The warming from 1915 to about 1940 is generally believed to be due to a slight increase in solar output and a reduction in volcanism. The recent warming from 1975 is due to the build-up of greenhouse gases, as they begin to dominate over sulphate aerosol releases. The slight cooling from 1940 to 1975 is thought to be due to industrial development (using lots of dirty coal) increasing aerosol emissions.
Nature: LIGO may have seen echoes from a black hole firewall
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