Drought, by Benjamin I. Cook (preface)

DROU G H T An Interdisciplinary Perspective

BENJAMIN I. COOK

Preface

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rought is an extreme event, a relatively rare occurrence that nonetheless can have widespread and devastating consequences for people and ecosystems. Compared to many climate and weather extremes, however, drought events have an impact that may not always be immediate or obvious. A wildfire, for example, can consume a housing development or an entire forest in a matter of hours or days, whereas intense precipitation and floods can destroy a bridge or road in minutes. Droughts, by contrast, are “slow-moving� disasters, with effects that accumulate incrementally over weeks, months, and even years as moisture deficits propagate through ecosystems and the hydrologic cycle. Despite this difference in temporal scale, drought impacts on agricultural productivity, ecosystem health, and even human morbidity make these events among the most expensive and damaging climate extremes in the world.

Indeed, one of the worst natural disasters in the history of the United States was the Dust Bowl drought of the 1930s. This nearly decade-long event devastated the center of the country, created almost unprecedented levels of land degradation and wind erosion, and caused the mass migration of millions of people and the abandonment of thousands of farms (B. Cook et al., 2009; Hansen & Libecap, 2004; Lee & Gill, 2015; Schubert et al., 2004). Further back, history is replete with examples of droughts contributing to the collapse of ancient societies, including the Ancestral Puebloans of vii

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southwestern North America (Benson, Petersen, & Stein, 2007), the Maya in Central America (Medina-Elizalde & Rohling, 2012), and the Angkor in modern-day Cambodia (Buckley et al., 2010). Even modern agricultural and political systems, with all our technological advances, can show remarkable vulnerability to drought. The recent drought in the eastern Mediterranean has been linked to the political instability in the region and has even been raised as a possible contributing factor to the Syrian civil war (Gleick, 2014; Kelley et al., 2015). A drought that only recently ended in California (as of this writing in spring 2018) had devastating consequences for the state’s agriculture, ecosystems, and water resources (He et al., 2017; Howitt et al., 2015). In some cases, drought impacts may even be realized globally. In the early 2000s, for example, a series of contemporaneous regional droughts suppressed vegetation productivity at a magnitude sufficient to cause a global-scale reduction in the terrestrial carbon sink (Zhao & Running, 2010).

Comprehensive discussions of drought dynamics and impacts are often difficult, however, even in the academic literature and especially within the context of climate change. Much of this difficulty arises because of the fundamentally interdisciplinary nature of drought, a phenomenon sitting at the intersection of climatology, meteorology, hydrology, ecology, agronomy, and even economics. Different disciplines will frequently view the same drought from different perspectives, often with little effort made to synthesize understanding of a given event across the entire system. A meteorologist, for example, might investigate the precipitation deficits and associated atmospheric circulation anomalies that caused a drought. Alternatively, a hydrologist might choose to focus on soil moisture or streamflow during the same drought, variables where additional processes (e.g., infiltration, runoff, and evapotranspiration) are important. Neither approach is wrong, per se, but such disciplinary separation can result in biased interpretations and sometimes opposing conclusions about the same event simply because the definition of drought is different in different fields.

As a result, there are no straightforward answers to some of the most pressing questions surrounding drought and climate change: How does this drought compare to droughts in the past? Is climate change making droughts worse? How do we know when a drought is over? Will there be enough water for everyone in the future? My goal with this book is to provide an accessible

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resource to students and researchers across environmental disciplines, showcasing drought as a unique interdisciplinary phenomenon. Because of my own background, much of the focus will be on drought and hydroclimate from a climatological perspective. Included are discussions of how the global climate system shapes patterns of aridity and regional drought variability, a survey of major drought events over the past 12,000 years, and an exploration of the effects of climate change on drought dynamics now and in the future. Beyond this, however, I will explore the impacts of drought on water resources, people, and ecological systems. Thus, I include cases studies of important past events (the Dust Bowl drought of the 1930s and the Sahel drought of the 1970s), an examination of the role of drought in desertification and land degradation, and a review of the development of irrigation and groundwater exploitation as tools to address water shortages.

