For the first time, scientists have traced the north-south shifts of the northern-most edge of the tropics back 800 years. The movement of the tropical boundary affects the locations of Northern Hemisphere deserts including the Sonoran, Mohave and Saharan. The Earth's climate system affects the movement of the tropics, which have been expanding since the 1970s. The research team found that in the past, periods of tropical expansion coincided with severe droughts.