In the event of a major drought, could technology make it rain?

L’story happened last year in Mexico. Farmers in the province of Puebla have been ligués to sue Volkswagen. The reasons for their anger: the car company next to their corn plantation diverted the rain. The farmers are convinced that Volkswagen ruined their crops by installing "hail cannons". These cause shock waves in the atmosphere to prevent hailstorms from damaging cars coming off the production line. The devices are blamed for causing months of drought, while farmers near the German automaker's plant expected a lot of rain.

This jaquerie is one of the most recent episodes in humanity's attempts to control the weather, a new kind of "rain dance" that raises questions and concerns.

Already in 2005, Nissan had installed hail guns near one of its Mississippi plants, scaring the neighbourhood with one shot every six seconds. Experts doubted from the outset that these measures were effective, and the World Meteorological Organization itself considered all these technical efforts to be a waste of time and money.

Nevertheless, many geoengineering techniques, designed to modify the normal weather process, have been developed in recent years. Cloud seeding has proven to be one of the most effective techniques for controlling precipitation. It involves shooting chemicals into clouds - often from a small plane - to cause rain or snow. This is done in more than 50 countries for a variety of reasons, including dispersing fog at airports, reducing property damage caused by giant hail in Canada, and increasing snowfall in Colorado and summer rainfall in Texas. Experts such as William Cotton, professor of atmospheric science at Colorado State University, esteems that "Seeding mountain clouds in winter can increase precipitation by 6 to 8 %. That's enough to make a large number of water users happy and willing to pay for it. »

No more than large states had to embark on this new rain dance with great means. The seeding of clouds to make rain fall is happening today and in some parts of the world on a dizzying scale.

For example, the Chinese government is currently developing the cloud seeding projectThe aim is to boost rainfall on the Tibetan plateau, an area of more than 1.6 million km2 , three times the size of Spain.

The state-owned China Aerospace Science and Technology Corporation has designed and built caissons that use military rocket engine technology and burn fuel to produce silver iodide, a cloud seeding agent. When the caissons are installed on mountain ridges, the particles move through the clouds and trigger rainfall in one of the driest places on Earth. This method is used locally, but the size of the Chinese experiment makes it seem like a geoengineering project. We are witnessing a veritable scallion race in many laboratories around the world.

Several technologies are currently being investigated to reduce the sun's radiation in order to temper the climate. Some of these methods are considered by the Academy of Technology to be unrealistic or dangerous:

- The installation of space reflectors to reflect some of the solar energy. This solution is mentioned as a possibility. However, the deployment of such reflectors has not been studied in detail, no doubt because of the low degree of realism of this technology, confirmed by a recent report by the National Centre for Space Studies.

- Stratospheric injection of sulphur aerosols, or aerosol precursors, is probably the methodology which, while not desirable, seems the most credible from a technological point of view. Such technologies for injecting sulphur aerosols do not exist, but seem, according to the authors of the report, to be inexpensively developable, at least in a fairly "rustic" manner. However, the Academy of Technologies points out that the effects induced by this method are not well known. The climatic changes induced by the injection of stratospheric aerosols are relatively difficult to control and their effects on the physico-chemistry of the stratosphere, in particular on the ozone balance, are still poorly known.

- Tropospheric seeding, to increase the reflectivity of low clouds, has also been proposed, especially over oceans. According to the report, climate effects are even less well controlled than with stratospheric injection. These technologies are not simple. They involve the spraying of large quantities of seawater, which contains impurities, and evaporative cooling of the injected sea salt aerosols, which cause the air mass to fall back.

- Increasing surface albedo is another method discussed in the report. Albedo is the reflectivity of a surface. For example, snow-covered areas reflect more solar radiation, amplifying the cooling of the earth's surface. The Technology Academy reporters distinguish here between marine surfaces (where an increase in reflectivity by microbubbles appears unrealistic) and continental surfaces where a more localized increase may be akin to adaptive techniques (more reflective crops, white-painted buildings).

In any case, a climate regulated by such solar radiation management technologies would nevertheless experience fairly strong residual changes on a regional scale, particularly in terms of possible changes in precipitation patterns.

So far, there have only been a handful of small experiments, but there is still a lot of research to be done if it is to be used as a technique for cooling the Earth.

This way of interfering in the climate is highly controversial and raises all kinds of ethical and logistical questions.

" It would be the most global undertaking that mankind has ever undertaken. Who's going to make the decision to do it? Trump? The head of Exxon? The U.N. General Assembly?"said in an interview with the Guardian Janos Pasztor, Executive Director of the Carnegie Climate Geoengineering Governance Initiative, which seeks to create effective governance for climate engineering technologies. He urges governments to develop policies to prevent a country of crooked billionaires from taking matters into their own hands. « We only have one atmosphere, so we have to do it right. ", he assures.