With the world's population being predominantly urban, the challenges of sustainability in cities have increased, as have those of education for sustainable cities. The French curriculum has taken up the subject from the mid-2015s. Teachers are asked to teach the issues of the sustainable city and to mobilize a prospective approach, i.e. to design scenarios for the desirable evolution of cities. The idea seems appealing, yet many teachers are powerless in the face of this teaching, which confronts them with a socially vivid question (Legardez & Simmoneaux, 2006), uncertainty, and foresight, which is a form of modelling for which they have little training.
Training teachers in sustainable city education raises a threefold training issue:

• An epistemological issue: The aim is to help teachers understand the debates surrounding sustainability in cities, the uncertain nature of knowledge and the values at stake. This issue relates to the relationships that teachers, often historians by training, have with knowledge (rapport aux savoirs (Charlot, 1999)).

• A didactic issue: The aim is to enable teachers to deal with socially lively issues in the classroom, that is, to be able to think up didactic approaches that do not conceal the uncertainty surrounding the sustainable city.
• A professional development issue: In the longer term, the challenge is to change teaching practices in geography, which are still largely transmissive and dialogical (Audigier, 1996; Tutiaux-Guillon, 2004).

The research question is how to develop teacher training that responds to this triple challenge. We have hypothesized that training based on experiential geography would make it possible to meet this challenge.

Experiential geography is based on experiential learning developed by Kolb (1984) in line with previous work (Dewey, 1938; Lewin, 1951; Piaget, 1968; Vygotskii et al., 2012). The Spatial Thinking research group modelled an experiential learning approach: the 4I's: Immersion, interaction, institutionalization and implementation (Leininger-Frézal & Gaujal, 2018). It is on this model that the design based research presented in the following section is based.

The immersion phase aims to involve learners in a situation in which they are confronted with a spatial practice. This situation can take different forms: field trip, role play, case study etc. The next phase, interaction, allows students to confront their experiences and understand what is common and what is different. This phase allows the construction of the knowledge at stake, which is then formalized in the next phase, i.e. institutionalization. The last phase is the one that allows to reinvest the learning achieved in other situations. 

We have developed a design based research during one year with 15 teachers in initial and continuing education within the framework of the master of didactics of the University of Paris and the master of geography education in Hambourg university. The experimentation took place in the framework of a project funded by the DAAD (German research funding agency) in collaboration with Sandra Sprenger (University of Hamburg). The experiment is still and will be structured in several phases. Immersion: Initially, students will carry out sensitive visits (Gille-Gaujal, 2016) to different urban spaces that they consider relevant to urban sustainability issues. A sensitive outing is a way of doing fieldwork in which the student apprehends the space surveyed by the senses. The students survey a delineated space autonomously in pairs. They note on a field notebook their sensations, smells, colors, their feelings. Students can alternate modes of travel: public transport, bicycle, on foot and stop to observe the passers-by and the landscape. Two visits will be carried out: one in the university district and the other in a space chosen by the student to be subjected to sustainability issues. Interaction: In a second step, the students will be confronted with the information collected in their field notebook for the outing to the library. They will explain the choice of the second field and discuss the reasons for their choice. The challenge will be to discuss sustainability issues in the city and to establish a panorama of the issues raised by urban sustainability. Institutionalization: In a third phase, the trainer Sandra Sprenger, will take up the questions identified and conduct a conference on the sustainable city in geography. At the end of this conference, the teachers will build a mental map of the sustainable city for them. Implementation: As a final step, the students will conduct a virtual excursion on the explored terrains, to be used with their high school students and to carry out an analysis of its use in class. We will analyze the students' productions, i.e. the virtual excursions and the mental maps produced on the sustainable city with regard to the following three questions: What vision of the city is constructed? What conception of sustainable development is conveyed? How do teachers take up ESD?

As the process is ongoing, it is difficult to present the results accurately at this time. We hope to show that experiential geography is a relevant approach to training teachers in sustainable city education. We hope to demonstrate that teachers are able to conceptualize what a sustainable city is in all its complexity and with the uncertainties involved in thinking about the future. Nevertheless, we expect that teachers will encounter difficulties in dealing with this complexity and uncertainty in the classroom. We will analyze these difficulties and try to see how to overcome them.

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