"In the 1930s and 1950s, the central United States experience two periods of extreme drought. The 1980s and 1990s had ten of the warmest years this century and possibly since the Little Climatic Optimum. Proxy and instrumental data indicate that 1998 was the warmest year globally in 1200 years of Earth history. In the following year, a La Nina developed and global temperatures dropped slightly. Nevertheless, the mean global temperatures recorded for this year was the sixth highest measurement since 1880. Many scientists believe the warmer temperatures of the 20th century are being caused by an enhancement of the Earth's greenhouse effect. "