Drought can bring negative effects in any region of the United States. It can bring long-lasting and severe impacts on the environment, agriculture, energy production, as well as health and safety. It can affect human activities and other factors of the economic and social well-being of our country. The likelihood of experiencing severe droughts in the coming years has raised several debates in Congress.
Such issues include how one can respond during drought incidents and how to prepare for future incidents of droughts. Furthermore, the debates include how to coordinate in the various agencies.
But, in this article, we will understand the causes of drought in the United States. We will determine how the drought has affected America in the past years. Moreover, we will learn how drought can affect us all in the future if we fail to mitigate its effect and to take action immediately.
An Overview of Drought in America
Through time, drought has affected several areas in North America. During the 13th century, severe and long-lasting droughts are one of the major factors of the disintegration of Pueblo society. This is true for the Southwest portion and the Mississippi Valley communities during the 14th and 16th centuries.
In the 20th 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—many relocating to California’s Central Valley—and revealing shortcomings in agricultural and land-use practices.
Today, the National Drought Mitigation Center (NDMC) monitors and reports on drought conditions across the nation. Drought conditions are broadly grouped into five categories: D0 (abnormally dry), D1 (moderate), D2 (severe), D3 (extreme), and D4 (exceptional).
Some part of the country is almost always experiencing drought at some level. Since 2000, some portion of the land area of the United States has experienced a drought of at least moderate intensity (D1) each year. The land area affected by drought can vary widely by year and also within a particular year.
For example, in May 2017, only 3.8% of the total U.S. land area was affected by a drought of at least moderate intensity (D1). In contrast, in September 2012, 55% of the nation faced a drought of at least moderate intensity, and 35% of the country was under severe drought (D2) conditions at that time.
Based on weekly estimates of drought conditions since 2000, on average, about 26% of the land area across the United States experiences at least moderate intensity in any given year.
There is particular concern about locations experiencing the most intense drought conditions: extreme and exceptional drought. Nearly every year, extreme drought (D3) affects some portion of the country. Since 2000, extreme drought or drier conditions have affected approximately 6.5% of the nation on average. Since 2000, exceptional drought (D4) conditions have affected approximately 1.4% of the nation on average.
Of particular note were the conditions between June 2011 and October 2011: exceptional drought (D4) occurred over the largest land area—greater than 9%—during those months over the period starting January 2000 until the present, with the affected areas concentrated in Texas. The following year, during August 2012, extreme and exceptional drought extended over 20% of the country and was concentrated in the central United States.
What Is Drought?
Drought has a number of definitions; the simplest may be a deficiency of precipitation over an extended period of time, usually a season or more. Conceptually, it may be easier to understand drought through its impacts.
For example, when evaluating the impact on agriculture, drought could be defined as a protracted period of deficient precipitation resulting in extensive damage to crops, resulting in loss of yield. Drought is usually considered relative to some long-term average condition or balance between precipitation, evaporation, and transpiration by plants (evaporation and transpiration are typically combined into one term: evapotranspiration).
An imbalance could result from a decrease in precipitation, an increase in evapotranspiration (from drier conditions, higher temperatures, higher winds), or both. It is important to distinguish between drought, which has a beginning and an end, and aridity, which is restricted to low rainfall regions and is a relatively permanent feature of an area’s climate (e.g., deserts are regions of relatively permanent aridity).
Higher demand for water for human activities and vegetation in areas of limited water supply increases the severity of the drought. For example, drought during the growing season likely would be considered more severe—in terms of its impacts—than similar conditions when cropland lies dormant.
For policy purposes, drought often becomes an issue when it results in a water supply deficiency. During these deficiencies, less than the average amount of water is available for irrigation, municipal and industrial (M&I) supply, energy production, preservation of endangered species, and other needs. These impacts can occur through multiple mechanisms, such as:
- decreased precipitation and soil moisture affecting dryland farming;
- low reservoir levels reducing allocations for multiple purposes (including irrigation, navigation, energy production, recreation, fish and wildlife needs, and other water supplies);
- low stream flows limiting withdrawals for multiple purposes, including M&I supplies, among others; and
- decreased exchange of water in lakes resulting in water quality problems limiting recreation (e.g., blue-green algae restrictions in multiple lakes in Oklahoma and Texas during 2011 and 2012 drought conditions).
Drought also can relate and contribute to other phenomena, such as wildfires and heatwaves.
To assess and classify the intensity and type of drought, certain measures, or drought indicators, are typically used. Drought intensity, in turn, can be a trigger for local, state, and federal responses to drought. The classification of drought intensity may depend on a single indicator or several indicators, often combined with expert opinion from the academic, public, and private sectors.
For example, the U.S. Drought Monitor uses five key indicators, together with expert opinion, with indicators to account for conditions in the West where snowpack is relatively important and with other indicators used mainly during the growing season The U.S. Drought Monitor intensity scheme—D0 to D4—is used to depict broad-scale conditions but not necessarily drought circumstances at the local scale.
