Increasing prevalence of hot drought across western North America since the 16th century

Abstract

Across western North America (WNA), 20th-21st century anthropogenic warming has increased the prevalence and severity of concurrent drought and heat events, also termed hot droughts. However, the lack of independent spatial reconstructions of both soil moisture and temperature limits the potential to identify these events in the past and to place them in a long-term context. We develop the Western North American Temperature Atlas (WNATA), a data-independent 0.5° gridded reconstruction of summer maximum temperatures back to the 16th century. Our evaluation of the WNATA with existing hydroclimate reconstructions reveals an increasing association between maximum temperature and drought severity in recent decades, relative to the past five centuries. The synthesis of these paleo-reconstructions indicates that the amplification of the modern WNA megadrought by increased temperatures and the frequency and spatial extent of compound hot and dry conditions in the 21st century are likely unprecedented since at least the 16th century.

Placing 21st Century Warming in Southern California, USA in a Multi-Century Historical Context

“…Exceptional warming in western North America has been linked to a combinat natural variability and anthropogenic forcing [1,18,28,[53][54][55], and many of these fa likely contributed to the temperature variability (Figure 3) at MSJSP, but at differen poral scales. For natural forcing, we found that during 1960-2020, late spring (April-PDO conditions were positively associated with instrumental Tmin (r = 0.54, p < Instrumental Tmin also is positively associated (r = 0.43, p < 0.05) with ENSO cond during April-July.…”

“…Summertime (June-August) temperatures in the southwestern US during the first two decades of the 21st century have been unmatched since at least the mid-1500s and are likely a consequence of anthropogenic warming [1]. The human and environmental consequences of increasing temperatures in this region are broad and ongoing and include changes to fire activity, snowpack and water resources, growing season length, and drought frequency.…”

“…Warming associated with greenhouse gas emissions is recognized as a significant contributor to drought conditions in the southwestern United States, with terms such as "globalchange-type-drought" [15], "anthropogenic drought" [16], and "hot drought" [1,17,18] Atmosphere 2024, 15, 649 2 of 9 frequently used in reference to anthropogenic changes to drought climatology. However, 21st century warming in southern California is likely associated with both natural and anthropogenic forcing mechanisms, and dissociating these mechanisms is problematic because of their interconnectedness.…”

“…Numerous tree ring reconstructions have been conducted in the southwestern United States using variable month/monthly combinations to identify annual temperature. These studies include reconstructions of mean [25,26] and maximum temperatures [1]. Additionally, investigations in precipitation patterns [27] and soil moisture indices [28][29][30] in the western United States also have been explored.…”

“…Moreover, these studies did not focus on the examination of minimum temperature. Because of this gap in research examining radial growth and minimum temperature in the southwest mountains of California, our ability to fully grasp the interplay between 21st century anthropogenic warming [1,18] and ecosystem dynamics of mountainous regions in southern California is limited. In this study, we explore the historical temperature variations of southern California, particularly focusing on the region's high-elevation old-growth forests within Mt.…”