**Creation of the “Institute of the Mathematics for Planet Earth”**

**Laure Saint-Raymond**. After five years at the ENS of Lyon, Laure Saint-Raymond will join the Institut des Hautes Etudes Scientifiques as a permanent professor in September 2021. Her work focuses on the asymptotic analysis of systems of partial differential equations, in particular those governing the dynamics of gases, plasmas or fluids. In particular, she has made important contributions to Hilbert's sixth problem concerning the axiomatization of mechanics. In parallel, she works on fluid mechanics models describing ocean currents, and studies the influence of rotation and stratification on wave propagation and boundary layer phenomena.

**Arnaud Guillin. **Arnaud Guillin is a Professor of Mathematics in Clermont Auvergne University (France), currently also Senior Member of the Institut Universitaire de France. He is specialized in probability, statistics, and their interactions with partial differential inequalities. More precisely he studies the long time behavior of Markov processes (Markov chains, diffusions, PDMP, ...) through coupling or functional inequalities. He has since a few years developed numerous collaborations, mainly based on novel statistics methodology or probabilistic models, in medicine, biology/ecology and earth science (ash falls from volcanoes, runout distances of pyroclastic currents, ...).

**You are director and executive director of the newly created "****Institute of the Mathematics for Planet Earth****" - IMPE (In French: ***Institut des Mathématiques pour la Planète Terre - IMPT*)**. What are the mathematics for planet Earth?**

We do not know if there is really a precise delimitation of mathematics for the planet Earth.

Our conviction (as mathematicians!) is that mathematics offers an abstract framework to better understand complex systems. The tools are diverse: statistics, probabilities, asymptotic analysis, dynamical systems, partial differential equations, game theory... and we have no preconceived ideas about the methods that will have to be used to best describe our physical environment (climate, oceans, atmosphere, glaciers, ...), the different biological ecosystems and their interactions with humans.

But the collaboration between mathematicians and scientists from different backgrounds can only be fruitful, to advance the modeling, the theoretical understanding of models, their simulation.... and also to imagine new mathematics!

**What is the goal of this new institute?**

The goal of the institute is to give a framework to these collaborations (of which there are already many individual examples) and to create a multidisciplinary community with a real coherence around a project with a strong societal stake.

The scientific community is traditionally organized along disciplinary lines, and it is important that each researcher maintains a strong expertise in one discipline. But it is also important to have common references in order to move forward together. The idea is therefore to propose continuing education for researchers who want to get involved in the project, through schools and by co-supervising students (PhD students, post-doctoral fellows, ...) on research projects intimately linking mathematics and at least one other science.

**What are the organizations that support it?**

For the moment, the institutions that carry this project are the CNRS (the main French research institution) and its thematic institutes -- more specifically, the mathematics, ecology and environment, physics, and sciences of the universe -- together with the Ecole Normale Supérieure of Lyon and several universities (University of Clermont Auvergne, University of Grenoble Alpes, University Claude Bernard Lyon 1, University Jean Monnet Saint-Etienne, University Savoie Mont-Blanc).

But in the long term, the institute has a national vocation and should federate other universities and research institutes such as INRIA (numerical sciences), INRAE (agriculture) or INSERM (health sciences). We would like the administration to be as light as possible and for all efforts to be focused on the scientific dynamics.

**This is not the first initiative that promotes the mathematics for planet earth. In 2013 for instance, the IMU, ICIAM and UNESCO (among others) launched the initiative "Mathematics for Planet Earth 2013". This was almost a decade ago; I guess that you got some useful feedback from that experience...**

The project was born in part from the work that had been done by INSMI during the year of Mathematics of the Planet Earth in 2013 (supported by the National Research Agency).

This initiative allowed to identify a number of interdisciplinary collaborations around these themes, and to make an inventory of the situation leading in particular to a large report (which unfortunately remained rather confidential) and to a series of small popularization articles "Brèves de maths" which is very successful.

The IMPT wants to keep this mission of scientific diffusion and is engaged in a collaboration with the Musée des Confluences in Lyon. But it goes further in that it also wants to be a place of exchange and training for researchers, not only an observer but a real actor of the research.

**Is there a coordination at the international level between your initiative and similar ones in other countries? Do you think that the ICIAM for instance could play a role in this regard ?**

There are of course climate-related research all over the world, and thematic programs that foster interdisciplinary collaborations. But we are aware of very few similar math-centered and cross-disciplinary (physics - geophysics - biology - economics) initiatives abroad.

For the moment, the institute is ambitious in its objectives but relatively modest in its size (it will finance between 5 and 10 collaborative projects each year). It will certainly expand in the future. For this, the scientific council - which has been carefully chosen - will be a major asset. The colleagues who sit on it, in addition to their expertise in various disciplines, represent an opening to other networks: European Weather Center, IPCC, World Climate Research Program...but also to private foundations that set up chairs.

We hope that scientific societies and international unions of researchers will seize this dynamic and promote the emergence and recognition of this interdisciplinary community at the international level.

**As we can see with the pandemic or climate change for instance, the outcomes of mathematical modelling are politically in the middle of the fray. The IMPT is therefore likely to be solicited to participate to the public debate about these issues of tremendous importance. This is quite an unusual role for a mathematician...**

It is important that on these subjects, the public debate can be enlightened by real scientific arguments. This is essential for democracy: otherwise we are not in a constructive dialogue but in a struggle for influence.

The mathematician has a particular capacity to summarize the data of a problem, and to implement a reasoning based on logic. Generally speaking, the scientist is not there to make decisions in place of politicians, nor to tell everyone what to think, but to give as objectively as possible all the elements to take into account.

To keep its credibility and the confidence of society, scientists must also be clear about the uncertainties, the margins of error, etc. and not venture into subjects that are outside their field of competence: another interest in the emergence of multidisciplinary structures with a collective reflection!