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An Information Statement of the American Meteorological Society
The American Meteorological Society recommends that appropriate institutions expand drought planning, preparedness, research, coordination, early warning, and communication efforts at the local, state, regional, tribal, federal, and international levels. Although we have made considerable progress in understanding and monitoring drought, notable gaps remain despite advancing knowledge and information services about climate variability, drought impacts, mitigation technologies, and societal response such as conservation and preparedness strategies. This statement describes drought and its challenges, centering on concerns in the United States while addressing underlying issues that are relevant globally.
Not to be confused with aridity, which is a permanent feature of some regional climates, drought is a natural temporary feature of the climate cycle that quietly wreaks havoc in most regions of the globe. It is the unavoidable result of our climate’s variability — variability that can impair or deprive local-to-national water supplies for weeks to years at a time. Drought is more than just a simple moisture deficit, however. It is the result of a complex interplay between (1) natural precipitation deficiencies, or excessive evapotranspiration over varying time periods and different areal extents, and (2) the demands of human and environmental water use that may be exacerbated by inefficiencies in water distribution, planning, and management. Drought affects various sectors of society and the natural environment in multifaceted ways, and thus has many definitions and metrics. Yet it is the most far-reaching climate-related disaster each year, causing hardship to millions of people. According to the National Climatic Data Center’s “Billion Dollar U.S. Weather Disasters,” economic losses from drought account for roughly 24 percent of all losses from major weather events, including floods, hurricanes, and severe storms. This impact is on par, per event, with hurricanes as the most costly of natural hazards in the United States. Drought leads to famine, death, and political unrest in many parts of the world, and greatly affects prosperity and quality of life in the United States.
Drought as a Natural Disaster
Most natural hazards are relatively quick-striking and immediately detectable events that cause structural damage and human injury. Tornadoes, for instance, evolve, are observed, and dissipate on timescales of minutes or hours. In contrast, drought exhibits a longer prologue — a gradual accumulation of deficits in precipitation and water supply followed by a trail of impacts in various economic sectors. These impacts may linger well after the driving physical aspects of the drought have abated, cascading through water management infrastructure or via persistence in the land-surface water balance. Drought-related agricultural and wildfire impacts can be swift and severe, whereas other impacts such as soil erosion and desertification typically take place more gradually. Surface and groundwater shortages that accumulate gradually can nonetheless cause sudden and profound impacts when water levels drop below critical thresholds. With exceptions such as wildfire impacts, droughts typically do not cause large-scale damage to built structures, although it is certainly possible with regard to foundations and infrastructure like water and/or other utility lines and roads due to the settling of contracting soils. For these reasons, the characterization of drought impacts and provision of disaster relief is a complex task.
Drought Types and Definitions
To facilitate communication, management, and response, drought often is categorized into four general types: 1) meteorological or climatological, 2) agricultural, 3) hydrological, and 4) socioeconomic. The first three types are defined by physical, hydrometeorological, or biological parameters, while the fourth centers on the impacts of drought on society. Meteorological or climatological drought is defined simply in terms of the magnitude and duration of a precipitation shortfall. Agricultural drought links meteorological drought characteristics to agricultural impacts, associating precipitation shortages most immediately with higher evapotranspiration levels and soil moisture deficits. Agricultural drought affects both irrigated and dryland crop production, as well as livestock industries that rely on nonirrigated pastures or surface runoff. Hydrological droughts arise from precipitation shortfalls that deplete surface or subsurface water supplies and, like agricultural droughts, can be exacerbated by anomalies in other meteorological variables that affect evapotranspiration (e.g., temperature, humidity, wind). The phenomena associated with and impacts of hydrological drought often lag behind those of meteorological droughts, but may precede those of agricultural droughts, such as where irrigation that depends on hydrologic runoff is necessary for crop growth. Socioeconomic drought is driven by imbalances in supply and demand of economic goods due to the physical characteristics of drought. Economic impacts include both direct effects, such as lost income from crop reduction, and secondary effects such as resulting reduced spending in rural communities. Social impacts can be health-related (physical and mental) and in some cases involve mass migration. These considerations argue for expanding the time periods and spatial scales that define drought, to account comprehensively for the ripple of geophysical effects through social and economic systems.
