This report discusses how drought is defined (e.g., why drought in one region of the country is different from drought in another region), and why drought occurs in the United States. How droughts are classified, and what is meant by moderate, severe, and extreme drought classifications, are also discussed. The report briefly describes periods of drought in the country's past that equaled or exceeded drought conditions experienced during the 20 th century. This is followed by a discussion of the future prospects for a climate in the western United States that might be drier than the average 20 th -century climate. The report concludes with a primer on policy challenges for Congress, such as the existing federal/nonfederal split in drought response and management and the patchwork of drought programs subject to oversight by multiple congressional committees. Following are brief answers to frequently asked questions related to drought. What is drought? Drought is commonly defined as a lack of precipitation over an extended period, usually a season or more, relative to some long-term average condition. History suggests that severe and extended droughts are inevitable and part of natural climate cycles. While forecast technology and science have improved, regional predictions remain limited to a few months in advance. What causes drought? The physical conditions causing drought in the United States are increasingly understood to be linked to sea surface temperatures (SSTs) in the tropical Pacific Ocean. Studies indicate that cooler-than-average SSTs have been connected to the severe western drought in the first decade of the 21 st century, severe droughts of the late 19 th century, and precolonial North American "megadroughts." The 2011 severe drought in Texas is thought to be linked to La Niña conditions (cooler-than-average SSTs) in the Pacific Ocean. What is the future of drought in the United States ? The prospect of extended droughts and more arid baseline conditions in parts of the United States could suggest new challenges to federal programs and water projects, which were conceived or constructed largely on the basis of 20 th -century climate conditions. Some studies suggest a transitioning of the American West to a more arid climate, possibly resulting from the buildup of greenhouse gases in the atmosphere, raising concerns that the region may become more prone to extreme drought than it was in the 20 th century. Some models of future climate conditions also predict greater fluctuations in wet and dry years; however, the net effect of such fluctuations is difficult to predict. What is federal drought policy? Although drought impacts can be significant, no comprehensive national drought policy exists. Developing a national policy would be challenging because of split federal and nonfederal responsibilities; the existing patchwork of federal programs; and differences in regional conditions, risks, and available responses. In 2000, the National Drought Policy Commission provided recommendations to Congress to improve drought policy. Congress has acted on some of the recommendations (e.g., authorizing the National Integrated Drought Information System), but not others (e.g., creation of a National Drought Council and a fund to support drought planning). Given current conditions, Congress may review the functioning and adequacy of existing federal responses and programs (e.g., access to and level of assistance provided, incentives for mitigation of drought risk, and preparedness of federal facilities). The likelihood of extended periods of severe drought, similar to conditions experienced centuries ago, and its effects on 21 st -century society in the United States raise several issues for Congress. These issues include how to respond to recurrent drought incidents, how to prepare for future drought, and how to coordinate federal agency actions. For example, drought often results in agricultural losses, which can have local, regional, and national effects. It also can affect other industries and services, including power and energy resource production, navigation, recreation, municipal water supplies, and natural resources such as fisheries, aquatic species, and water quality. How to address these impacts is an often recurring issue for Congress. Addressing drought on an emergency basis is costly to individuals, communities, and businesses. Additionally, millions and sometimes billions of dollars in federal assistance can be expended in response to drought's social consequences. Thus, another recurrent policy issue is how to prepare and mitigate future drought impacts and how to do so efficiently across the many federal agencies with various and sometimes overlapping drought responsibilities. Drought has afflicted portions of North America for thousands of years. Severe, long-lasting droughts may have been a factor in the disintegration of Pueblo society in the Southwest during the 13 th century, and in the demise of central and lower Mississippi Valley societies in the 14 th through 16 th centuries. In the 20 th century, droughts in the 1930s (Dust Bowl era) and 1950s were particularly severe and widespread. In 1934, 65% of the contiguous United States was affected by severe to extreme drought, resulting