In this letter, we recall that young people's concerns about climate change are based on our best scientific knowledge and that the measures taken so far by our authorities to protect the climate and the biosphere are far from up to the threat. We therefore call for immediate action to redress the playing field, to discourage climate-damaging activities and to encourage positive action. The young people have grasped the problem and are right to mobilize, their future is at stake. This letter has already been signed by 4,426 scientists from all over the world by April 2019.