L’impératif d’émissions nettes nulles

OXFORD – Le monde vient de conclure un accord historique concernant les changements climatiques. L’accord conclu à la Conférence des Nations Unies sur les changements climatiques à Paris engage les pays à prendre des mesures pour limiter le réchauffement planétaire à des températures « bien en deçà » du 2 °C par rapport au niveau préindustriel et à poursuivre des « initiatives » pour limiter le réchauffement à 1,5 °C. Il exige également des pays développés qu’ils assurent le financement de 100 milliards $ par année en guise d’aide aux pays en voie de développement. Mais, malheureusement, les négociations de dernière heure ont mis de côté le chiffre qui importe vraiment pour notre planète : zéro.

Il s’agit là du volume net de dioxyde de carbone que l’on peut émettre pour voir un jour la température de la planète se stabiliser à un niveau ou à un autre. Zéro, rien, nada. Le système atmosphère-océan de la Terre est comme une baignoire qui se remplit de CO2 et d’autres gaz à effet de serre : plus ce niveau est élevé, plus la planète se réchauffera.

Le robinet des émissions doit être fermé dès que la baignoire atteint un niveau corrélé à un certain niveau de réchauffement – disons, 2 °C, au-dessus duquel, selon le consensus quai général des scientifiques, les risques deviennent graves, les points de bascule deviennent possibles et la capacité d’adaptation de la civilisation n’est plus assurée. Autrement, la baignoire de l’atmosphère continuera de se remplir, réchauffant la planète de 3 °C, 4 °C, 5 °C et ainsi de suite, jusqu’à ce que les émissions cessent éventuellement d’elles-mêmes – ou bien lorsque notre espèce s’éteindra. Plus vite ce robinet se fermera, plus basse sera la température à laquelle le climat planétaire se stabilisera, moins les risques seront présents et plus faible sera le coût que nous aurons à assumer dans l’adaptation à une planète réchauffée.

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