(March 2014) Dramatic and spontaneous natural disasters garner substantial humanitarian aid—as they should. But long-term chronic environmental pressures such as heat stress also put tremendous strain on rural households, especially households in less developed countries that rely on agriculture.

People migrate in response to immediate disasters as well as to longer-term environmental strains. Humanitarian aid can potentially reduce the need for both kinds of migration.

Migration From Environmental Stress

Environmental factors have always shaped migration. In the ancient Sahara, archaeological and anthropological evidence suggest that humans migrated as shifts in the monsoon patterns allowed them to move deeper into the desert. Alternatively, periods of droughts forced people to leave the desert in search of water and pasture land for their animals—much like contemporary nomadic populations.1

More recent examples are also useful. The dust bowl in the United States from 1931 to 1939 led to massive soil erosion and a drastic reduction in soil productivity. Billowing clouds of dust buried farm equipment and buildings. Hundreds of thousands of residents had no choice but to leave the Great Plains.2

Humanitarian Aid and Migration

Today, environmental catastrophes fill the news. Typhoon Haiyan devastated the Philippines in November 2013; and a massive tornado carved a 12-mile path through Oklahoma City in May 2013. Over $4 billion are spent on emergency relief for natural disasters every year.3 But dramatic environmental disasters aren't the only reasons people migrate.

Comparing Floods and Heat Stress in Pakistan

To shed light on migration in the face of different environmental challenges, a recent study in Nature Climate Change focused on rural Pakistan over a 21-year period. The migration histories of over 4,000 individuals between 1991 and 2012 were examined by researchers Valerie Mueller from the International Food Policy Research Institute, and Clark Gray and Katrina Kosec from the University of North Carolina's Population Center. They linked rainfall and temperature data with migration data.

Their research found that heat stress rather than floods was most related to migration. Heat stress in rural Pakistan is intense during the wheat-growing season, from winter to spring. Also, poor households tended to move longer distances, suggesting that migration is important as an adaptive response, especially among the most vulnerable populations.4

Environmental Pressures Act on a Continuum

Research in other regions has also documented the migratory impact of long-term chronic environmental stress. In dry rural regions of Mexico, migration to the United States increases in times of low rainfall.5 In India, a severe drought in 2003 led nearly all households in the small village of Khaliakani to send at least one migrant in search of income elsewhere.6

An environmental driver of migration, drought, as a longer-term chronic stress, is at one end of a continuum. Sudden catastrophic events are at the other end. In the case of chronic long-term migration, a family may not move but will send someone from the household to seek labor elsewhere and send remittances home. On the other hand, acute and sudden natural disasters may result in refugee streams, although relocation may be short term.

Difference Pressures Require Different Policies

Humanitarian aid in the wake of natural disasters is essential. Yet the impacts on rural livelihoods of longer-term environmental pressures should also be recognized. Sustainable development requires policies and programs to help rural residents adapt to weather-related risks. Such responses might include capturing rainfall for irrigation ("rainwater harvesting"), using heat-resistant seed varieties, providing agricultural insurance, and improving weather forecasting.

Lori M. Hunter is an associate professor of sociology, Institute of Behavioral Science, Programs on Population, Environment and Society, at the University of Colorado Boulder. She is also editor-in-chief of Population and Environment. This article is part of PRB's CPIPR project, funded by a grant from the Eunice Kennedy Shriver National Institute of Child Health and Human Development. Hunter is liaison from the University of Colorado's Population Center to CPIPR. NICHD-funded researchers highlighted in this article are Clark Gray of the Carolina Population Center at the University of North Carolina Chapel Hill; Myron Gutmann from the University of Michigan's Population Studies Center; and Hunter and colleagues Fernando Riosmena and Raphael Nawortzki from the CU Population Center at the University of Colorado Boulder.


  1. Nick Brooks et al., "The Climate-Environment-Society Nexus in the Sahara From Prehistoric Times to the Present Day," The Journal of North African Studies 10, no. 3-4 (2005): 253-92; and Oscar Alvarez Gila, Ana Ugalde Zaratiegui, and Virginia López De Maturana Diéguez, "Western Sahara: Migration, Exile, and Environment," International Migration 49, no. s1 (2011): e146-63.
  2. Myron P. Gutmann and Vincenzo Field, "Katrina in Historical Context: Environment and Migration in the U.S.," Population and Environment 31, no. 1-3 (2007): 3-19.
  3. David Stromberg, "Natural Disasters, Economic Development, and Humanitarian Aid," Journal of Economic Perspectives 21, no. 3 (2007): 199-222.
  4. Valerie Mueller, Clark Gray, and Katrina Kosec, "Heat Stress Increases Long-Term Human Migration in Rural Pakistan," Nature Climate Change 4 (Jan. 26, 2014), accessed online at www.nature.com/nclimate/journal/v4/n3/full/nclimate2103.html, on Feb. 4, 2014.
  5. Raphael J. Nawrotzki, Fernando Riosmena, and Lori M. Hunter, "Do Rainfall Deficits Predict U.S.-Bound Migration From Rural Mexico? Evidence From the Mexican Census," Population Research and Policy Review 32, no. 1 (2013): 129-58.
  6. Sebastian Jülich, "Drought Triggered Temporary Migration in an East Indian Village," International Migration 49, no. S1 (2011): e189-99.