Princeton ecologist John P. Caspersen and his colleagues failed to detect signals of CO2 fertilization when they analyzed growth rates of more than 20,000 forest plots in five eastern U.S. states. Even if CO2 fertilization does occur, according to a recent report in Nature, the benefit doesn't last forever. Peter M. Cox of the Hadley Center for Climate Prediction and Research in Berkshire, England, and his colleagues conducted a global climate simulation and found that by the middle of the century, the land becomes a source of CO2 rather than a sink. Apparently, the estimated atmospheric warming of about 5.5 degrees Celsius over land would invigorate soil microbes that give off CO2 as they digest fallen tree leaves and other organic material. The CO2 output of the soil microbes would eventually outpace trees' ability to absorb carbon, and by 2100 an extra six billion metric tons of carbon will remain in the atmosphere every year.
Carbon sinks can vary dramatically on short timescales as well.
Even in the five years of the Kyoto Protocol's first commitment period, land-based ecosystems could give off more carbon dioxide than they absorb, Solomon says. A year of rampant wildfires, for instance, would release loads of extra CO2 as the forests and grasslands turned to ash. In the end, scientific uncertainty took a backseat to politics and economics in the Kyoto negotiations. And many scientists question the idea of using carbon sinks as emissions credits in the first place. I don't think sinks make a good deal of sense, because they don't solve the problem.