Le cycle du carbone dans les forêts et le changement climatique: Comprendre le passé pour s'adapter au futur

A series of extreme events affecting forests that are perhaps related to climate change - pests, storms, fires and droughts - has recently received much coverage in the media and in scientific publications. These documents describe forests as sources of carbon. In contrast, all the methods for estimating the carbon balance of forests worldwide show a net absorption, a global sink of over a billion tons of carbon per year (1 PgC/yr.). Changing land use strongly affects the global balance, especially in the tropics, highlighting that an even greater "natural" forest sink is necessary to ensure consistency with the values recorded in the atmosphere. The four main determining factors for this "natural" forest sink are: climate change, increased carbon dioxide (CO₂) concentration in the atmosphere, increased nitrogen deposition and changes in the intensiveness of forest management. Even though our understanding of the causes of forest sinks is still imperfect, it appears that the concentration of CO₂ and nitrogen deposition are the major causes on the global scale. The effect of CO₂ is particularly important in the tropics while nitrogen deposition is thought to be predominant everywhere else. Simultaneous assessment of different adaptation options for forest management in terms of their benefits in the areas of adaptation and mitigation shows up examples of "no regrets" measures in spite of the uncertainty connected with climate risk.