But water vapor is not considered a forcing. Nor is it assigned a global-warming potential, because it has a relatively short residence time of about 10 days in the atmosphere, compared with a century or longer for CO2.

温室气体分子吸收光能、再把热量传递给周围分子,只是瞬间的事情,和residence time完全不是一个数量级,为什么要考虑residence time?