Assessment of Climate Change in the Southwest United States: A Report Prepared for the National Climate Assessment

Assessment of Climate Change in the Southwest United States: A Report Prepared for the National Climate Assessment


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Product Details

ISBN-13: 9781610914468
Publisher: Island Press
Publication date: 05/09/2013
Series: NCA Regional Input Reports Series
Pages: 528
Product dimensions: 7.90(w) x 9.80(h) x 1.20(d)

About the Author

Gregg Garfin is Deputy Director for Science Translation & Outreach, Institute of the Environment, and Assistant Professor in the School of Natural Resources and the Environment at the University of Arizona. Angela Jardine has been an environmental scientist for over ten years. She is currently at the Amazon rainforest at the Instituto Nacional de Pesquisas da Amazônia in Manaus, Brazil. Robert Merideth is editor in chief and a senior researcher at the Udall Center for Studies in Public Policy at the University of Arizona. Mary Black has worked as an editor, writer, anthropological linguist, and librarian for the University of Arizona. She currently serves as a liaison with federal agencies, tribes, and scientists. Sarah LeRoy is a Research and Outreach Scientist for the Institute of the Environment at the University of Arizona where she is the editor for the Southwest Climate Change Network.

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Assessment of Climate Change in the Southwest United States

A Report Prepared for the National Climate Assessment

By Gregg Garfin, Angela Jardine, Robert Merideth, Mary Black, Sarah LeRoy


Copyright © 2013 Institute of the Environment
All rights reserved.
ISBN: 978-1-61091-446-8


Summary for Decision Makers

1.1 Introduction

Natural climate variability is a prominent factor that affects many aspects of life, livelihoods, landscapes, and decision-making across the Southwestern U.S. (Arizona, California, Colorado, Nevada, New Mexico, and Utah; included are the adjacent United States-Mexico border and Southwest Native Nations land). These natural fluctuations have caused droughts, floods, heat waves, cold snaps, heavy snow falls, severe winds, intense storms, the battering of coastal areas, and acute air-quality conditions. And as a region that has experienced—within the relatively short time span of several decades—rapid increases in human population (Figure 1.1), significant alterations in land use and land cover, limits on the supplies of water, long-term drought, and other climatic changes, the Southwest can be considered to be one of the most "climate-challenged" regions of North America. This document summarizes current understanding of climate variability, climate change, climate impacts, and possible solution choices for the climate challenge, all issues that are covered in greater depth in Assessment of Climate Change in the Southwest United States.

The juxtaposition of the Southwest's many landscapes—mountains, valleys, plateaus, canyons, and plains—affect both the region's climate and its response to climate change. Whether human and natural systems are able to adapt to changes in climate will be influenced by many factors, including the complex topographic pattern of land ownership and the associated policies and management goals. Moreover, the human population in the region will likely grow, primarily in urban areas, from a population of about 56 million in 2010 to an estimated 94 million by 2050 (Figure 1.1). [Chapter 3]

The Southwest climate is highly variable across space and over time related to such factors as ocean-land contrasts, mountains and valleys, the position of jet streams, the North American monsoon, and proximity to the Pacific Ocean, Gulf of California, and Gulf of Mexico. The Mojave and Sonoran Deserts of Southern California, Nevada, and Arizona are the hottest (based on July maximum temperatures), driest regions of the contiguous United States. Coastal zones of California and northwestern Mexico have large temperature gradients and other properties from the shore to inland. Mountain regions are much cooler and usually much wetter regions of the Southwest, with the Sierra Nevada and mountains of Utah and Colorado receiving nearly half of their annual precipitation in the form of snow. The resulting mountain snowpack provides much of the surface water for the region, in the form of spring runoff. [Chapter 4]

There is mounting scientific evidence that climate is changing and will continue to change. There is also considerable agreement—at varying levels of confidence sufficient to support decision making—regarding why the climate is changing, or will change [Chapter 19]. Readers of this summary may wish to review all or parts of the complete report, Assessment of Climate Change in the Southwest United States, to learn more about the region's climate, and its likely changes and effects.