What Causes Drought in the United States?
The immediate cause of drought is the predominant sinking motion of air (subsidence) that results in compressional warming or high pressure, which inhibits cloud formation and results in lower relative humidity and less precipitation. Regions under the influence of semipermanent high pressure during all or a major portion of the year are usually deserts, such as the Sahara and Kalahari deserts of Africa and the Gobi Desert of Asia.
Prolonged droughts occur when these atmospheric conditions persist abnormally for months or years over a certain region.
Predicting drought is difficult because the ability to forecast surface temperature and precipitation depend on a number of key variables, such as air-sea interactions, topography, soil moisture, land surface processes, and other weather system dynamics.
Scientists seek to understand how all these variables interact and to further the ability to predict sustained and severe droughts beyond a season or two, which is the limit of drought forecasting abilities today.
In the tropics, a major portion of the atmospheric variability over months or years seems to be associated with variations in sea surface temperatures (SSTs). Since the mid-to-late 1990s, scientists have increasingly linked drought in the United States to SSTs in the tropical Pacific Ocean.
Cooler-than-average SSTs in the eastern tropical Pacific region—“La Niña-like” conditions—have been shown to be correlated with persistently strong drought conditions over parts of the country, particularly the West.20 A number of studies have made the connection between cooler SSTs in the eastern Pacific Ocean and the 1998-2004 western drought, three widespread and persistent droughts of the late 19th century, and past North American megadroughts that recurred between approximately 900 and 1300 A.D.
The precolonial megadroughts apparently lasted longer and were more extreme than any U.S. droughts since 1850 when instrumental records began. Some modeling studies suggest that within a few decades, the western United States may again face higher base levels of dryness or aridity, akin to the 900-1300 A.D. period. The Southwest as a whole, in fact, has experienced a drought of fluctuating severity for the past 16 years, possibly foreshadowing conditions that could become more common in the coming decades.
Although the relationship between cooler-than-normal eastern tropical Pacific SSTs (La Niña-like conditions) and drought in the United States is becoming more firmly established, meteorological drought is probably never the result of a single cause.
What is emerging from the scientific study of drought is an improved understanding of global linkages—called teleconnections by scientists—among interacting weather systems, such as the El Niño-Southern Oscillation, or ENSO. (See box for a description of ENSO.) For example, some scientists link La Niña conditions between 1998 and 2002 to the occurrence of near-simultaneous drought in the southern United States, Southern Europe, and Southwest Asia.
Drought and Climate Change
The relationship between climate change and future trends in droughts is complex, and the scientific understanding of this relationship is evolving. In 2007, the Intergovernmental Panel on Climate Change (IPCC) released its Fourth Assessment Report, which stated that, globally, very dry areas have more than doubled since the 1970s due to a combination of ENSO events and global surface warming.
The 2007 IPCC report added that very wet areas declined by about 5% globally. The report asserted that documented trends in severe droughts and heavy rains showed that hydrological conditions were becoming more intense in some regions.
In 2012, the IPCC issued a new report stating that “there are still large uncertainties regarding observed global-scale trends in droughts.” The newer report noted that although its earlier assessment had stated that very dry areas have more than doubled since the 1970s, that observation was based largely on only one study, which relied on a measurement primarily related to temperature, not moisture.
A different study, which looked at soil moisture simulations, found that global trends in drought duration, intensity, and severity predominantly were decreasing, not increasing, but with strong regional variation.
The 2012 IPCC report assigned medium confidence that there has been a slight overall tendency toward less dryness in North America (i.e., a wetting trend with increasing soil moisture and runoff). It noted that the most severe droughts in the 20th century occurred in the 1930s and 1950s, where the 1930s drought was the most intense and the 1950s drought was the most persistent.
In comparison to the severe megadroughts that occurred in North America hundreds and thousands of years ago, as documented using paleoclimate evidence (discussed earlier in this report), these recent droughts were not unprecedented, according to the 2012 IPCC report.
The 2012 IPCC report concluded that despite new studies that have furthered the understanding of mechanisms leading to drought, there is still limited evidence or ability to attribute observed changes. The IPCC assessed that there was medium confidence that anthropogenic influence has contributed to changes in drought patterns in the second half of the 20th century, but gave low confidence to the attribution of changes in drought patterns at the regional level.
The report noted that some regions of the world had experienced trends toward more intense and longer droughts, such as southern Europe and West Africa. In other regions, such as central North America and northwestern Australia, droughts have become less frequent, less intense, or shorter.
In 2014, the IPCC released its most recent climate assessment, which stated that for North America, decreases in snowpack already are influencing seasonal stream flows. However, the report had medium-to-high confidence that recent droughts (and floods, and changes in mean streamflow conditions) cannot yet be attributed to climate change.
Further, the report stated that it is not yet possible to attribute changes in drought frequency in North America to anthropogenic climate change. The report noted that changes in these events, however, may be indicative of future conditions.