The limits of CO2 fertilization as a benefit of global warming

More plants sounds good. But which plants, and will they remove enough carbon dioxide to make a difference? Those are the questions that turn an initially promising idea into something much less reassuring.

The opening predicts a large increase in atmospheric CO2 and explains why that could warm the Earth: CO2 traps energy being released as heat from the surface. At this point, I expect a discussion of the damage. Instead, we get a possible silver lining. More CO2 could help plants photosynthesize, producing more vegetation and more crops. That extra vegetation could then draw more CO2 out of the atmosphere.

So there are two proposed benefits: agricultural abundance and a slower rate of warming. Notice what the second claim actually promises. Temperatures could still rise, just more slowly than they otherwise would. Slowing warming isn't the same as stopping or reversing it.

That sounds encouraging, but how much CO2 would the extra plants remove? Removing a tiny fraction and removing a large fraction would have very different implications. More uptake alone doesn't tell us whether the change is enough to offset the problem. Keep that question in mind.

Paragraph 2 adds a different complication: plants don't all respond equally. My immediate thought is weeds. What if they get a much bigger boost than the crops we want? Of course, the reverse would sound promising: perhaps the crops benefit more. We need to see which way the evidence goes.

Unfortunately for the optimistic picture, corn and sugarcane may lose their current advantage. Patterson and Flint's research suggests that certain weeds perform better and reduce the crops' yields. Picture a cornfield in which the extra CO2 helps the corn grow but helps the weeds even more. More vigorous plant growth doesn't necessarily mean more corn to harvest. Competition matters.

The rangeland example extends the concern beyond farming. One weedy grass grows much better than three other grasses when CO2 is plentiful. That grass also makes land more prone to burning, so its spread could mean more frequent and more severe wildfires. A greener landscape isn't automatically a healthier or more useful one. Which species gain ground matters.

But wait: even weeds take CO2 out of the atmosphere. Perhaps the farming benefit disappoints while the climate benefit survives. The final paragraph asks exactly that. Given the direction of the discussion, I expect another problem. One possible guess is that the plants gaining ground won't remove as much CO2 as the plants they displace. But that's a guess, and the actual research gives us a different explanation.

Billings simulated warming in wet tundra grasslands. Plant growth increased, yet CO2 levels also increased. Why? As the permafrost thawed, more peat—accumulated dead plant material—decomposed. That decomposition released CO2. The living plants removed some CO2, but they couldn't compensate for the increased release from the dead material. We're comparing what comes out with what goes in, not asking whether plants absorb any CO2 at all.

Billings estimated that a four-degree Celsius rise in summer temperatures would cause the tundra to release 50 percent more CO2. Four degrees sounds like a substantial change, but don't turn that condition into a definite prediction for the whole planet. The estimate tells us what would happen if those summer temperatures rose that much. The final sentence also explains why the tundra deserves attention: high-latitude habitats are expected to have the greatest temperature increases.

The conclusion isn't simply that CO2 is bad. It's that the proposed benefits of extra plant growth are unreliable. Faster growth can favor weeds over crops, and extra CO2 absorption can be outweighed by greater CO2 release. The initial mechanism can be real while the optimistic conclusion goes too far.