Thursday, November 29, 2012

New paper finds Arizona droughts were less frequent and less extreme during 20th century

A new paper published in Climatic Change reconstructs droughts in NE Arizona over the past 400 years and finds the 20th century had "fewer multiyear [severe droughts] than any other century" and that "Perhaps of greatest relevance, this study suggests that severe and sustained episodes of dual-season drought, which are largely missing from the instrumental period, have occurred multiple times in the past (e.g., 1660s, 1740s, 1890s)." The paper adds to multiple others demonstrating that global warming does not increase the frequency or severity of droughts, floods, cyclones, or extreme weather.

From the paper: 

"the longest continuous run of seasonal drought during the modern period was 5 years, whereas the pre-modern period contained episodes of six, seven and eight consecutive-year droughts. Similarly, the 20th century, with only 9 winter and 12 summer droughts, contained fewer multiyear events than any other century. The uniqueness of the 20th century is further highlighted by the marked occurrence of protracted dual-season pluvials, rather than drought, over this period. Perhaps of greatest relevance, this study suggests that severe and sustained episodes of dual-season drought, which are largely missing from the instrumental period, have occurred multiple times in the past (e.g., 1660s, 1740s, 1890s)."


 


Reconstructed cool- and warm-season precipitation over the tribal lands of northeastern Arizona

Abstract

For over a decade, the Hopi Tribe and Navajo Nation of northeastern Arizona have suffered the effects of persistent drought conditions. Severe dry spells have critically impacted natural ecosystems, water resources, and regional livelihoods including dryland farming and ranching. Drought planning and resource management efforts in the region are based largely on the instrumental climate record, which contains a limited number of severe, sustained droughts. In this study, a new network of moisture-sensitive tree-ring chronologies provides the basis for evaluating the longer-term temporal variability of precipitation in the Four Corners region. By analyzing the earlywood and latewood components within each annual tree ring, we are able to generate separate, centuries-long reconstructions of both cool- (October-April) and warm-season (July-August) precipitation. These proxy records offer new insights into seasonal drought characteristics and indicate that the instrumental record fails to adequately represent precipitation variability over the past 400 years. Through the use of two different analysis techniques, we identify multiyear and decadal-scale drought events more severe than any in the modern era. Furthermore, the reconstructions suggest that many of the historically significant droughts of the past (e.g., 17th century Puebloan drought) were not merely winter phenomena, but persisted through the summer season as well. By comparing these proxy records with historical documents, we are able to independently validate the reconstructions and better understand the socioeconomic and environmental significance of past climate anomalies on the tribal lands of northeastern Arizona.

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