During the 20th century, the average temperature of the continental United States rose by almost 1 degree Fahrenheit (0.5 degree Celsius)—everywhere, that is, except in the Southeast. There, until the 1980s, the temperature actually decreased slightly. Climate scientists dubbed this peculiar phenomenon the “warming hole,” and it was the cause of much speculation. But beginning in the 1990s, temperatures in the Southeast began to warm again, and in the early years of the 21st century this warming has accelerated.