Drought

Introduction

Drought does not happen suddenly. It evolves in a region when weather conditions produce declining rainfall amounts that cause vegetation to dry, soil moisture to be reduced, and surface water sources to decline and ultimately to dry up. The length of a drought depends entirely on weather conditions. It can be only a few weeks in duration or it can last years to become a serious weather-related catastrophe. Satellite images of vegetation in a region can provide evidence of long-term drought conditions where they exist. As vegetation dries up, wildfires become more frequent and intense.

About 10% to 15% of global land areas were considered to be undergoing moderate to severe drought conditions in the 1970s. That number rose to around 30% by 2002. Changing climate conditions that have increased global temperatures are considered by atmospheric scientists to be the cause for at least half of the increase in droughts around the world. All climate models used by the scientists indicate that global warming will increase. Scientists also are predicting more severe drying conditions if the concentration of greenhouse gases increases in the atmosphere.

Historical Background and Scientific Foundations

Historical records of weather events have been developed from studies of tree rings, ice core samples from glaciers, and studies of sediment layers in lakes. However, systematic instrumental records of weather events go back only a little over 100 years. Over the past 50 years, climatologists have discovered the relationship between regional climate events, such as droughts, and much larger interactions between the atmosphere and oceans.

The occurrence of major droughts is believed to be related to ocean-atmosphere oscillation cycles, such as the El Niño/Southern Oscillation (ENSO) cycle. It was not until the 1980s that climatologists saw that El Niño, (an upwelling of warm water in the Pacific Ocean) and La Niña (an upwelling of cold water in the Pacific Ocean) were related to the see-saw atmospheric pressure changes known as the Southern Oscillation. The ENSO cycle affects climate across North America. Similar oscillation patterns in other atmosphere-ocean systems are now considered to contribute to weather conditions that cause major droughts around the world.

Global warming is impacting the oscillation cycles and affecting the location and severity of droughts. Warming temperatures will increase evaporation from ocean surfaces. The extra atmospheric moisture is expected to increase average rainfall overall but not uniformly around the world. Warmer temperatures also will cause the land to dry more quickly where rainfall is less plentiful. Although some regions will experience greater rainfall, other regions will suffer more severe droughts. The percent of land area that suffers severe droughts is expected to continue to rise.

WORDS TO KNOW

DIPOLE: A system that has two equal but opposite characteristics separated by a distance.

EL NIÑO: A warming of the surface waters of the eastern equatorial Pacific that occurs at irregular intervals of 2 to 7 years, usually lasting 1 to 2 years. Along the west coast of South America, southerly winds promote the upwelling of cold, nutrient-rich water that sustains large fish populations, that sustain abundant sea birds, whose droppings support the fertilizer industry. Near the end of each calendar year, a warm current of nutrient-poor tropical water replaces the cold, nutrient-rich surface water. Because this condition often occurs around Christmas, it was named El Niño (Spanish for boy child, referring to the Christ child). In most years the warming lasts only a few weeks or a month, after which the weather patterns return to normal and fishing improves. However, when El Niño conditions last for many months, more extensive ocean warming occurs and economic results can be disastrous. El Niño has been linked to wetter, colder winters in the United States; drier, hotter summers in South America and Europe; and drought in Africa.

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

LA NIÑA: A period of stronger-than-normal trade winds and unusually low sea-surface temperatures in the central and eastern tropical Pacific Ocean; the opposite of El Niño.

SAHEL: The transition zone in Africa between the Sahara Desert to the north and tropical forests to the south. This dry land belt stretches across Africa and is under stress from land use and climate variability.

SOUTHERN OSCILLATION: A large-scale atmospheric and hydro-spheric fluctuation centered in the equatorial Pacific Ocean. It exhibits a nearly annual pressure anomaly, alternatively high over the Indian Ocean and over the South Pacific. Its period is slightly variable, averaging 2.33 years. The variation in pressure is accompanied by variations in wind strengths, ocean currents, sea-surface temperatures, and precipitation in the surrounding areas. El Niño and La Nin˜a occurrences are associated with the phenomenon.

TREE RINGS: Marks left in the trunks of woody plants by the annual growth of a new coat or sheath of material. Tree rings provide a straightforward way of dating organic material stored in a tree trunk. Tree-ring thickness provides proxy data about climate conditions: most trees put on thicker rings in warm, wet conditions than in cool, dry conditions.

The Standard Precipitation Index (SPI) was developed in 1993 to provide early warning of drought conditions and to assess the extent of a drought. SPI values are determined from data collected on precipitation in a specific region for a specified length of time. The SPI index is keyed to reflect local drought conditions. If low precipitation conditions persist for three to six months, agriculture is impacted. Longer low precipitation periods can result in reduced groundwater supplies and increased potential for severe wildfires. The severity of wildfires depends on the terrain of the region and the type of vegetation.

An older, more complex assessment tool for drought conditions is the Palmer Drought Severity Index, developed in the United States in 1968. It is based on a supply-and-demand concept for water use in addition to precipitation measurements. The Palmer index is used by relief agencies to determine where emergency assistance may be needed due to extreme drought conditions. The Palmer index also classifies excess rainfall conditions.

