The D Y Facts

OUTLOOK AGRICULTURE AND DROUGHT THE DY FACTS Drought has wreaked havoc throughout history, destroying crops and causing famine and conﬂict. And it could be getting worse. By Olive Heﬀernan. DEFINING DOUGHT Characterized by unusual and persistent dry weather, drought is caused by shifts in global weather patterns such as El Niño, METEOOLOGICAL but also increasingly by human-induced climate change. DOUGHT Meteorological Identifying drought is dicult, however, so there is considerable Occurs when a persistent drought uncertainty about whether it is getting any worse. decline in 1 precipitation causes both reduces the water available soil-moisture on land in snowpack 2 , and hydrological ice sheets, lakes and drought. rivers 3 . 1 SOIL-MOISTUE DOUGHT Occurs when the soil dries Snow Rain out as water is evaporated back to the atmosphere 4 , drains deeper into the soil 5 and is extracted by humans 6 . Runo Evapotranspiration 2 HYDOLOGICAL Soil moisture DOUGHT 4 Occurs when water Groundwater 3 Evaporation reserves in aquifers, 7 lakes and reservoirs 7 Inltration fall below average levels Percolation owing to high human 5 demand or low rainfall. Groundwater ow 6 A LONG HISTOY Serious droughts have occurred in Australia and Africa, among other regions, in recent decades, but more severe ‘megadroughts’ have occurred on virtually every continent throughout history. Global Megadrought The Great Famine The US Dust Bowl Sahel Drought 4,200 YEAS AGO 876–78 90 930s 940s 970s–980s GLOBAL MEGADROUGHT THE GREAT FAMINE FEDERATION DROUGHT THE US DUST BOWL CENTRAL EUROPEAN SAHEL DROUGHT Where: North America Where: Began in Where: Australia. It Where: Centred on the DROUGHT Where: In 1983 it covered but spread to Europe, southern India and covered half the Great Plains but covered Where: Central and 65% of the Sahel, or Africa and Asia. Lasted spread to the tropics continent by 1901. 60% of the United States. Eastern Europe in around 8 million km2. for several centuries. and China. Cause: Lack of rainfall. Lasted all decade. 1945–47. Cause: Probably natural Cause: Cooling of the Cause: Severe El Niño Impact: Reduced cattle Cause: Low rainfall and Cause: Low rainfall. variation in ocean North Atlantic, which that led to the failure of from 12 million to 7 poor land management Impact: Devastated temperatures and reduced rainfall by as the Asian monsoon. Food million and sheep from (deep ploughing). crops. In some countries, atmospheric dynamics, much as 30%. shortages for the poor 91 million to 54 million. Impact: Agricultural such as Romania, coupled with human- Impact: Linked to demise worsened by Colonial-era Led to a massive failure production fell by 17%. 90% of the population driven climate change. of Akkadian Empire and imperialism. of the wheat crop and Cost the US government went hungry. In parts Impact: Famine led to civilizations in Greece, Impact: More than caused the Darling River US$13 billion in aid and of the Czech Republic, 600,000 deaths in Egypt and the Indus 5 million deaths in India in New South Wales to 2.5 million people left the the cereal yield fell by 1972–75, and again Valley of Pakistan. and 30 million in total. dry up. aected states. about 30%. in 1984–85. ZUMA PRESS/ALAMY / SEAN SEXTON COLLECTION/CORBIS / CORBIS ALAIN NOGUES/SYGMA/CORBIS COLLECTION/CORBIS / SEAN SEXTON ZUMA PRESS/ALAMY S2 | NATURE | VOL 501 | 26 SEPTEMBER 2013 AGRICULTURE AND DROUGHT OUTLOOK ECENT DOUGHTS In 2011 and 2012, drought occurred on almost every continent, partly because of an unusually strong La Niña that caused record rainfall in Australia and led to severe water shortages in Sudan. Hundreds of millions of hectares of crops were destroyed, from wheat in Russia to sugar cane in India. The map shows how rainfall deviated from the norm in this period and highlights the regions most aected by drought — Maize Soy Wheat Rice many of which are major growing regions for the world's top four staple crops. The top four crops account for 45% of all cultivated land and at least 40% of human calorie intake. The main growing regions for each are indicated on the map. Black Sea region Tens of millions of hectares of wheat damage worth US$1 billion in Russia alone. China Millions of people and livestock left Sahara without drinking In Mali, hundreds water, and 5 million of thousands of hectares of crops people driven damaged. from their homes by crop failure and high cereal prices, intensied by conict. United States and Mexico Severe drought cost billions of US dollars in the United States and led to food Argentina and Brazil shortages in Mexico. By early 2013, 80% Yield losses of 40% 30% of the contiguous US in some maize-growing was aected. regions. Loss of 30% 20% of sugar cane in some Horn of Africa parts of Brazil. Fall in 10% Up to half of crops and livestock lost. soybean yield in Human mortalities number 0% both countries. 50,000–100,000, mainly in Somalia, Ethiopia and Kenya. –10% Precipitation anomaly Precipitation –20% –30% LAND UNDE THEAT In many regions, climate change is expected to increase the amount of land at risk from drought and heat, and will threaten more arable areas. Each 1°C rise in global warming could cut grain yield by as much as 5%. SHEFFIELD, J. & WOOD, E. F. DROUGHT: PAST PROBLEMS AND FUTURE SCENARIOS (ROUTLEDGE, 2011) PAST DROUGHT: E. F. SHEFFIELD, J. & WOOD, 50 160 40 1971–2000 140 Maximum 2071–2100 40 35 120 Wheat is more sensitive to 100 30 drought than 30 to heat stress 80 Optimum range 20 25 60 Temperature (ºC) Temperature 40 10 20 20 Spatial extent of monthly drought (% of land) of monthly drought Spatial extent 0 0 15 1900 1950 2000 2050 2100 Year (millions of hectares) heat stress Suitable land at risk from World (average) West Africa Amazon Mediterranean Southeast Asia Central America Northern Europe Central North America Rice Maize Soy Wheat Rice Maize Soy Wheat TEIXEIRA, E. I. ET AL. AGRICULTURAL AND FOREST METEOROLOGY 170, 206–215 (2013) METEOROLOGY AND FOREST AL. AGRICULTURAL TEIXEIRA, E. I. ET 26 SEPTEMBER 2013 | VOL 501 | NATURE | S3.

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