The spate of recent controversies about climate research has given fresh voice to a group of scientists who question the mainstream view that human activity is warming the planet to dangerous levels.

Very few scientists disagree that the earth's climate has warmed since 1850. But some have long argued that there are too many uncertainties about man's role in the warming, and that other factors, such as solar activity and the greenhouse effect of clouds, could account for a large part of the observed warming trend. Among this...