What was done
The authors studied the orientation, morphology and internal structure of dunes in the easternmost (wettest) portion of the Nebraska Sand Hills, where shallow core and outcrop samples indicate the dunes were formed some 800 to 1000 years ago, when aridity was widespread and persistent across western North America. In addition, based on wind data obtained from six meteorological stations in and near the Nebraska Sand Hills, they employed a computer program to calculate the sand-drift vectors of dunes that would form today if the sand was free to move and not held in place by protective prairie grass.

What was learned
Sridhar et al. determined that the current configuration of the Sand Hill dunes could not have been created by the region's present wind regime, in which air currents from the south in the spring and summer bring moist air from the Gulf of Mexico to the U.S. Great Plains. Instead, their work revealed that the spring and summer winds that formed the dunes some 800 to 1000 years ago had to have come from the southwest, bringing much drier and hotter-than-current air from the deserts of Mexico, along with greatly reduced opportunities for rain.

What it means
The findings of this important study clearly suggest that much of western North America was likely both drier and hotter than it is today some 800 to 1000 years ago (and possibly earlier as well), during the global Medieval Warm Period (MWP). As Sridhar et al. describe it, "the dunes record a historically unprecedented [our italics] large-scale [our italics] shift of circulation that removed the source of moisture from the region during the growing season." What is more, the resultant drier and warmer conditions may then have been further "enhanced and prolonged," as they phrase it, "by reduced soil moisture and related surface-heating effects," which effects, we might add, are not operative in our day to the degree they were 800 to 1000 years ago, as was demonstrated by still other of Sridhar et al.'s computer analyses. Hence, their work provides compelling evidence that great portions of western North America were warmer than they are today during the central portion of the Medieval Warm Period, as defined by the global MWP frequency distribution at the bottom of the Interactive Map and Time Domain Plot of our Medieval Warm Period Project.