NORTH AMERICAN DROUGHT RECONSTRUCTIONS FROM LONG TREE-RING RECORDS

A large part of the western USA has been experiencing unusually severe and persistent drought over the past 5 years. This drought highlights both the vulnerability of this semi-arid region to shortfalls in rainfall and the need to better understand long-term drought variability and its causes in North America. To this end, centuries long, annually-resolved tree-ring records have been used to reconstruct annual changes in both drought and wetness over large portions of North America. The drought metric used for this purpose is the summer-season Palmer Drought Severity Index (PDSI). The PDSI reconstructions have been produced on a 297-point 2.5x2.5° regular grid that covers most of North America, a substantial expansion of an earlier 155-point 2° x 3° grid that only covered the coterminous USA. This new generation of North American drought reconstructions also extends back over 1,000 years into the past in parts of North America, a substantial increase over the ca. 300 years available in the previous reconstructions. An analysis of these reconstructions in the western USA reveals a persistent increase in aridity over the AD 900-1300 interval, a timing that is remarkably similar to that ascribed elsewhere to the “Medieval Warm Period” (MWP). This period of elevated aridity is also found in independent records of increased fire frequency and reduced lake levels in western North America. This was followed by a long-term reduction in aridity beginning around AD 1300 that persisted up to roughly AD 1850, a possible expression of cooler/wetter “Little Ice Age” conditions there. During the 20th century, relative aridity has increased in the western USA to the point where it is now above the long-term average, but not yet at the level seen during much of the MWP. If the MWP can be used as an analog for future “warm-world” droughts in the western USA, greenhouse warming is likely to increase both drought frequency and severity there in the coming decades. However, there is also evidence for a long-term solar forcing effect on drought in the western USA that needs to be taken into account. The way in which this mode of climate forcing behaves in the future is likely to affect drought frequency and severity as well.