claim: Globally averaged thermometers show two periods of warming since 1900 : a half-degree from natural causes in the first half of the 20th century, before there was an increase in industrial carbon dioxide that was enough to produce it, and another half-degree in the last quarter of the century.
evidence: The average global temperature on Earth has increased by about 0.8° Celsius ( 1.4° Fahrenheit ) since 1880; Two-thirds of the warming has occurred since 1975, at a rate of roughly 0.15-0.20 °C per decade.