance toward carbon release are found in the tropics, in deserts, and during hot summer days further north or south. In other words, it does not have to be all that hot. The kinds of temperatures parts of the globe face with global warming — for instance over 60 days’ worth of higher-than-100F temperatures in North Texas (or whatever the exact numbers turn out to be) — are plenty to cause that effect. Plants adapted to the tropics or to hot summers such as some grasses, have special (C4) photosynthesis to reduce that back reaction. Desert plants have an even more thorough adaptation called CAM photosynthesis. The back reaction is a big deal, as you can see by the fact that plants have evolved to handle it as much as possible. It’s not some rare event we don’t need to worry about.

So, yes, under current rather cool conditions, carbon fixation increases with some increased atmospheric carbon. It won’t take much warming before there’s more back reaction than forward and plants make the CO2 problem worse. (They’ll also die, which will make it even worse, but that’s another whole topic.)

That’s why the next sentence in Leake’s article makes me want to smack him for its lethal irresponsibility: “Some researchers believe the phenomenon is strong enough to buy humanity some extra years in which to try to reverse the growth of greenhouse gas emissions.” He follows it with the usual caveat, “However, few dispute that this will provide anything more than a temporary reprieve.”

That last is not just a boring standard disclaimer, it’s also wrong. It will not buy us time in the future. It has bought us time. As the Nature article says, extra photosynthesis could be soaking up as much as a tenth of our yearly carbon emissions. It is buying us time right now. Time we are squandering.
If he’d known anything about the topic, that’s the point he would have made.

global warming, carbon dioxide, photosynthesis