1.2 Observed Recent Climatic Change in the Southwest

The climate of the Southwest is already changing in ways that can be attributed to human-caused emissions of greenhouse gases, or that are outcomes or expressions consistent with such emissions—with these notable observations:

The Southwest is warming. Average daily temperatures for the 2001–2010 decade were the highest (Figure 1.2) in the Southwest from 1901 through 2010. Fewer cold waves and more heat waves occurred over the Southwest during 2001–2010 compared to average decadal occurrences in the twentieth century. The period since 1950 has been warmer than any period of comparable length in at least 600 years, as estimated on the basis of paleoclimatic tree-ring reconstructions of past temperatures. [Chapter 5]

Recent drought has been unusually severe relative to droughts of the last century, but some droughts in the paleoclimate record were much more severe. The areal extent of drought over the Southwest during 2001–2010 was the second largest observed for any decade from 1901 to 2010. However, the most severe and sustained droughts during 1901–2010 were exceeded in severity and duration by multiple drought events in the preceding 2,000 years (Figure 1.3). [Chapter 5]

Recent flows in the four major drainage basins of the Southwest have been lower than their twentieth century averages. Streamflow totals in the Sacramento-San Joaquin Rivers, Upper Colorado, Rio Grande, and Great Basin were 5% to 37% lower during 2001–2010 than their twentieth century average flows. Moreover, streamflow and snowmelt in many snowmelt-fed streams of the Southwest tended to arrive earlier in the year during the late twentieth century than earlier in the twentieth century and up to 60% of the change in arrival time has been attributed to increasing greenhouse-gas concentrations in the atmosphere (Figure 1.4). [Chapter 5]

1.3 Projected Future Climatic Change in the Southwest

Climate scientists have high confidence that the climate of the Southwest will continue to change through the twenty-first century and beyond, in response to human-generated greenhouse gas emissions, and will continue to vary in ways that can be observed in historic and paleoclimate records (Table 1.1). However, not all aspects of the climate change or variation can be projected with equal confidence. The highest confidence is associated with projections that are consistent among climate models and with observed changes, such as those described in the previous section. The magnitude and duration of future change depends most on the amount of greenhouse gases emitted to the atmosphere, particularly carbon dioxide emitted by the burning of coal, oil, and natural gas. Much of the future change will be irreversible for centuries after substantial anthropogenic carbon dioxide emissions have ceased.

Warming will continue, with longer and hotter heat waves in summer. Surface temperatures in the Southwest will continue to increase substantially over the twenty-first century (high confidence), with more warming in summer and fallthan winter and spring (medium-high confidence) (Figures 1.5 and 1.6). Summer heat waves will become longer and hotter (high confidence). Winter cold snaps will become less frequent but not necessarily less severe (medium-high confidence). [Chapter 6 and 7]

Average precipitation will decrease in the southern Southwest and perhaps increase in northern Southwest. Precipitation will decline in the southern portion of the Southwest region, and change little or increase in the northern portion (medium-low confidence) (Figure 1.6). [Chapter 6]

Precipitation extremes in winter will become more frequent and more intense (i.e., more precipitation per hour) (medium-high confidence). Precipitation extremes in summer have not been adequately studied. [Chapter 7]

Late-season snowpack will continue to decrease.Late winter-spring mountain snowpack in the Southwest will continue to decline over the twenty-first century, mostly because temperature will increase (high confidence) (Figure 1.7). [Chapter 6]

Declines in river flow and soil moisture will continue. Substantial portions of the Southwest will experience reductions in runoff, streamflow, and soil moisture in the mid- to late-twenty-first century (medium-high confidence) (Figure 1.7). [Chapter 6]

Flooding will become more frequent and intense in some seasons and some parts of the Southwest, and less frequent and intense in other seasons and locations. More frequent and intense flooding in winter is projected for the western slopes of the Sierra Nevada (medium-high confidence), whereas snowmelt-driven spring and summer flooding could diminish in that mountain range (high confidence). [Chapter 7]

Droughts in parts of the Southwest will become hotter, more severe, and more frequent (high confidence). Drought, as defined by Colorado River flow amount, is projected to become more frequent, more intense, and more prolonged, resulting in water deficits in excess of those during the last 110 years (high confidence). However, northern Sierra Nevada watersheds may become wetter with climate change (low confidence). [Chapter 7]

1.4 Recent and Future Effects of Climatic Change in the Southwest

Terrestrial and freshwater ecosystems

Natural ecosystems are being affected by climate change in noticeable ways, which may lead to their inhabitants needing to adapt, change, or move:

The distributions of plant and animal species will be affected by climate change. Observed changes in climate are associated strongly with some observed changes in geographic distributions of species in the Southwest (high confidence). [Chapter 8]

Ecosystem function and the functional roles of resident species will be affected. Observed changes in climate are associated strongly with some observed changes in the timing of seasonal events in the life cycles of species in the region (high confidence). [Chapter 8]

Changes in land cover will be substantial. Observed changes in climate are affecting vegetation and ecosystem disturbance (Figure 1.8). Among those disturbances are increases in wildfire and outbreak of forest pests and disease. Death of plants in some areas of the Southwest also is associated with increases in temperature and decreases in precipitation (high confidence). [Chapter 8]

Climate change will affect ecosystems on the U.S.-Mexico border. Potential changes to ecosystems that transect the international border are often not explicitly considered in the public policy exposing these sensitive ecosystems to climate change impacts (high confidence). [Chapter 16]

Coastal systems

Coastal California is already being affected by climate change, and future climate-related change will become more notable if greenhouse-gas emissions are not substantially reduced:

Coastal hazards, including coastal erosion, flooding, storm surges and other changes to the shoreline will increase in magnitude as sea level continues to rise (high confidence). Sea levels along the California coast have risen less than a foot since 1900, but could rise another two feet (high confidence), three feet (medium-high confidence), or possibly more (medium-low confidence) by the end of the twenty-first century (Figure 1.9). [Chapter 9]

Effects of coastal storms will increase. Increased intensity (medium- low confidence) and frequency (medium-low confidence) of storm events will further change shorelines, near-shore ecosystems, and runoff. In many regions along the coast, storms coupled with rising sea levels will increase the exposure to waves and storm surges (medium-high confidence). [Chapter 9]

Economic effects of coastal climate change will be large. Between 2050 and 2100, or when sea levels are approximately 14–16 inches higher than in 2000, the combined effects of sea-level rise and large waves will result in property damage, erosion, and economic losses far greater than currently experienced (high confidence). [Chapter 9]

Coastal ecosystems and their benefits to society will be affected. Ocean warming, reduced oxygen content, and sea-level rise will affect marine ecosystems, abundances of fishes, wetlands, and coastal communities (medium-high confidence). However, there is uncertainty in how and by how much coastal ecosystems will be affected. [Chapter 9]

Ocean acidification is taking place. Many marine ecosystems will be negatively affected by ocean acidification that is driven by increased levels of atmospheric carbon dioxide (high confidence). But there is substantial uncertainty about the effects of acidification on specific coastal fisheries and marine food webs. [Chapter 9]


Water is the limiting resource in the Southwest, and climate variability and change will continue to have substantial effects on water across much of the region. Reduction in water supplies can lead to undesirable changes in almost all human and natural systems including agriculture, energy, industry forestry, and recreation. In particular:

Climate change could further limit water availability in much of the Southwest. A large portion of the Southwest, including most of the region's major river systems (e.g., Rio Grande, Colorado, and San Joaquin), is expected to experience reductions in streamflows and other limitations on water availability in the twenty-first century (medium-high confidence) (Figure 1.7). [Chapters 5, 6, 7, and 10]

Water availability could be decreased even more by unusually warm, decadeslong periods of drought. Much of the Southwest, including major river systems such as the Colorado and Rio Grande, has experienced decades-long drought repeatedly over the last 1,000 to 2,000 years. Similar exceptional droughts could occur in the future, but temperatures are expected to be substantially hotter than in the past (high confidence) (Figure 1.3). [Chapters 5, 6, 7, and 10]

The past will no longer provide an adequate guide to project the future. Twentieth-century water management has traditionally been based in part on the principle of "stationarity," which assumes that future climate variations are similar to past variations. As climate changes, temperature will increase substantially and some areas of the Southwest will become more arid than in the past (high confidence). [Chapters 6 and 10]

Surface water quality will be affected by climate change. In some areas, surface water quality will be affected by scarcity of water, higher rates of evaporation, higher runoff due to increased precipitation intensity flooding, and wildfire (high confidence). [Chapter 10]

