The authors propose a way to synthesize different approaches to globally map land degradation by combining vegetation and soil indicators into a consistent framework for assessing land degradation as an environmental ‘debt’. our combined approach reveals a broader lens for land degradation through global change, in particular, identifying hot-spots for the different kinds of land degradation.
Earth's life support systems require a healthy biosphere with diverse ecosystems. Degradation of these ecosystems and the soils that support their functioning is a threat to human activity and wildlife. About 95% of our food is produced on land. This led Warren Buffet, the ‘oracle of Omaha’ and famed investor, to opine that land is a more desirable investment than gold (Buffet, 2018), which is supported by recent economic analysis suggesting returns on investment in soil health may run into the trillions of US$ (Schindler, 2020).

With growing populations and affluence, agriculture will need to considerably expand its capacity to meet demand (Hunter et al., 2017). At the same time, the most productive soils are already in use (Ramankutty et al., 2008). Currently (2018), FAO estimates ~12% of land is cultivated and ~25% used as meadow or pasture, and only ~21% of world soils are without major soil constraints for cultivation (Bot et al., 2000). This nexus brings the issue of land into sharp policy focus; more so given an estimated third of land was already degraded at the beginning of the 1990s (Oldeman et al., 1991) and an estimated 1.3 billion people lived on degraded land in 2010 (Barbier & Hochard, 2018), and both numbers keep growing. The challenge and opportunity for policy makers is now to solve land scarcity issues by halting and reversing global land degradation, to meet the increasing global demand for agriculture, provide food security and mitigate the environmental impact that follows land degradation, such as biodiversity loss and climate change (Smith et al., 2016).

Major global initiatives recognize the prescience of the issue. Most prominently, the United Nations (U.N.) sustainable development goals (SDGs) have land degradation neutrality by 2030 as a target (indicator 15.3.1; Sachs et al., 2019). Yet, what exactly is land degradation remains an unresolved issue. To date, indicators of degradation have tended to focus on either above- or below-ground characteristics of the biosphere. Here we propose a way to synthesize these approaches combining vegetation and soil indicators into a consistent framework for assessing land degradation as an environmental ‘debt’. Single indicators can give a limited lens through which we see an issue and underestimate the level of debt, whereas our combined approach reveals a broader lens for land degradation through global change, in particular, identifying hot-spots for the different kinds of land degradation. We propose that this approach advances our ability to assess degradation and should serve to provide important measures by which success in reversing it can be better quantified.

Some years ago, Gibbs and Salmon (2015) already raised this issue, by comparing maps of the world's degraded lands using different approaches, including expert opinion on soil status (e.g. GLASOD), satellite observation of plant productivity (e.g. GLADA), biophysical models (e.g. potential vegetative productivity) and remote sensing classifications of land abandonment. As to be expected, there are many regions in the world that are degraded, or are being degraded, according to some measure but not another.
The United Nations is a good example of an organization using multiple definitions (see discussion in Supporting Information). The different definitions provide valuable, but divergent, spatial assessments of land degradation. It is apparent, that combining them would build upon their individual strengths and compensate their individual limitations. We demonstrate how this can be achieved using a natural resource ‘debt’ approach to capture global change. In particular, what we propose:

Combining the current SDG indicators—land use, carbon stocks above- and below-ground—with globally modelled soil erosion by water using the Global Soil Erosion Modelling platform approach already used by IPCC,
Capturing land degradation more comprehensively, as status and trend, and
Building on the idea of a ‘land, soil and carbon debt’, defined as the difference in each land degradation indicator's current value and what it would be without human intervention, or in a native condition. This utilizes recent advances in remote sensing, machine learning, and computational resources, and can now be implemented in a straightforward manner at the global scale. This considerably extends, for example, the utility of the 15.3.1 indicators in an important way: It lets us distinguish between the natural and the man-made change. As a framework, it can also, easily be augmented to include additional physical processes, such as phosphorus depletion, acidification, diffuse contamination, and others.

The degradation debts accounting approach should assist in achieving the long-term sustainability of terrestrial ecosystems.