It’s so easy to slip into debt, but so hard to dig oneself out. Just ask the typical wage earner—even business and national leaders. People who know better still wait for that next paycheck, assumed pay raise or small miracle to help them catch up. As any accountant will tell you, accumulating debt is not a sound financial management policy.

If you want to grow your savings and avoid going into financial debt, you need to spend your money no faster than you earn it. To do this, you need to know exactly how much money you make and spend over a given period, and compare the two. You can then create a budget for continuing to live within your means by predicting how your future earning and spending may change.

Similarly, if you want to ensure that you are not overusing ecological assets—which provide the ecological services on which all human activities, including the economy, depend—you need to know how productive these assets are, and the rate at which you use them. Biocapacity, a measure of ecological productivity, reflects the rate at which ecosystems renew and regenerate. Most fundamentally, it encompasses the biomass useful to humans, including renewable resources like food, fiber and timber, and waste absorptive services, such as sequestering carbon from burnt fossil fuel.

Biocapacity can be compared with the rate at which you are using these ecological services—your “Ecological Footprint.” “You” could be an individual, a country or even humanity as a whole. The comparison between the two figures—your Footprint and your territory’s biocapacity—shows whether you are living within your territory’s ecological means or exceeding them. If the latter, you are either importing renewable resources from elsewhere, depleting your own ecological assets by harvesting them faster than they are replenished or using the global commons. For instance, you can use the global commons by fishing international waters or emitting carbon dioxide that you lack the capacity to sequester into the global atmosphere.

At a global scale, because there is nowhere else to obtain renewable resources, a biocapacity deficit means depleting ecosystems and/or dumping excess carbon into the atmosphere, where it then accumulates. Either option is obviously unsustainable—thus, the importance of knowing when you are exceeding capacity.

Once you know your current ecological balance, even as a first approximation, you can then develop scenarios of how this may change in the future. As with financial projections, the accuracy of ecological budget projections depends on how well all the trends that will affect the planet’s productivity and human consumption can be specified. To avoid speculation, Ecological Footprint and biocapacity accounts are by design historical rather than predictive—they measure current and past productive capacity and use of resources based on empirical data. They do not include predictions about whether contemporary ecosystem management practices, such as more fertilizer or less tilling, will impact this balance.

Of course, Ecological Footprint and biocapacity measurements are only as accurate as the underlying data, assumptions and methodological decisions. Because the most comprehensive and most widely accepted data is generally at the national level, Global Footprint Network, a nonprofit organization, calculates the Footprint accounts annually for most countries in the world and aggregates these results to generate global Footprint and biocapacity values. The primary data for these annual calculations come from sources that are as official as it gets: statistics from the U.N. and related international agencies. For example, data on agricultural and livestock production come from the Food and Agriculture Organization of the United Nations, and is the same data the World Bank and other global institutions rely on; carbon emissions data come from the International Energy Agency; estimates of average values of carbon sequestration by the world’s forests are those used in Intergovernmental Panel on Climate Change reports. These data sets, although certainly not perfect, are the most widely accepted ones; as they improve, so will the accuracy of the Footprint accounts. Global Footprint Network provides the core Footprint accounts used internationally.

Again by design, when there is uncertainty involved, the methodology used to generate the Footprint accounts tends to err on the side of underestimating demand and overestimating supply. This conservative approach helps ensure that, if humanity is exceeding ecological limits, the extent of the problem will not be exaggerated. Even given this conservative methodology, the accounts indicate that at the global scale, the rate at which humanity is using ecological resources currently is overshooting their regeneration by more than 50 percent. This is sometimes expressed as “humanity is using the capacity of 1.5 Earths.”

Given that there is limited area on the surface of the planet that is biologically productive—cropland, grazing land, fishing grounds and forest—overshoot means that there is not sufficient productive area to support our current level of consumption. People’s various demands on nature compete for use of this limited area, and these demands can be added up.

Ecological Footprint accounting measures the area of productive surface required to regenerate the biomass needed to keep up with demand, whether for fiber or timber, food or land to sequester carbon emissions from fossil fuel. Some types of productive area, such as cropland, are typically used at full capacity—we essentially harvest all the resources that are produced. In other areas, such as fishing grounds and forest, resources may be harvested at a rate faster than they are renewed. This is possible because there is a stock of fish or timber that exceeds the amount that is regenerated ever year. (For fisheries in particular, although we suspect this is already the case not just locally but also globally, the available data are not yet sufficiently complete for the Footprint accounts to be able to reflect this—again, an example of the conservative nature of the accounts.)