Human health

The Southwest's highly complex and often extreme geography and climate increase the probability that climate change will affect public health. Several potential drivers of increased health risk exist only or primarily in the Southwest, and there is substantial variation in the sensitivity exposure, and adaptive capacity of individuals and groups of people within the Southwest to climate change-related increases in health risks:

Climate change will drive a wide range of changes in illness and mortality. In particular, climate change will exacerbate heat-related human morbidity and mortality and lead to increased concentrations of airborne particulates and pollutants from wildfires and dust storms. Climate change may affect the extent to which organisms such as mosquitoes and rodents can carry pathogens (e.g., bacteria and viruses) and transmit disease from one host to another (medium-high confidence). [Chapter 15]

Allergies and asthma will increase in some areas. On the basis of data showing earlier and longer spring flower bloom, allergies and asthma may worsen for individual sufferers or become more widespread through the human population as temperature increases (medium-low confidence). [Chapter 15]

Disadvantaged populations will probably suffer most. The health of individuals who are elderly infirm, or economically disadvantaged is expected to decrease disproportionately to that of the general population (high confidence), due to their increased exposure to extreme heat and other climate hazards. [Chapter 15]

Additional effects of climate change

Climate change has the potential to affect many other sectors and populations within the Southwest. For example:

Agriculture will be affected by climate change. Effects of climate change and associated variability on production of both crops and livestock could be long-lasting, with short-term reductions in profitability (medium-low confidence). [Chapter 11]

Energy supplies will become less reliable as climate changes and climate change will drive increasing energy demand in some areas. Delivery of electricity may become more vulnerable to disruption due to extreme heat and drought events that increase demand for home and commercial cooling, reduce thermal power plant efficiency or ability to operate, reduce hydropower production, or reduce or disrupt transmission of energy (medium-high confidence) (Figure 1.10). [Chapter 12]

Climate change will affect urban areas in differing ways depending on their locations and on their response or adaptive capacities. Climate change will affect cities in the Southwest in different ways depending on their geographic locations. Local capacity to address effects of climate change will also vary depending on governmental, institutional, and fiscal factors. Incidences of air pollution related to increased heat are likely to increase, and water supplies will become less reliable (medium-high confidence). [Chapter 13]

Reliability of transportation systems will decrease. Climate change will affect transportation systems in different ways depending on their geographic location (e.g., changing sea level and storm surge affect coastal roads and airports), potentially impeding the movement of passengers and goods (medium-high confidence). [Chapter 14]

Climate change may disproportionately affect human populations along the U.S.-Mexico border.Climate changes will stress on already severely limited water systems, reducing the reliability of energy infrastructure, agricultural production, food security and ability to maintain traditional ways of life in the border region (medium-high confidence). [Chapter 16]


Excerpted from Assessment of Climate Change in the Southwest United States by Gregg Garfin, Angela Jardine, Robert Merideth, Mary Black, Sarah LeRoy. Copyright © 2013 Institute of the Environment. Excerpted by permission of ISLAND PRESS.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents

