Trees Will Save Us From Global Warming? Scratch That

For the couple of decades the Greening Earth Society, a creation of the coal industry, has been happily insisting that the more carbon dioxide we pump into the atmosphere the lusher and more verdant the world will be. As far as climate change goes, their attitude is Alfred E. Neuman’s: what, me worry?

So it is always amusing when even the most straightforward assertions break down. In the climate-change field, one such assertion is that, since plants breathe in carbon dioxide, surely in a world with higher concentrations of CO² plants will flourish and suck up lots of the stuff. We call that a negative feedback. Unfortunately, a study in this week's Nature finds that, after exceptional warming, an ecosystem anchored by a tallgrass prairie actually takes up less carbon dioxide than it did before the warming.

For their study, scientists led by John Arnone of the Desert Research Institute and David Schimel of the National Center for Atmospheric Research turned up the heat in large controlled-environment chambers housing a tallgrass prairie. It was 4 degrees C. (7 degrees F.) warmer than normal. The researchers collected data over four years (the simulated heat wave occurred in year two), and measured what happened to CO2 uptake. Result: warming decreased the ecosystem’s carbon uptake in both the hot year and the following year.

The problem—and plants that absorb less carbon definitely present a problem, since it is carbon uptake that serves as the basis for biomass, otherwise known as food, fuel and fiber—was that the heat triggered a drought in the year it occurred. That suppressed what’s called primary productivity, or how much plants grow as a result of carbon uptake. In all, carbon sequestration fell threefold over the study period of four years. It took two years for carbon uptake to return to what it was pre-heat wave. As the scientists put it, “more frequent anomalously warm years, a possible consequence of increasing anthropogenic carbon dioxide levels, may lead to a sustained decrease in carbon dioxide uptake by terrestrial ecosystems.”

Now, a rise of 4 degrees C. is more than global warming is expected to bring in this century as measured by global median temperatures. But local spikes of that magnitude are in the cards, and already happening. As the world warms, the scientists warn, "an increase in frequency and intensity of anomalously warm years may decrease the ability of terrestrial ecosystems to absorb CO² and store carbon.”

There is no small irony in the finding that warmer conditions cut carbon storage. One of the last great hopes for avoiding dangerous global warming is for plants to suck up more and more of the CO² the industrial world produces. Oh well.