Wednesday, June 13, 2007

Planting trees

A rigorous conclusion about increased CO2 levels in the atmosphere is that plants will benefit. While getting less light, they will receive more CO2 to breathe and more water precipitation. All plants will benefit from the latter; but certain plants - the kind that like to hang out in the shade of other plants, low-light types - will be selectively more favored. The relatively larger favoring of low-light plants is a case of Darwinian selection, but overall, the effect is anti-Malthusian. And since we humans tend to have a landlubber bias, it should be mentioned: "plants" here very much includes ocean plankton and the like.

The exact size of the plant enhancement and its effect in turn on removing some of the CO2 are hard to estimate. However, it is clear that plants are the most significant sink of CO2 in the atmosphere. They experience one to two growing seasons a year, which is a much faster response time than the nearest competing CO2 sink, ocean absorption (decades or longer). This geophysical effect is less important, although easier to estimate: the CO2 diffuses into ocean water, then dissolves and enters the carbonate cycle (time scale: about 10 years); the carbonates then sink to the deep ocean (time scale: 100 years), where they sediment out and are subducted into the Earth's crust (time scale: few 1000 years).

But the sheer impact of plants on the atmosphere is best gauged if you consider the following facts.
  • Essentially all of the oxygen (O2) in the atmosphere is due to plant exhalation. Molecular oxygen is a highly reactive compound; without a source of continual replenishment, the O2 in the Earth's atmosphere would disappear quickly by oxidation reactions (like rust) with rocks and soil.*
  • The famous Mauna Loa graph measuring atmospheric CO2 concentrations from 1957 (the first International Geophysical Year) shows the impact of plant growth in spring and summer in the significant cyclical squiggle imposed, year by year, on the rising CO2 concentration. In some years, the annual variability (which is mostly due to plants) is larger than the annual mean increase itself.
It's tempting to relate the CO2 concentration directly to plant growth in this way, but it must be kept in mind that both plant growth and the CO2 level are affected by multiple factors. For example, if the amount of plant respiration is lower in one year or a series of years for some reason other than reduced CO2 concentration (say, less rainfall or lower temperatures), then the CO2 concentration will rise - fewer plants means less less plant respiration and less CO2 converted to O2. The atmosphere, geosphere, and biosphere are linked by multiple and generally stabilizing feedback mechanisms of this sort.

Freeman Dyson, one of the last surviving giants of the heroic mid-century age of physics, discusses plants and their effect on atmospheric CO2, among other things, on these YouTube interviews here and here.

So plant trees. Actually, plants will respond on their own just fine to enhanced CO2 and precipitation; human planting will simply add to what the plants will already do by themselves. My real point is, planting is not expensive - and it is good for a lot of other, more tangible reasons: trees look nice, they provide shade (especially for urban heat islands), and (most importantly) they are excellent for holding soil and water.

In the meantime: relax. The human effect on climate variability needs to be viewed as important, but not urgent.** There's plenty of time to do more pressing things: understand the climate better, especially the Earth's CO2 and carbon budgets, which are now measured very imperfectly - more things climatologists should be doing instead of wasting their time with questionable computer models. Instead of guessing, imagining, and frightening ourselves, a better approach would be to measure lots of things, especially related to plants, that are not understood.

Advanced countries will adopt new energy technologies in any case. Here the political and economic need for change is far more pressing than the ecological one, operating on a time scale of decades, not centuries. A couple centuries from now, when manmade CO2 increases in the atmosphere will probably start to have a noticeable impact on climate, our descendants will thank us if we do good things (understand the climate better, develop reasonable and sustainable alternative energy technologies) but also if we don't do stupid things (wasting resources or destroying modern civilization in a fit of hysteria). Our legacy to them will be not just what we do; it will also be what we don't do - the dumb things we avoided.

I've ducked addressing the plant respiration subject in a quantitative fashion for just the reasons Dyson mentions: the whole question has been broached scientifically only in the last decade, and there's a lot to be learned. But there's lots of time to study it.
* Indeed, in the next generation of searches for planets revolving around other stars, a key goal is to detect the presence of oxygen in their atmospheres, by looking for distinctive O2 spectral lines. Persistent O2 in planetary atmospheres would virtually clinch the case for the presence of plant life on those planets.

** As readers of The Seven Habits of Highly Effective Planets will recognize.

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