How to keep accounts?

During the presidential campaign, a group of elected officials issued “manifestos for candidates”, specifically calling for the “application of zero net artificialization” (or ZAN) to sustainably revive tourism in their region.

This appeal again testifies to the concern that this kind of purpose (which involves re-naturalizing as much as artificially created on a national scale) creates within the community.

ZAN’s challenge

However, at first glance ZAN seems to be a big step in considering environmental issues in land use planning. Indeed, the loss and fragmentation of natural habitat is now recognized as a major cause of biodiversity decline. Urban sprawl and intensification of land use also have negative effects on climate, social and economic conditions.

In France, 20 to 30,000 hectares of natural, agricultural and forestry (ENAF) disappear each year due to urban sprawl, including areas facing declining population. If we consider in more detail the whole of the intensive occupation of space by human activity, about 47% of the national territory can be considered ethnographic.

If land use control is gradually taken over by city planning laws, then the Climate and Sustainability Act of 2021 has the potential to clarify the need for land acquisition control along with biodiversity conservation. It thus defines land acquisition considering the environmental functions performed by the soil (essential for maintaining biodiversity) and sets the ZAN target for 2050.

However, the effect of a law strongly depends on how it is applied. This is especially the case with the decree of the Council of State which has to be fixed (for a certain period of time for an unknown period of time).

How are we going to calculate the artificialization?

The method of calculating land acquisition is an important issue, as it determines the level of policy ambition. Specifically, the draft decree relating to this naming was submitted for public consultation in March 2022.

We learn that planning for artificialization and urban planning will be accounted for in the document and it is limited to the land area. At the technical level, the accounts will be kept from 2031 “Large Land Use” (OCSGE), a database of the National Institute for Geographic and Forest Information (IGN) based on a model that separates land cover. And its use.

This data, currently being produced, will have the advantage of being spatially precise (up to 200 meters)2 For built-up area and 2500 m2 For objects located outside the built-up area) and regularly updated, a good compromise for monitoring development on the scale of town planning documents.

This land use monitoring system can distinguish 14 types of cover (from impenetrable built-up area to leafy tree formation) and 17 land use categories (for example agriculture, road or logistics service use). The choice to classify categories as “artificial” or “non-artificial” then results in a link between the coverage level and usage… but also political discourse.

Urban green space, natural or artificial?

For example, in the draft decree, treeless green space for urban use (including the lawn area of ​​an urban park, but also a grass playground) is considered artificial.

We can then legitimately think that this choice will not discourage nature promotion initiatives in the city. If building a concrete space (such as a car park) for an urban garden is not considered a degradation on ZAN’s balance sheet, why would communities be involved?

This ambiguity is specifically explained by the limitations of OCSGE. Treeless plants are described in two categories: “herbal texture” and “other non-woody texture” (which have higher vegetation but no trees). Data from the OCSGE coverage model does not distinguish more or less the natural character of these spaces. For example, a grass sports stadium would be defined as an area of ​​medicinal plants. Would we consider transforming a car park into a football field an act of dehumanization?


The Arcachon Bay SCOT dataset allows testing in a variety of situations. On the left, green spaces without trees are counted as artificial for urban use. On the right, they omit. This choice has a strong effect on the goal of reducing land acquisition.
Brian Padilla and colleague / MNHN, Contributed by the author

The choice to classify treeless green spaces for urban use among artificial spaces reminds us that, depending on the ecological structure and their integration into management practices, urban green spaces are not always conducive to biodiversity.

Quarries, non-artificial space?

The draft decree offers even more surprising choices: surfaces related to excavation activities will be considered non-artificial. However, regardless of the type of exploitation, these activities usually involve snatching extra burden to lift the deposit.

The duration of the operation is usually between 20 and 30 years, but is extended after many operations, sometimes up to 100 years. Throughout this time, soil works are minimized, so much so that it seems unimaginable that these surfaces are not considered artificial under the law.

In all, as a whole, about 3,300 mines represent an area of ​​about 110,000 hectares of land surface. A comfortable envelope would be excluded from the ambitious purpose of the law if the draft decree is as such.

Risk of simplicity: Finding the wrong purpose

In order to achieve ZAN, it is necessary to keep accounts: which places have been artificially created, which ones have been restored for biodiversity?

Having data on a national scale is a resource to better regionalize this ambitious objective, but be aware of the limitations imposed by practice: models that describe soil cover and usage will evolve over time.

However, the tools available offer options that must be considered. For example, it would be relevant not only to calculate in terms of surface area, but also to add an artificialization coefficient to each section to reflect the intensity of artificialization and the corresponding soil performance. The artificial character will then no longer be two-way, but will follow a gradient. This, for example, can give a greater weight to the ZAN account in heavily sealed areas (built-up areas) than in urban green areas.

If a binary and a surface approach have the qualities of simplicity, it comes up against one big disadvantage: biodiversity is a complex object. Although the purpose of ZAN is to stem its decline, one caveat is: if we over-simplify, we can, again, achieve ZAN at the accounting level by forgetting to save the living space.

The expected decree was finally adopted by the Council of State on Friday, April 29, after writing this article. The choices discussed here are fixed: treeless urban green spaces are classified as artificial soils, where ongoing mines are classified as artificial soils.

Brian Padilla, Environmental Research and Specialist Engineer, National Museum of Natural History (MNHN); Fanny Gillette, Sociologist, CNRS Researcher, National Museum of Natural History (MNHN) And research and expertise within the framework of Salomi Gellet, ERC Sequence and ZAN, National Museum of Natural History (MNHN)

This article has been republished from Conversations under a Creative Commons license. Read the original article.

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