Given the breadth of the topic and the targeted audience for this book, I will by necessity simplify certain discussions and presentations (e.g., the description of the Hadley circulation) and instead focus on the points most relevant for drought and hydroclimate. Drought, ultimately, is a complex concept that can be defined in a variety of ways. No two droughts are the same across regions or time, and the severity of their impacts, now and in the future, will depend on many different physical, biological, and social factors. This book will capture such diversity and showcase the importance of embracing this broad perspective as we consider how to make sense of droughts in a world that, because of climate change and development, will look very different in the future compared to the past.

Acknowledgments This book is easily the most difficult writing task I have ever undertaken.

(Even my dissertation felt easier.) Few things are quite as intimidating as staring at a blank page and knowing it is up to you to fill those pages with useful insights, edit them down to something approaching sensibility, and decide when it is finally done. It is an immense privilege. Few people have the opportunity and platform to synthesize and share their thoughts and ideas surrounding the topics they have dedicated much of their career to understanding. It is beyond humbling. Also, it’s nice having a place to

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write down all the water cycle numbers that I was tired of looking up in the literature.

Thank you to Patrick Fitzgerald for first approaching me with the idea to write a book on drought, and for remaining endlessly patient as I slogged away at it these last few years. Thank you to Kevin Anchukaitis, Ron Miller, Michael Puma, Richard Seager, Jason Smerdon, Park Williams, Lizzie Wolkovich, and all my other collaborators past and present who have made me a better scientist. Thank you as well to Michael Mann, Howie Epstein, and Paolo D’Odorico for taking a chance on me at the University of Virginia when nobody else would.

And thank you to my parents, Rori, and Luca for their endless love and patience.

—BRIAN WARDLOW, director and professor, Center for Advanced Land Management Information Technologies and the School of Natural Resources, University of Nebraska-Lincoln

—JONATHAN T. OVERPECK, William B. Stapp Collegiate Professor and Samuel A. Graham Dean of the University of Michigan’s School for Environment and Sustainability

“A first-principles, comprehensive, and up-to-date exposition of drought, including its drivers and consequences, by a major player working at the cutting edge of interdisciplinary science. Drought is perfectly organized, written, and illustrated, with the early chapters on hydrology and climate laying the needed groundwork for the reader to truly appreciate the later chapters on the history and future of drought and its impacts. In my estimation, this is easily the most important and useful book ever published on the phenomenon of drought.”

DROUGHT An Interdisciplinary Perspective

An Interdisciplinary Perspective

“Drought, aridity, and hydroclimatic stress are major concerns worldwide, and climate change is already making the situation worse. This book provides a foundation that many—whether interested in the basic science, the human impacts, or the impacts on natural systems—will find useful. Rarely are relevant insights from the recent geologic past woven together so well with knowledge gained from the instrumental and satellite era to illuminate the challenges that lie ahead. The evidence provided in this book highlights how serious the threat to both humans and nature will be. A must-read.”

DROUG H T

“This book presents an interesting, multidisciplinary perspective on the various dimensions of drought, which is a complex natural hazard of global importance.”

COOK

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n this innovative book, Benjamin I. Cook brings together climate science, hydrology, and ecology to provide a synthetic overview of drought and its environmental and social consequences. A complex phenomenon at the intersection of a range of scientific disciplines and public policy issues, drought is one of the many problems anthropogenic climate change may exacerbate. A scientifically comprehensive and approachable overview of water issues throughout the world, Drought is an essential text for a broad range of students in earth science and environmental and sustainability studies.

BENJAMIN I. COOK is a research scientist at the NASA Goddard Institute for Space Studies and at the Lamont-Doherty Earth Observatory of Columbia University. He also teaches on drought and climate science in Columbia’s School of Professional Studies.

Cover design: Milenda Nan Ok Lee Cover photo: Scott London / © Alamy

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—JULIO L. BETANCOURT, visiting scientist, Earth System Science Interdisciplinary Center

BENJAMIN I. COOK