Pete Smith

Terrestrial sinks are not permanent. If a land management or land use change is reversed, the carbon accumulated will be lost. New C stocks, once established, need to be preserved in perpetuity. A number of current biosphere C stocks are vulnerable to current and future climatic change and human activity. Because losses from these will militate against activities to increase sink strength in the future, potential carbon losses from land need to be considered when estimating the potential for increasing land carbon sinks.

Under the Kyoto Protocol, sinks include those arising from afforestation and reforestation (deforestation also needs to be accounted for), forest management, cropland management, grazing land management, and revegetation. Estimates for the sequestration potential of these activities range from about 30 to 80 grams of carbon per square meter per year (g C m-2 y-1). The best options for enhancing sinks occur where there are co-benefits associated with a sink-enhancing activity—"win-win" management options have the greatest potential for implementation. In many cases the techniques to enhance carbon sinks are well known, but economic and educational constraints present the greatest barriers to their implementation. Terrestrial C sink enhancement needs to be tackled hand-in-hand with related environmental, political, and socioeconomic problems.

Engineered Biological Carbon Sinks on Land— Their Place in the Global C Cycle

During the 1980s and 1990s fossil fuel-combustion and cement production emitted 5.9 petagrams (Pg) C y-1 to the atmosphere, while land use change emitted 12 PgC y-1 (Sabine et al., Chapter 2, this volume). Atmospheric C increased at a rate of 3.2 ±0.1 PgC y-1, the oceans absorbed ~2.3 PgC y-1, and the estimated terrestrial sink was 1.8

The size of the biological terrestrial carbon pools are 2,300 PgC for soil organic car bon and 650 PgC for vegetation (Sabine et al., Chapter 2). The annual fluxes between land and atmosphere (global net primary production [NPP], respiration, and fire) are of the order of 60 PgC y-1 (Sabine et al., Chapter 2). The pool sizes are therefore large compared with both the gross and net annual fluxes of carbon to and from the terrestrial biosphere. Manipulating the size of these terrestrial carbon pools is at the heart of efforts to engineer carbon sinks on land.

Engineered sinks on land are typically designed to replace stocks that were present before human intervention. Historically, for example, soils have lost between 40 and 90 PgC globally through cultivation and disturbance (Schimel 1995; Houghton 1999; Lal 1999) with total system losses closer to 180 PgC (DeFries et al. 1999; Houghton et al. 1999). The engineered soil C sink is therefore replacing the stock lost through earlier human activities. The goal of engineering biological sinks on land is to increase the C stock relative to the current C stock.

Biological carbon sinks on land can be engineered by increasing the size of the soil organic carbon or vegetation pools. This is achieved by increasing the net flux of carbon from the atmosphere to the terrestrial biosphere by increasing global NPP, by storing more of the carbon from NPP in the longer-term carbon pools in the soil, or by slowing decomposition. In essence, engineered carbon pools on land rely on either increasing the standing stock of carbon in vegetation or sequestering carbon in soil.

For vegetation carbon sinks, the best (but not the only) option for carbon storage is via tree planting (afforestation, reforestation). For soil carbon sinks, the best options are to increase C stocks in soils that have been depleted in carbon, that is, agricultural soils and degraded soils. Other options are available, and these will be listed later.

Estimates for soil carbon sequestration potential vary widely. Based on studies in European cropland (Smith et al. 2000a), U.S. cropland (Lal et al. 1998), global degraded lands (Lal 2003), and global estimates (Cole et al. 1996; Watson et al. 2000), global soil carbon sequestration potential is estimated to be 0.9 ± 0.3 PgC y-1 (Lal 2004), between one-third and one-quarter of the annual increase in atmospheric carbon levels. Over 50 years, the level of C sequestration suggested by Lal (2004) would restore a large part of the carbon lost from soils historically. Estimates based on Europe and North America by Smith et al. (2000b) suggest a lower potential (one-third to one-half of historical soil C loss over 100 years).

Estimates for the carbon sequestration potential for forests range between about 1 PgC y-1 (the lower figure of IPCC 1997) and 2 PgC y-1 (Trexler 1988 [cited in Metting et al. 1999]), between one-third and two-thirds of the annual increase in atmospheric carbon levels. As well as absorbing CO2 from the atmosphere, a change in the balance between deforestation and afforestation/reforestation would have the added benefit of reducing the net efflux of CO2 to the atmosphere from land use change, that is, it would decrease the size of a current C source as well as increase the size of a potential C sink.

The estimate for total terrestrial C sequestration potential by IPCC (2000) was about 2.5 PgC y-1. This estimate includes sinks from the management of agricultural

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