how to keep accounts?

During the presidential campaign, a collective of elected officials published a “manifesto for candidates” that particularly called for “adapting the use of Zero Net Artificialization” (or ZAN) to revive tourism in their territories in a sustainable way.

This appeal once again testifies to the concern created by this national goal (which involves renaturalizing as much space as those artificialized at the national level) among societies.

The challenges of ZAN

However, at first glance, ZAN seems to be a big step forward in terms of taking ecological issues into account in spatial planning. In fact, the loss and fragmentation of natural habitats is now recognized as the main factor in the collapse of biodiversity. Urban sprawl and intensification of land use also have negative effects in climatic, social and economic terms.

In France, between 20 and 30,000 hectares of nature, agriculture and forestry (ENAF) areas disappear every year due to urban sprawl, also in areas experiencing population decline. If we look more broadly at the entire intensive occupation of space by human activities, almost 47% of the national territory can be considered anthropized.

If the regulation of land use has been gradually taken over by laws relating to urban planning, the Climate and Resilience Act of 2021 has the advantage that it formulates the need to control land use while conserving biodiversity. It thus defines land use by taking into account the ecological functions that the soil performs (essential for the conservation of biodiversity), and sets the ZAN target for 2050.

However, the effects of a law depend heavily on the way it is applied. It is especially by decree of the Council of State to determine (by a deadline for the unknown time) “the nomenclature of the artificial lands as well as the scale on which the artificialization of the land is to be appreciated in the documents of planning and urbanism”.

How should we count the artificialization?

The method of calculating land use is a crucial issue because it determines the level of ambition of the policy. Exactly, the draft decree on this nomenclature was sent for public consultation in March 2022.

We learn that artificialization will be taken into account in planning and urban planning documents and that it is limited to rural areas. At the technical level, the accounts from 2031 are kept thanks to “large-scale land use” (OCSGE), a database of the National Institute for Geographic and Forest Information (IGN), which is based on a model that separates land cover and its use.

These data currently being produced will have the advantage of being spatially accurate (from 200 m2 for buildings and 2500 m2 for objects located outside the built-up area) and regularly updated, a good compromise for monitoring developments in the scope of urban planning documents.

This land use monitoring system makes it possible to distinguish between 14 types of cover (from impenetrable built-up areas to green tree formations) and 17 land use categories (eg agricultural, road or logistics services). The choice to classify the categories as “artificial” or “non-artificial” is then the result of the intersection between the coverage layer and applications … but also of political negotiations.

The city’s green areas, natural or artificial?

In the draft decree, e.g. non-wooded green areas for urban use (which include lawns in urban parks but also grass sports grounds) as artificial.

We can then rightly wonder whether this election would not deter initiatives that promote nature in the city. If deconstruction of a concrete space (such as a parking lot) to install a city garden that does not count as a disartificialization in ZAN’s balance, why should communities engage in this way?

This ambiguity is explained in particular by the limits of OCSGE. Non-tree vegetation is described in two categories: “herbaceous formations” and “other non-tree-like formations” (which include areas with higher vegetation but devoid of trees). The data from the OCSGE coverage model do not distinguish the more or less natural nature of these spaces. For example, a grassy sports stadium will be interpreted as a space with herbaceous vegetation. Will we consider that the conversion of a car park into a football pitch is an act of de-partitioning?

Aerial maps whose land areas corresponding to the different spaces are colored;  the card on the left is almost completely colored, the one on the right is almost half as much
The Arcachon Bay SCOT dataset allows different scenarios to be tested. To the left, green areas without trees for urban use count as artificial. To the right, they are excluded. This choice has a strong impact on the goals of reducing land use.
Brian Padilla and colleagues / MNHN, Provided by the author

The choice to classify non-wooded green areas for urban use among artificial spaces also reminds us that depending on their integration into ecological frameworks and their management methods, urban green areas are not always favorable for biodiversity.

Quarries, non-artificial spaces?

The draft decree proposes a much more surprising choice: the surfaces corresponding to quarrying activities will be considered non-artificial. However, regardless of the type of exploitation, these activities generally involve removing the congestion to extract the occurrence.

The duration of the operations usually lasts between 20 and 30 years, but many operations are extended far beyond, sometimes up to more than 100 years. Throughout this period, the functions of the earth have been reduced to a minimum, so much so that it seems inconceivable that these surfaces are not considered artificial by law.

All the more so as there are a total of almost 3,300 quarries representing around 110,000 hectares of surface area. A comfortable envelope to be drawn from the ambitious goals of the law if the draft decree remains as it is.

The risk of simplicity: getting the wrong goal

In order to achieve ZAN, it is necessary to keep accounts: which spaces have been artificialized, which, on the contrary, have been restored for the benefit of biodiversity?

Having data at the national level is an asset to better territorialize this ambitious goal, but one must be aware of the boundaries that the exercise imposes: the models that describe coverage and use of the land will evolve over time.

However, the tools available offer options that need to be considered. It will e.g. be relevant no longer to count only in surface area, but to attach an artificialization coefficient to each of the categories so that it reflects the intensity of the artificialization and the functionality of the soils in question. The artificial character would then no longer be dichotomous, but would follow a gradient. This could, for example, give a greater weight in the ZAN accounts for densely populated areas (buildings) compared to urban green areas.

If the simplicity of a binary approach and surface approach has its advantages, it encounters a major pitfall: biodiversity is a complex object. While the purpose of ZAN aims to stop its collapse, a warning is in order: By oversimplifying, we can achieve ZAN at the accounting level by again forgetting to preserve the favorable areas for the living.

Edit: the expected decrees were finally adopted by the Council of State on Friday, April 29, after the writing of this article. The choices discussed here are fixed: The city’s green areas without trees are classified as artificial lands, while quarries in operation are classified as non-artificial.

Brian Padilla, Organic Research and Expertise Engineer, National Museum of Natural History (MNHN); Fanny Guillet, sociologist, researcher at CNRS, National Museum of Natural History (MNHN) and Salomée Gelot, Research and Expertise in the Framework of the ERC Sequence and the ZAN, National Museum of Natural History (MNHN)

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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