in widespread economic disruption and displacement of populations from the U.S. heartland. Drought conditions are broadly grouped into five categories: (1) abnormally dry, (2) moderate, (3) severe, (4) extreme, and (5) exceptional. Some part of the country is almost always experiencing drought at some level. Since 2000, no less than 6.6% of the land area of the United States has experienced drought of at least moderate intensity each year. The land area affected by drought of at least moderate intensity varies by year and also within a particular year. For example, since 2000, the total U.S. land area affected by drought of at least moderate intensity has varied from as little as 6.6% (July 6, 2010) to as much as 55% (September 25, 2012). Based on weekly estimates of the areal extent of drought conditions since 2000, the average amount of land area across the United States affected by at least moderate-intensity drought has been 26%. While the previous percentages refer to the extent of drought nationally, there is particular concern about those locations experiencing the most intense drought conditions. Nearly every year, extreme drought affects some portion of the country. Since 2000, extreme drought or drier conditions have affected approximately 6.4% of the nation on average. During August 2012, extreme drought extended over 20% of the country. Since 2000, exceptional drought conditions have affected approximately 1.4% of the nation on average. Of particular note were the conditions between June and October 2011; exceptional drought occurred over the largest land area—greater than 9%—during those months, with the affected areas concentrated in Texas. This year, 2013, is likely to be another exceptional year in terms of the breadth of drought conditions throughout the country, particularly in the Great Plains and eastern portions of the Midwest. The severe to exceptional drought conditions throughout the central and western parts of the United States appear to be persisting during spring 2013. (See Figure 1 .) In mid-August 2012, the extent of the drought conditions was significant: over 70% of the land area of the United States (including Alaska and Hawaii) was affected by abnormally dry and drought conditions. The percentage of land area affected by abnormally dry or drought conditions stayed at or above 65% through February 2013, and remains above 60% as of mid-April, 2013. The intensity of the drought varied across the country, but the regions of extreme and exceptional drought were clustered across the Midwest, Great Plains, Southwest, and in the Southeast, particularly Georgia in 2012. The widespread nature of drought and excessive heat conditions in the Midwest and Great Plains in the summer of 2012 contributed to sharply lower yields for major crops, including corn and soybeans. For example, the average U.S. corn yield was 123.4 bushels per acre, down more than 40 bushels from expectations before the onset of drought. For all crops, crop insurance indemnities for 2012 losses totaled $16 billion. In 2013, continuation of drought in the Great Plains and western parts of the Midwest is affecting prospects for wheat and other crops as well as pasture and forage conditions for livestock producers. Figure 2 shows that Texas and portions of Florida and Georgia experienced exceptional drought conditions (the worst category of drought) in early 2012, while the upper Midwest, including most of the Mississippi Valley experienced normal conditions. A year later, in early 2013, the drought had eased somewhat in portions of Georgia and Florida, but intensified throughout the center of the country from Texas to the Canadian border. Nearly 12% of the contiguous United States was in exceptional drought conditions from late June 2011 through October 2011, compared to approximately 6% of the country the following year. However, exceptional drought conditions persisted over nearly 6% of the contiguous United States from mid-August 2012 through mid-February 2013. Although less severe for portions of the country, such as Texas and Florida, the 2012-2013 drought affected broader swaths of the agricultural heartland compared to 2011 ( Figure 2 ). The 2012-2013 experience illustrates that the extent, timing, and particular features of areas affected by drought—dryland versus irrigated farm regions, or regions that are still recovering from previous droughts—are important in addition to the relative severity of drought conditions. Figure 2 shows a snapshot of drought conditions for March in North America for 2010-2013. In 2010 most of North America and the United States were experiencing near-normal conditions. The extent and severity of the 2012-2013 drought raised questions regarding its origin, and whether the drought was consistent with the range of natural variability in the U.S. Midwest and Plains, or whether it was linked to longer-term changes in the Earth's climate system, such as human-induced global warming. Although the images presented in Figure 2 may seem to indicate a steady progression of drought in the middle portion of the country from near-normal conditions in 2010 to widespread and intense drought in 2012-2013, a March 2013 analysis concludes that the 2012 intense drought was a discrete extreme event.