Land provides the most fundamental resources for humankind: food, fiber, energy, shelter, and a host of ecological services (e.g., climate regulation, air and water purification, and carbon storage) that are critical for the functioning of Earth as a system. The amount of land on Earth is finite. On this finite resource is a rapidly growing population that increasingly modifies and demands more of it. Indeed, there are already many disturbing trends that can be detected. Humans are appropriating land to meet their resource needs, from wholesale conversion to modification of vital support services of the land. More than half of Earth's terrestrial surface has been modifi ed by humans, and nearly one-third of the world's land surface is being used to grow food.

This poses an important question: Can Earth's land resources support current and future population and consumption levels? The answer will depend on the physical quantity of land, the quality of land, and the geographic distribution of land. Although intrinsically a renewable source of resources, land degradation, erosion, salinization, and other overuses of land can turn it into a nonrenewable resource (NRR). Moreover, our ability to use the land may be constrained by water availability or by the availability of cheap, reliable, and sustainable sources of energy. Even if we have enough high-quality land, whether or not we can sustainably support humankind will also depend on the standard of living to which we aspire, the diets we consume, and the ways in which we use land. Land sustainability will depend on whether diets evolve to be primarily plant- or animal-based, the location and form of human settlements, and how we allocate land to competing uses. Land sustainability will also require measurement tools that conceptualize and quantify land in ways that allow us to evaluate whether land is being used sustainably.

In this chapter, we evaluate current trends in land use and explore opportunities for alternative conceptualizations of land that allow for measurements of sustainability. Our goal is to provide a framework to measure land sustainabil-ity that will ensure that this limited resource remains permanently available, fertile, and renewable for generations to come.

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