Bryson's main contribution to the debate on climate change was the idea of "the human volcano" causing global cooling, via an increase in aerosol loading.[2] This idea was sparked in 1962 by his own observation, while flying across India en route to a conference, that his view of the ground was blocked not by clouds but by dust. At the time, the instrumental temperature record did not show unambiguous warming and the view that the earth might be cooling, and heading for further cooling, was not unreasonable. Others, including Hubert Lamb, who created a Dust Veil Index,[3] thought volcanoes were more responsible for global-scale aerosol.[4]
In later years, when it was clear that the climate was indeed warming, Bryson argued that while climate change and a global increase in temperature are real, he did not believe that they are caused by human activity. Rather, he argued that they are part of natural global climate cycles, particularly the end of the Little Ice Age:
"All this argument is the temperature going up or not, it’s absurd," Bryson continues. "Of course it’s going up. It has gone up since the early 1800s, before the Industrial Revolution, because we’re coming out of the Little Ice Age, not because we’re putting more carbon dioxide into the air."