Chapter 1. Summary for Decision Makers
-Observed Recent Climatic Change in the Southwest
-Projected Future Climatic Change in the Southwest
-Recent and Future Effects of Climatic Change in the Southwest  
-Choices for Adjusting to Climate and Climate Change
-Key Unknowns 
PART I. Context
Chapter 2. Overview
-Context and Scope
-Other Southwest Region Climate Assessments
-Sponsors and Authors of the 2012 SRCA
-Characterizing and Communicating Uncertainty
-Accountability and Review
-Organization of this Report
Chapter 3. The Changing Southwest
-Lay of the Land: Geographical Themes and Features
-Land Use and Land Cover
PART II. Weather and Climate of the Southwest
Chapter 4. Present Weather and Climate: Average Conditions
-General Climate Characteristics
-Major Climate and Weather Events
-Climate Variability
Chapter 5. Present Weather and Climate: Evolving Conditions
-Climate of the First Decade of the Twenty-first Century 
-Climate Trends for 1901-2010
-Extreme Weather Variability During 1901-2010
-Summertime Drought During 1901-2010
-Hydroclimatic Variability During 1901-2010
-Paleoclimate of the Southwest United States
-Future Monitoring and Science Needs
Chapter 6. Future Climate: Projected Average
-Global Climate Models: Statistical and Dynamical Downscaling
-Climate Scenarios
-Data Sources
-Temperature Projections
-Projections of Other Temperature Variables
-Precipitation Projections
-Atmospheric Circulation Changes
-North American Monsoon
-Changes in Precipitation-related Measures
Chapter 7. Future Climate: Projected Extremes 
-Heat Waves
-Wintertime Cold Outbreaks
-Surface Hydrology
-Fire Weather
-Discussion and Key Uncertainties
PART III. Effects of Climate Change in the Southwest
Chapter 8. Natural Ecosystems
-Introduction: Climate, Climate Change, and Ecosystems of the Southwest
-Phenology and Species Interactions
-Southwestern Deserts
-Southwestern Forests
Chapter 9. Coastal Issues
-Coastal Assets
-Observed Threats
-Ocean and Coastal Impacts to Ecosystems
-Coastal Impacts to Communities
-Managing Coastal Climate Risks
Chapter 10. Water: Impacts, Risks, and Adaptation
-Physical Changes to the Water Cycle
-Human and Natural Systems Impacts, Risks and Vulnerabilities
-Water Sector Adaptation Activities
-Planning Techniques and Stationarity
-Potential Supply and Demand Strategies and Solutions
-Barriers to Climate Change Adaptation
-Federal Adaptation Initiatives
-SECURE Water Act Overview
-Western States Federal Agency Support Team (WestFAST)
-Climate Change and Water Working Group (CCAWWG)
-State Adaptation Efforts
-Regional and Municipal Adaptation Efforts
Chapter 11. Agriculture and Ranching
-Distinctive Features of Southwestern Agriculture
-Implications for Specialty Crops
-On-farm Water Management
-System-wide Water Management: Lessons from Programming Models of  Water Allocation
-Ranching Adaptations to Multi-year Drought
-Disaster Relief Programs and Climate Adaptation
Chapter 12. Energy: Supply, Demand, and Impacts
-Energy in the Southwest: Past and Present
-Potential Climate Impacts on Energy
Chapter 13. Urban Areas
-Cities in the Southwest
-Pathways Through which Climate Change Will Affect Cities in the Southwest
-Critical Missing Data and Monitoring in Cities
Chapter 14. Transportation
-Passenger Transportation Trends in the Southwest
-Freight Movement in the Southwest
-Impacts of Climate Change
-Major Vulnerabilities and Uncertainties
Chapter 15. Human Health
-Current Climate-Related Health Concerns in the Southwest
-Climate Change and Potential Health Implications 
-Observed and Predicted Effects on Health from Climate Change
-Public Health Planning for Climate Change
Chapter 16. Climate Change and U.S.-Mexico Border Communities
-Definition of the Border Region
-Border Region Climate Variability, Climate Change, and Impacts
-Understanding Vulnerability, Risk, and Adaptive Capacity in the Border  Region
-Sectoral Analysis of Border Vulnerability
Chapter 17. Unique Challenges Facing Southwestern Tribes
-The Effects of Marginal Living Conditions and Extreme Climatic  Environments
-Current Impacts on Native lands
-Potential Rangeland Impacts
-Adaptation Strategies and Adaptation Planning
-Challenges for Adaptation Planning
-Vulnerability from Economic, Political, and Legal Stresses
- Climate Change Mitigation Strategies
PART IV. Options and Research Needs
Chapter 18. Climate Choices for a Sustainable Southwest
-Defining a Sustainable Approach to Climate Change in the Southwest
-Making a Sustainable Living in the Southwest: Lessons from History
-Limiting Emissions in the Southwest
-Adaptation Options in the Southwest
-Linking Mitigation and Adaptation
-Barriers to Planning for and Implementing Climate Solutions
-Coping with the Risks of Rapid Climate Changes
-Research Gaps
Chapter 19. Moving Forward with Imperfect Information
-Uncertainty Typologies
-Confidence and Uncertainty
-What Is Known and Not Known About Climate in the Southwest
-Moving Forward
Chapter 20. Research Strategies for Addressing Uncertainties
-Developing Research Strategies from Information Needs
-Research Strategies Derived from the Southwest Climate Assessment
-Research Strategies from Southwestern Ecoregional Initiatives
-Strategies to Improve Characterization in Climate and Hydrology
-Strategies to Improve Characterization of Impacts and Vulnerabilities

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