Characteristics of Drought
Characteristics of drought include impacts, intensity, duration, spatial extent, and timing. Intensity commonly refers to the severity of the precipitation deficit and how quickly it develops. Magnitude accounts for the combination of a drought’s intensity and duration. Each drought is unique, but common features of the most severe droughts that have far-reaching human and ecological impacts include long duration, large moisture deficits, and large areal extent, particularly when these impacts occur during a climatological wet season.
Drought impacts tend to follow predictable progressions that vary as a function of societal wealth and economic activity. In less developed regions of the world, the primary impacts are crop failures followed by food insecurity, clean drinking water shortages, and eventually water-related health problems, famine, energy shortages, mass migrations, and political unrest. In developed nations, food shortages and severe health hazards are rare; rather, impacts often are socioeconomic, including reduced crop production, fire, shellfish industry losses, higher costs for consumables (e.g., food, energy, transportation), reduced recreational opportunities, and domestic and industrial water restrictions. Lack of water can play a detrimental role in energy resource development and leads to reduced hydropower generation and lack of power-plant cooling capacity. It is also important to note that not all impacts are negative, as drought plays a critical role in sustaining ecosystem health for many regions around the world. In addition, many impacts are positive for certain sectors that rely on good weather for their productivity, such as construction or tourism.
Ecological impacts can be substantial but difficult to quantify. For example, the scope and structural nature of wildfire destruction may be assessed directly, but the alteration of watershed functioning is less readily measured. Similarly, the areal extent of coastal wetland loss due to saltwater intrusion may be measured, but other indirect consequences often are less obvious. For example, more interdisciplinary research and use of varied assessment techniques will be needed to identify the links between the wetland loss and reduced stream flows, increased water temperature, tree mortality, and impacts on native species through forage and water depletion, and spread of disease.
In the future, as now, drought impacts will be determined not only by the frequency and intensity of drought, but also by the number of people at risk, their degree of risk, and the resiliency of natural systems. Degree of risk is a function of exposure, vulnerability, and response. Where demand for water and other shared natural resources increases — driven in part by population growth in drought-prone areas, urbanization, government policies, land use changes, and other factors — future droughts are likely to produce increasingly negative impacts even without any physical changes in drought frequency or intensity.
Climate changes can shift expected patterns of hydrometeorological forcings on local to regional scales and thereby alter the nation's vulnerability to drought extremes. The 2012 AMS Information Statement on Climate Change attributes the “unequivocal” observed warming of the climate system and “inevitable” future warming to past and present human influences on the climate system. The observed impact of climate- system changes on droughts varies regionally, depending on changes to the drivers of drought such as precipitation and temperature and on regional features of the hydroclimatological system. Some drought-related consequences are well established, such as the reduction of dry-season river flow due to temperature-related reductions in seasonal snowpacks and glacier extent. Others are less certain, particularly where potential atmospheric moisture increases or rainfall pattern changes may balance increased evaporative losses or water demands. The IPCC Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX) asserts “medium confidence that some regions of the world have experienced more intense and longer droughts … but in some regions droughts have become less frequent, less intense, or shorter.” The US Global Change Research Program assessment, 2009 Global Climate Change Impacts in the United States, recognizes that climate change adds uncertainty to existing water management, design, and planning challenges, because it undermines the traditional assumption that observed hydrology and climate from the instrumental record are representative of the future. This added uncertainty may also increase our nation’s vulnerability to drought extremes.
Routine operational monitoring of all components of the hydrologic cycle is needed for quantifying drought and responding to impacts. A comprehensive perspective of drought includes climate variables, streamflow, groundwater and reservoir levels, snowpack, soil moisture, temperature, evapotranspiration, and vegetation conditions. Some of these variables, such as vegetation conditions, snow extent, and (in some areas) soil moisture, can now be monitored using satellite-based imagery. In recent decades, our ability to monitor, synthesize, and disseminate critical drought-related information also has been greatly advanced by use of automated weather stations, enhanced radar, supercomputers, computer mapping systems, land data assimilation systems, and improved communication and information delivery techniques. The latter include Drought Early Warning Systems (DEWS) via the US Drought Monitor (USDM), and the National Integrated Drought Information System (NIDIS). In addition, support of our nation’s monitoring networks, particularly our volunteer in situ observing network, is critical and essential as they serve as the historical backbone of our climate record, which gives us the proper historical framework from which decisions are made.
