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: Before the Industrial Revolution, naturally occurring amounts of greenhouse gases caused the air near the surface to be warmer by about 33 °C ( 59 °F ) than it would be in their absence.