Impacts and Issues

Severe droughts have social and economic consequences. In the 1930s, a seven-year drought in the Great Plains of the United States created such desperate conditions that millions of people migrated to other regions of the country, such as California, to find better living conditions. A writer for the Washington Evening Star coined the term “Dust Bowl” to describe the effects of this severe Great Plains drought. North America is one of the regions that has historically had severe droughts.

Climate change is expected to intensify droughts in all the regions that already experience periodic droughts. In addition to North America, parts of Africa and Australia are already experiencing exceptionally dry conditions. The Australian Commonwealth Scientific and Industrial Research Organisation issued a report in January 2007 warning the Sydney, Australia, region to expect temperatures to rise 9°F (5°C) above the global average in the next 20 years.

Sydney, Australia, could experience severe droughts nine out of every ten years by the second half of the century. The region is already dry and bush fires are frequent, but that scenario is expected to worsen, since scientists predict rainfall will decrease by 40% by 2070. Periodic extreme storms are predicted for the Australian coast to add to the country's inhospitable climate, if climate change continues as current studies predict.

Australia's climate is under the influence of three ocean-atmospheric systems: the El Niño/Southern Oscillation in the Pacific Ocean, the Indian Ocean Dipole, and the Antarctic Circumpolar Wave for the Southern Ocean. The Indian Ocean Dipole is a year-to-year climate pattern that involves alternating warmer and colder ocean surface temperatures in the Indian Ocean. The three systems create a complex weather model for Australia.

IN CONTEXT: U.S. SCIENTISTS CALL FOR NATIONAL DROUGHT POLICY

In a February 2007 meeting, members of the American Association for the Advancement of Science (AAAS) called upon the United States to adopt a national drought policy that plans for global climate change. The researchers asserted that parts of the United States, including some of the nation's most densely populated urban centers and agriculturally productive areas, needed long-term solutions for water management, desertification prevention, and drought adaptation. The group of scientists focused on the western United States, citing the region's recent history of drought and its decades-long battle over freshwater resources, especially those from the Colorado River.

Scientists note that the western United States has long experienced periods of drought, but that global climate change may increase the frequency, length, and severity of future droughts. Past droughts have highlighted the region's precarious balance of high water-needs but vulnerability to water scarcity. Water shortages during regional droughts have affected farms, fisheries, and residential households. Increased urban development in arid areas has increased demand for freshwater in areas where it is naturally scarce, requiring complex water-sharing agreements with more water-rich neighboring states. Furthermore, most of the population of the western United States is on or near the Pacific Coast, making large population centers vulnerable to sea-level rise and freshwater and soil salination, compounding the anticipated long-term effects of climate change in the region.

The AAAS researchers, calling for a national drought policy, noted that droughts are currently responded to on an ad-hoc basis, addressing issues only as they arise. The group asserted that current drought assistance programs, agricultural regulations, and land and water-use laws may prove insufficient to cope with the effects of climate change. Although some states have addressed the effects of climate change through land and resource management laws, federal leaders have been hesitant to adopt policies that acknowledge specific threats of global climate change. As of November 2007, no national policy taking into account the AAAS recommendations had been proposed.

Africa is another region already suffering from droughts that are thought to be tied to global warming. The Sahel region across Africa, from just below the Sahara desert in the east to the Atlantic coast on the west, and bordered by the wet tropical regions to the south, is being impacted by climate change. That region of Africa is influenced by the temperatures in the Indian Ocean. The Sahel region has had periods of adequate rainfall, but has suffered from drought in more recent years. The African droughts have created serious human crises that are frequently publicized in United Nations articles.

Southern Africa also is a drought-prone region likely to suffer more droughts if present climate changes continue. The gloomy outlook for droughts in southern Africa was highlighted by Lloyd's of London, the world's oldest insurer. In May 2007, Lloyd's cited global warming as the cause for drought that is increasing in many parts of the world, and particularly in southern Africa. Lloyd's has formed a partnership with other major groups, including Harvard University's Center for Health and Global Environment and the Insurance Information Institute, to study the severity and consequences of natural disasters that may arise in the future in many regions of the globe as a result of climate change.

See Also Desert and Desertification; Dust Storms; El Niño and La Niña; Ocean Circulation and Currents; Wildfires.

BIBLIOGRAPHY

Periodicals

Baines, Peter. “Australia's Climate Cerberus: The Puzzle of Three Oceans.” Ecos 97 (1998): 22–25.

Web Sites

“Climate Change Warning for Sydney.” BBC News, January 31, 2007. < http://news.bbc.co.uk/2/hi/asia-pacific/6315885.stm> (accessed September 1, 2007).

“Climate Change ‘Will Dry Africa.’” BBC News, November 29, 2005. < http://news.bbc.co.uk/1/hi/sci/tech/4479640.stm> (accessed September1, 2007).

“Drought and Wildland Fire.” UCAR:The University Corporation for Atmospheric Research. < http://www.ucar.edu/research/climate/drought.jsp> (accessed September 1, 2007).