Clouds of (slightly less) unknowing

摘要

Clouds and aerosols have long been two of the more mysterious forces in the climate. They sometimes warm and sometimes cool the Earth. The net effect, it was thought, was that they offset part of the overall warming trend, which would have been greater had it not been for their influence. But the details were obscure. They still are, but much less so. Scientists' improved understanding of clouds and aerosols is, says Piers Forster of the University of Leeds, "the most interesting development since AR4". That acronym refers to the fourth assessment report of the Intergovernmental Panel on Climate Change (ipcc), which was published in 2007. The first parts of its successor, the fifth assessment, were published on September 27th and 30th. And the upshot of its assessment of clouds and aerosols is that their cooling effect looks less strong than it did.