Under current conditions, competing land uses result in different trade-offs that place pressure on land resources. Here we highlight the various types of land uses, their current conditions, and potential future scenarios.
Ecosystem services are the multiple benefits that people receive from nature, such as water purification and flood control by wetlands. Earth's ability to provide these services has diminished because land has been modified and transformed. This has led to a host of environmental and societal problems, including reduced water and air quality, soil erosion, and loss of biodiversity (MEA 2005a; Vitousek et al. 1997b). In addition to the direct impact on well-being (e.g., decreased water quality or fewer pristine hiking locations), lost services can also impact communities indirectly through higher costs, such as replacing an ecosystem service with built infrastructure (e.g., destruction of wetlands increases the need for dike construction to control storm surges).
Forest ecosystems provide important services via carbon sequestration. One might imagine using the sequestration capacity of forest ecosystems as a service to regulate global carbon emissions. Of the 9 billion petagrams (Pg) of global carbon emissions, 7.5 Pg come from fossil fuels combustion and about 1.5 Pg from land use change (Canadell et al. 2007). Allowing for total current land and ocean sinks of 5 Pg C/yr (Canadell et al. 2007) yields a net release into the atmosphere of approximately 4 Pg C/yr. Assuming that we are free to plant new forests to sequester the excess carbon, and that the global mean sequestration rate is approximately 2-3 tons C/km2/yr, then we would require on the order of 1-2 billion ha of land. This is equivalent to the current extent of global cropland (see Ramankutty, this volume). Clearly, allocating land to global crop production precludes using forest carbon sequestration services to sequester excess carbon emission and vice versa. Moreover, the forest carbon sequestration will saturate after ~100 years when the trees reach maturity, and an additional 1-2 billion km2 will need to be planted after that to continue the sequestration.
Another example is the Brazilian Cerrado, one of 25 global biodiversity hotspots. The original extent of the Cerrado was over 2 million square kilometers and home to 4400 endemic species of plants and animals (Myers et al. 2000). Since the 1970s, aggressive land conversion for agriculture has resulted in a net loss of 79% of the land base. The Cerrado grows 58% of Brazil's soybean crop and 41.5% of the area is used to pasture 40 million head of cattle (Hooper et al. 2005).
The loss of the Cerrado ecosystem introduces the issue of nonsubstitutabil-ity of land use raised earlier in our discussion about the conceptualization of land, namely that in a global context one cannot simply add loss and gains in land use for specific activities in a simple land-accounting approach. The Cerrado ecosystem is only represented in one geographic location globally. Its complete loss would result in the extinction of a critical ecosystem service.
On the other hand, reconsidering contextual dependency and substitutabil-ity means that (a) soybean production can occur in other regions where it does not lead to the demise of specific ecosystem services or (b) a substitute oil crop can be grown in another geographic location. The former requires looking at land in terms of geographic portfolios of land use options that allow sustainable trade-offs. The latter requires recognizing that there is a need to protect global crop biodiversity, again to create a portfolio of crop use options that are adaptable to different biophysical conditions, within different geographic regions of the globe.
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