Clear-cut carbon

While re-growing, razed forests may sequester enough CO₂ to offset a substantial amount of the carbon lost to logging and other land-use change, a new model shows. Through practices such as clearing forests and cultivating cropland, humans have altered 42-68 per cent of the Earth’s surface and added over a hundred billion tonnes of carbon dioxide to the atmosphere.

Now Elena Shevliakova of Princeton University, New Jersey, and colleagues provide the first global estimate of how much of the CO₂ emitted from land has been offset by re-growth, using a model of CO₂ sources and sinks in terrestrial ecosystems. To cover a range of possible land-use changes that may have taken place, they used four different scenarios. They found that even extensive human interference caused the net loss of only 1.1-1.3 billion tonnes of carbon per year in the 1990s — about half of previous estimates. One factor that may explain this, they say, is the 0.35-0.6 billion tonnes of carbon absorbed annually by plants growing back after disturbance, mostly in tropical forests.

The researchers suggest that replenishing of forests could be one of the ‘missing’ sinks that scientists have been seeking to help them balance the global carbon budget.

Shevliakova, E., S. W. Pacala, S. Malyshev, G. C. Hurtt, P.C.D. Milly, J. P. Caspersen, L. T. Sentman, J. P. Fisk, C. Wirth, and C. Crevoisier (2009), Carbon Cycling under 300 Years of Land-use Change: the Importance of the Secondary Vegetation Sink, Global Biogeochem. Cycles, doi:10.1029/2007GB003176.

Appears in Nature Reports Climate Change | Vol 3 | May 2009.