The quantity and quality of land use directly and indirectly relates to many “grand challenges” in sustainability science (Vitousek, 1997; Rindfuss et al., 2004; Global Land Project, 2005; Steffen et al., 2007; Turner et al., 2007). Land use is a major driver for habitat encroachment and biodiversity loss (Sala et al., 2000), for the alterations of global biogeochemical cycles (Gruber and Galloway, 2008; Postel et al., 1996; Vitousek et al., 1997) and for soil degradation (Lal, 2004). Changes in land use and subsequent changes in land cover play a central role in the global carbon cycle and significantly contribute to anthropogenic climate change (Brovkin et al., 2004; Canadell et al., 2007; McGuire et al., 2001; Watson et al., 2000). On the other hand, land use provides the nutritional basis for humans and thus of any socioeconomic system, and is intrinsically linked to food security (Ayres, 2007; Foley et al., 2005; Millennium Ecosystem Assessment, 2005). Research on global land use has a long tradition, reaching back to the work of G.P. Marsh (1865) and A. Von Humboldt (1849). It gained momentum in sustainability research in the mid-1970s, when the impact of land use on the global surface albedo was recognized (Lambin et al., 2006). Since then, many aspects of land use have been assessed, quantified and mapped across spatio-temporal scales. Two aspects of land use changes can be distinguished: (a) Changes in land cover, i.e. alterations of biophysical characteristics of the Earth's surface, e.g. by expansion or contraction of a certain land use type; a prominent example would be the expansion of agricultural fields into pristine forests. (b) Changes in land use intensity, denoting changes in the levels of socioeconomic inputs (e.g., labour, resources, water, energy or capital) and/or altered output (value or quantity) per unit area and time. Changes in intensity need not result in changes in land cover, but cause ecological changes within the same land cover type. Increasing land use intensity stands in an inverse relation to land expansion for increasing production. Consequently, a major effect of intensification may be to “spare” land, e.g. for wilderness conservation, by concentrating production on other areas (Tilman, 2001). Indeed, this effect is often assumed to be essential for many sustainability aspects, as it allows to reduce area demand and avoid considerable carbon emissions from deforestation (Burney et al., 2010) or habitat encroachment (Green et al., 2005). In the future, safeguarding the land-sparing effect of intensification could become decisive, given the rising nutritional and energy demands of a growing world population, and the concomitant need to protect the shrinking untouched habitats of the Earth, rich in biodiversity and carbon. Moreover, many policies that aim at harnessing land use for the goals of climate change mitigation, such as strategies aimed at expanding bioenergy production, or at reducing greenhouse gas emissions from deforestation and forest degradation (REDD), will probably not be effective without the land sparing effect of intensification. On the other hand, many technologies required for intensification are associated with detrimental ecological impacts, such as the accumulation of toxins in food, ecosystem and soil degradation, groundwater and air pollution, or biodiversity loss (IAASTD, 2009; Matson et al., 1997; Millennium Ecosystem Assessment, 2005; Tilman, 2001). Such processes negatively affect the ability of ecosystems to sustain vital ecosystem services, thereby running the risk of jeopardizing human well-being in the long run (Foley et al., 2005). Thus, it will become imperative to find ways of sustainable intensification (Tilman et al., 2002) that allow reaping its land-sparing benefits while at the same time avoiding the detrimental social and ecological effects. However, the interrelation between intensification and expansion of land use is far from trivial. Empirical analyses of Rudel et al. (2009) on the interrelation between past trajectories in cropland expansion and intensification resulted in inconclusive findings. At the national scale, land use intensification was paired with a decline or stasis in cropland area between 1970 and 2005 only in countries that “externalized” agricultural production (e.g. grain imports) or preserved land with explicit land conservation programs (Rudel et al., 2009). These counterintuitive findings may be explained not only by large data gaps and uncertainties (Grainger, 2009), but also by feedback loops of higher order, such as a rebound effect of consumption to increased production, that overcompensated the land-sparing effect (Lambin and Meyfroidt, 2011). This altogether casts doubts on the straightforward interpretations or scenario-based extrapolations of the beneficial effects of land intensification strategies. These feedback loops of land transitions are active across a wide range of spatial and temporal scales (Global Land Project, 2005; Lambin and Geist, 2005; Bennett and Balvanera, 2007; Erb et al., 2009b; Lambin and Meyfroidt, 2011). To take such feedbacks into account is indispensable, but it poses a formidable challenge to land change science (Turner et al., 2007), as it requires innovative methods and new perspectives that allow for the construction of sound causal chains between the various factors, mechanisms, determinants and constraints that underpin land-use intensification processes. In this commentary, I discuss the potential contribution of an extension of the socioeconomic metabolism concept (Ayers and Simonis, 1994; Ayres, 1989; Fischer-Kowalski and Huttler, 1998) by accounts that create an integrated picture of socio-ecological flows (Erb et al., 2008; Haberl et al., 2004; Krausmann et al., 2004) to global land system science. Such an approach could help to develop an analytical framework for conceptualizing and reporting on the complex, systemic interactions related to land use intensification, including feedbacks between production and consumption. It thus might give guidance for data collection and analysis, and so enhance the understanding of the interplay between land expansion and intensification.

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