For example, in California’s San Joaquin Valley, one of the U.S.’s most productive farming regions, the daytime temperature has increased little over the past 100 years, Christy says. But the nighttime temperatures of the valley are now about 4 °C higher than a century ago. Meanwhile, in the Sierra Nevada Mountain foothills to the east, the daytime and nighttime temperatures have essentially been static over the past century.


Christy says the shift in natural dry, brown scrub to irrigated green fields in the San Joaquin Valley and other large farming regions means less heat is absorbed, and it provides a cooling effect on daytime temperatures. In contrast, the additional moisture in the ground holds in heat that is released at night, leading to the warmer nighttime readings. This is not a heat island effect, Christy proposes, nor is it fully related to heat radiating back into space and being trapped by CO2. “In some areas, the observed warming is more consistent with land-use change than our understanding of greenhouse gas effects,” he believes.