Prediction and Warning
Extensive research during the past two decades clearly indicates the important influence of large-scale sea surface temperature (SST) variations on drought occurrence. SST anomaly patterns associated with the El Niño–Southern Oscillation provide the major forcing of interannual regional climate variations in many areas. Furthermore, recent research has shown that the Pacific Decadal Oscillation, Atlantic Multidecadal Oscillation, and North Atlantic Oscillation are associated with decadal-scale climate-system variability that promotes persistent, multiyear drought in some regions of the U.S. The effects of these ocean variations are transmitted to remote areas of the globe through recurrent but seasonally varying patterns of atmospheric circulation anomalies referred to as teleconnections. Teleconnections affect tropical precipitation regimes as well as large areas of the extratropics, including southern Australia, eastern Asia, parts of sub-Saharan Africa, and regions of both North and South America.
Operational coupled climate forecast models that simulate the interaction of the ocean, land, cryosphere, and atmosphere strive to capture the dynamical evolution of large-scale SST patterns and their associated teleconnections, affording useful predictability for some regions/seasons. Current operational climate forecasting offers seasonal predictions with up to a year of lead time, showing skill that is generally greater for temperature than for precipitation and that varies greatly by region and season. On a regional scale, model experiments indicate that drought conditions themselves, particularly in large multistate United States droughts, can help to perpetuate drought through reinforcing interactions between the dry land surface and the overlying warm atmosphere.
At present, the best approach for predicting the development, intensification, and demise of a drought is a two-fold strategy that combines the monitoring of both local water and climate conditions and large-scale wind patterns, including the comparison of current conditions to historical analogues, with the interpretation of computer forecasts. This strategy is employed by both the Monthly and Seasonal Drought Outlooks, which are issued monthly by the National Oceanic and Atmospheric Administration/National Weather Service/Climate Prediction Center as an operational effort geared toward infusing such advances into drought predictability. Although predicting drought on any scale remains a challenge, progress in understanding global-to-regional scale climate-system phenomena provides hope for improving drought prediction at longer lead times.
Early warning of drought onset, and characterization of its evolving environmental and economic impacts, can be further enhanced by the use of regional-scale early warning systems that promote sustained partnership networks linking meteorological and climatological information providers to water, agriculture, and other private and public management communities.
The hydrometeorological aspects of drought are unavoidable and often unpredictable, but the risks of socioeconomic drought impacts can be reduced through preparedness plans and proactive state and national drought policies. Such plans and policies can improve the coping capacities of governments on the local, state, tribal, and national scales, as well as those of foreign governments, and help reduce their associated costs. Since 1982, the number of states with drought plans has increased from 3 to 47, with a recent emphasis on proactive mitigation techniques. Most plans, however, still focus primarily on response (after the fact), limiting our country’s ability to proactively deal with drought in advance. Drought plans should include both tactical steps (such as the development of prediction-based triggers to conserve water in the face of elevated drought risk) and strategic measures (such as the institution of responsible water management). Tactical steps are supported by DEWS, which provide integrated monitoring and delivery systems for timely coordination, communication, and distribution of information to decision makers and the public. DEWS help to sharpen drought assessment methodologies by documenting drought impacts, and also provide a foundation for interpreting drought events against the backdrop of past extremes and longer-term climate change. Today’s policies that promote the development of regionally appropriate drought monitoring, prediction, and mitigation procedures have an important strategic component — they will help reduce the future costs and associated hardships of drought and provide resiliency in the face of changes in climate and other factors that may alter drought characteristics in the future.
[This statement is considered in force until September 2019 unless superseded by a new statement issued by the AMS Council before this date.]