A fading green hope for climate
US News & World Report, Feb. 10, 2002
It was a comforting dream while it lasted: Carbon dioxide spewed into the air from tailpipes and smokestacks would speed up the growth of forests. The forests in turn would store the carbon in wood and soil, staving off climate change. The theory even undergirded the Kyoto Protocol, which allows countries to meet greenhouse gas targets by planting trees as well as by trimming industrial emissions. But the latest research has delivered an unpleasant wake-up call.
Plants need carbon dioxide for photosynthesis and growth, so it wasn't unreasonable to imagine that rising carbon dioxide levels would act as a planetary fertilizer. In 1996, a team of researchers led by biogeochemist William Schlesinger of Duke University began testing the theory. They pumped tons of carbon dioxide daily from towers rising over an experimental forest of loblolly pines outside Chapel Hill, N.C. Meters measured gases entering and leaving the pine needles; bands on the tree trunks assessed their month-to-month growth. The initial results were reassuring: When the researchers increased ambient levels of the gas by about 50 percent, to levels expected by midcentury, tree growth jumped by up to 25 percent.
But the longer they studied the forest, the more complicated the picture looked. For starters, the growth spurt lasted just four years. Later, the trees settled back to growing only about 6 percent faster than their neighbors. "The trees quickly run down key nutrients in the soil," explains Schlesinger. Trees grown in carbon-dioxide-enriched air also compensated by pumping more of the carbon down through their roots to microbes in the soil. Instead of storing the carbon in the soil as humus, these organisms released much of it back into the air as carbon dioxide.
After seven years amid the loblolly pines, Schlesinger has concluded that we can't rely on the forests of the future to store our excess carbon dioxide. "I would count on nothing," he says flatly. To some scientists, that's an argument for taking matters into our own hands and looking for ways to bury the gas. To Schlesinger, though, it underscores the hazards of tinkering with natural systems. He thinks that the best solution to global warming is to burn less coal, oil, and natural gas. "Rather than trying to gather up marbles that have spilled, let's not spill 'em in the first place."