The day of the overshoot, an effective but imprecise indicator

This year the earth overshoot day » falls Thursday, July 28, announces the NGO Global Footprint Network and the World Wide Fund for Nature (WWF). This indicator, calculated annually by the Global Footprint Network, marks the date when humanity has consumed all the resources the Earth can generate in a year. Thus, during the five months remaining in the year 2022, we will live on credit, using resources that the planet is no longer able to produce.

The 1970s, the trend is undeniable: this date continues to advance. In 2022, the day of the overshoot thus arrives two months earlier than twenty years ago. The only lulls corresponding to economic crises (1970s, 2008) or health crises (in 2020).

The indicator has the advantage of highlighting the limits of our planet, but also the disproportionate burden that the police weighs on the environment in certain countries. In France, for example, Earth Overshoot Day took place on May 5. In the United States, it was March 13.

The concept of ecological footprint

To say that Overshoot Day marks when humanity has consumed all the resources the Earth can produce in a year is the definition for the general public. In reality, the calculation is more complex.

To determine this, the Global Footprint Network relies on the concept of ecological footprint, created in the early 1990s by researcher Mathis Wackernagel as part of his doctoral thesis. The ecological footprint corresponds to the area needed to renew the resources consumed by humanity », says Laetitia Mailhes, spokesperson for theNGO. She is composed of six surface categories: crops, pasture, forest areas necessary for foresters, fishing areas, built-up areas and forest areas necessary to absorb the carbon emitted by the combustion of fossil fuels ». The carbon footprint alone constitutes the major part of the global ecological footprint (60 %).

The ecological footprint of humanity, calculated by theNGO from a corpus of more than 15,000 data provided by United Nations agencies, is then reported to the biocapacity » of the planet, i.e. its ability to regenerate resources. To be able to compare them, these two data are expressed in a common unit of measurement: the global hectare [1].

Currently, the global ecological footprint is thus greater than 21 billion global hectares when biocapacity is estimated at only 12.1 billion global hectares, or 1.6 per person. To arrive at the date of the day of the exceedance, the ratio between the biocapacity and the ecological footprint is multiplied by 365, ie the number of days in a year.

The main interest of this indicator is that it takes into account all the competing demands of humanity on nature: on resources but also on ecosystem services such as carbon absorption, explains Laetitia Mailhes. This makes it possible to understand that climate change is not the problem but a symptom – the most important – of our evil. The basic problem is ecological. It is a question of mismatch between available resources and human activities ».

A great communication tool »

If the concept of ecological footprint was born in the 1990s, the Global Footprint Network began to communicate on the day of the overshoot only in 2006. Before that, theNGO created in 2003 was rather expressed on the number of planets necessary to maintain our way of life (1.75 in 2022). The message is the same but we want to communicate at least once a year to the general public. It was just a communication strategy », recognizes Laetitia Mailhes.

It’s a great communication tool for understanding planetary boundaries », abounds Aurélien Boutaud, associate researcher at theUMR environment-city-society of the National Center for Scientific Research (CNRS) and author of a book, in 2009, entitled The ecological footprint (Discovery). The unit of measurement in question, which corresponds to the area of ​​biosphere necessary to allow a certain way of life, is brilliant, it speaks to everyone. Instinctively, we understand that there are not infinite surfaces », pursue the geographer.

However, this index has a number of limits, says Aurélien Boutaud. First, the statistics on which the Global Footprint Network is based are more or less reliable depending on the country. You have to keep in mind that the statistical tool of a country like France is not the same as that of a poorer country ».

Furthermore, like any synthetic tool, the calculation of the ecological footprint seeks to simplify very complex information to make it accessible to as many people as possible. We have to make shortcuts and hypotheses that are not always visible in the final result », judge the researcher.

It is a tool that remains very anthropocentric »

The main example, in the case of the ecological footprint, is that of the calculation of the carbon footprint, which is probably underestimated. The bias is to convert the tonnes of greenhouse gases emitted into the minimum area that would be needed to absorb this CO2. The methodological assumption is already debatable in itself. But there, the choice that was made is to take a young forest with a very high capture rate of CO2 atmospheric. This is a hyper-optimistic assumption, which tends to underestimate the area that would really be needed », explains the researcher.

In addition, Laetitia Mailhes does not hide it, the ecological footprint has a certain number of blind spots. We cannot take into account all aspects of sustainable development and we do not claim to do so. For example, we do not measure the loss of biodiversity », she explains. It is a tool that remains very anthropocentric, confirms Aurélien Boutaud. It considers that all biocapacity is potentially available to meet the needs of humanity. Or, we know very well that we should probably not intervene on 10 to 20 % of the biosphere in order to maintain “hot spots” of biodiversity. »

We do not measure the loss of biodiversity. » © Didier Flury/Reporter

For the researcher, the concept of ecological footprint Reaches its limits when one wants to move from raising awareness to implementing policy actions. The greater the awareness, the more precise tools we will need ». Thus, since the end of the 2000s, a new framework makes it possible to think that the damage caused by humanity to a finite planet: that of the planetary limits. This time they refer to thresholds beyond the conditions of habitability of the Earth are called into question, which, again, are not taken into account by the concept of ecological footprint.

For Aurélien Boutaud, these two tools are not contradictory, they simply address different audiences: The planetary limits, which are more difficult to understand than the ecological footprint, complete this tool on its weak points. »

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