With global overshoot, we have reached the point where there is competition for the productive area required to meet our various consumption demands. For example, food production may be expanded by converting forest into cropland. But this comes at a cost—reduced production of forest resources, which mean either fewer trees available to harvest for wood and wood products and/or fewer trees left standing to absorb and incorporate our carbon emissions. Footprint accounting makes these trade-offs clear.

Some critics have suggested that the Ecological Footprint is inadequate as a metric of sustainability because it doesn’t take into account the future impact of current ecosystem management practices. For example, it doesn’t address whether the creation of monocropped forest plantations increases the risk of rapid proliferation of pathogens and thus of future reductions in forest productivity. Without doubt, there are numerous important issues to consider in developing scenarios of future sustainability, and no single metric can address them all. Ecological Footprint accounting is designed to address one necessary dimension of sustainability—whether the biosphere has sufficient regenerative capacity to keep up with human demands on this biocapacity. In doing so, it provides a key starting point; as with financial planning, you need to know where you are today before building projections of what might happen in the future. Additional variables quantifying assumptions about trends in other dimensions must be used in conjunction with Footprint data in building a more complete picture of whether any particular course of action is likely to prove sufficient in creating a sustainable future.

It is also sometimes claimed that the Footprint accounts lead to perverse conclusions, a consequence of methodological flaws. For example, in a recent paper by Blomqvist et al. (2013), the authors criticize the Footprint because “less than half the area of the United States planted with eucalyptus could essentially give us an Ecological Footprint equal to one Earth—an approach that no ecologist would recommend.” They suggest that because there may be impractical or undesirable strategies consistent with ending overshoot (although this particular strategy, contrary to the authors’ assertions, would not necessarily work) the Footprint is neither a “robust measure of ecological sustainability” nor does it offer good “guidance for policy makers in identifying and evaluating options to improve use and management of natural capital.”

But instead of exposing a weakness, this example actually illustrates one of the strengths of using a comprehensive metric like the Footprint as a—not the—measure of sustainability. By aggregating demand on different types of productive area using a common metric and a single unit of measurement, the Footprint also encourages the adoption of a systems perspective that helps ensure any proposed solution in fact yields a net benefit. Yes, planting half the United States with high–carbon sequestration tree plantations might seem to help reduce overshoot. But viewing this strategy through a Footprint lens immediately raises questions about potential trade-offs that in turn might decrease or even eliminate any net benefit of this strategy. How much fossil fuel will be used in planting and maintaining these forests? On what land will these plantations be located, and are there already productive ecosystems in these locations that would be displaced? If on cropland, where would food then be imported from, and how much would this increase demand on nondomestic ecosystems? Only once these trade-offs are taken into account can it be determined if a tree planting strategy will result in a net gain or a net loss of biocapacity, and in turn an increase or decrease in overshoot.

In addition, any proposed ecological management strategy—and more generally, any policy designed to address sustainability—must be evaluated not only in terms of its impact on overshoot, but also in terms of many other criteria. This is not just true of the Footprint, it is true of any metric designed to address a single aspect of a complex issue. Getting out of overshoot is a necessary condition for sustainability, but it is certainly not a sufficient one. What about human well-being, for example? This is why the Footprint metric is often used in conjunction with the U.N.’s Human Development Index to simultaneously define two minimum thresholds for a sustainable society—having access to sufficient biocapacity to accommodate the society’s Footprint, along with a human development score consistent with a decent standard of living. Just as no one would suggest that covering half the United States with eucalyptus is a great idea, no one would suggest that minimizing consumption to the point that most people live miserable lives is acceptable as a sustainable solution, regardless of whether or not this strategy would end overshoot.

To avoid sinking further into ecological debt, we need to rein in our ecological overspending. We can begin by measuring how much nature we have, and how much nature we use. This balance sheet, the Ecological Footprint accounting system, enables decision makers to budget thoughtfully within the means of biocapacity. As any CEO will testify, it is dangerous to operate without a balance sheet.