Potentially extreme migrations under climate change

Adam Sobel and I have a new paper on Potentially Extreme Population Displacement and Concentration in the Tropics Under Non-Extreme Warming.

The paper is a theoretical exercise making a simple point: tropical populations that migrate to maintain their temperature will have to move much further that their counterparts in middle latitudes. This results from well established atmospheric physics that aren't focused on by the researchers who usually think about climate change and migration. One reason this result is interesting is that, in a sense, the physics of the tropical atmosphere "amplify" the impact of warming (at least in this dimension) so that even small climate changes can produce really large population displacements in the tropics (theoretically).

Here's the abstract:

Evidence increasingly suggests that as climate warms, some plant, animal, and human populations may move to preserve their environmental temperature. The distances they must travel to do this depends on how much cooler nearby surfaces temperatures are. Because large-scale atmospheric dynamics constrain surface temperatures to be nearly uniform near the equator, these displacements can grow to extreme distances in the tropics, even under relatively mild warming scenarios.  Here we show that in order to preserve their annual mean temperatures, tropical populations must travel distances greater than 1000 km over less than a century if global mean temperature rises by 2C over the same period. The disproportionately rapid evacuation of the tropics under such a scenario causes migrants to concentrate in tropical margins and the subtropics, where population densities increase 300% or more. These results may have critical consequences for ecosystem and human wellbeing in tropical contexts where alternatives to geographic displacement are limited.

See my post here for a longer explanation of what's going on in the paper.

Hurricane-induced migration [Plot of the Week]

Impulse:

Hurricane Katrina, as pictured in the Gulf of Mexico at 21:45 UTC on August 28, 2005.

Response:

This map illustrates the national scope of the dispersion of refugees from Hurricane Katrina. It shows the location by zip code of the 800,000 displaced Louisiana residents who requested federal emergency assistance. The evacuees ended up dispersed across the entire nation, illustrating the wide-ranging impacts that can flow from extreme weather events, some of which are projected to increase in frequency and/or intensity as climate continues to change. (Source: Louisiana Geographic Information Center 2005)

"Our results add to an emerging literature that documents very high rates of return to small capital investments in developing countries"

Mushfiq Mobarak presented at the Columbia Sustainable Development seminar on Monday, and while I couldn't be there the paper (pdf here) seemed worth passing on:

Seasonal Migration and Risk Aversion 

Gharad Bryan, Shyamal Chowdhury, A. Mushfiq Mobarak 

Abstract: Pre-harvest lean seasons are widespread in the agrarian areas of Asia and Sub-Saharan Africa.  Every year, these seasonal famines force millions of people to succumb to poverty and hunger.  We randomly assign an $8.50 incentive to households in Bangladesh to out-migrate during the lean season, and document a set of striking facts.  The incentive induces 22% of households to send a seasonal migrant, consumption at the origin increases by 30% (550-700 calories per person per day) for the family members of induced migrants, and follow-up data show that treated households continue to re-migrate at a higher rate after the incentive is removed.  The migration rate is 10 percentage points higher in treatment areas a year later, and three years later it is still 8 percentage points higher. These facts can be explained by a model with three key elements: (a) experimenting with the new activity is risky, given uncertain prospects at the destination, (b) overcoming the risk requires individual-specific learning (e.g. resolving the uncertainty about matching to an employer), and (c) some migrants are close to subsistence and the risk of failure is very costly. We test a model with these features by examining heterogeneity in take-up and re-migration, and by conducting a new experiment with a migration insurance treatment.  We document several pieces of evidence consistent with the model.   

Yes, that's $8.50 and 550-700 calories per person per day for family members.

Using cell phones to track post-disaster population movements in Haiti

Predictability of population displacement after the 2010 Haiti earthquake

Xin Lu, Linus Bengtsson, and Petter Holme

Abstract: Most severe disasters cause large population movements. These movements make it difficult for relief organizations to efficiently reach people in need. Understanding and predicting the locations of affected people during disasters is key to effective humanitarian relief operations and to long-term societal reconstruction. We collaborated with the largest mobile phone operator in Haiti (Digicel) and analyzed the movements of 1.9 million mobile phone users during the period from 42 d before, to 341 d after the devastating Haiti earthquake of January 12, 2010. Nineteen days after the earthquake, population movements had caused the population of the capital Port-au-Prince to decrease by an estimated 23%. Both the travel distances and size of people’s movement trajectories grew after the earthquake. These findings, in combination with the disorder that was present after the disaster, suggest that people’s movements would have become less predictable. Instead, the predictability of people’s trajectories remained high and even increased slightly during the three-month period after the earthquake. Moreover, the destinations of people who left the capital during the first three weeks after the earthquake was highly correlated with their mobility patterns during normal times, and specifically with the locations in which people had significant social bonds. For the people who left Port-au-Prince, the duration of their stay outside the city, as well as the time for their return, all followed a skewed, fat-tailed distribution. The findings suggest that population movements during disasters may be significantly more predictable than previously thought.

h/t Kyle

An unusual number of goodies in Nature this week

A special Outlook issue reviewing the challenges of malaria control:

Nature Outlook: Malaria (Open access)

The war against the malaria parasite has raged for millennia, and still claims hundreds of thousands of lives each year. Resistance is a growing issue — for both the parasite to current therapy, and the mosquito to pesticides. Past attempts to eradicate malaria have failed. What will it take to finally subdue this deadly disease?

Commentary on the ivory tower:

Global issues: Make social sciences relevant
Luk Van Langenhove

Excerpt:

The social sciences are flourishing. As of 2005, there were almost half a million professional social scientists from all fields in the world, working both inside and outside academia. According to the World Social Science Report 2010 (ref. 1), the number of social-science students worldwide has swollen by about 11% every year since 2000, up to 22 million in 2006. 

Yet this enormous resource is not contributing enough to today's global challenges, including climate change, security, sustainable development and health. These issues all have root causes in human behaviour: all require behavioural change and social innovations, as well as technological development.... 

Despite these factors, many social scientists seem reluctant to tackle such issues. And in Europe, some are up in arms over a proposal to drop a specific funding category for social-science research and to integrate it within cross-cutting topics of sustainable development. This is a shame — the community should be grasping the opportunity to raise its influence in the real world.... 

Today, the social sciences are largely focused on disciplinary problems and internal scholarly debates, rather than on topics with external impact.... 

The main solution, however, is to change the mindset of the social-science community, and what it considers to be its main goal. If I were a student now, I would throw myself at global challenges and social innovations; I hope to encourage today's young researchers to do the same.

Meta-analysis of a famous question:

Comparing the yields of organic and conventional agriculture
Verena Seufert, Navin Ramankutty & Jonathan A. Foley

Abstract: Numerous reports have emphasized the need for major changes in the global food system: agriculture must meet the twin challenge of feeding a growing population, with rising demand for meat and high-calorie diets, while simultaneously minimizing its global environmental impacts1, 2. Organic farming—a system aimed at producing food with minimal harm to ecosystems, animals or humans—is often proposed as a solution3, 4. However, critics argue that organic agriculture may have lower yields and would therefore need more land to produce the same amount of food as conventional farms, resulting in more widespread deforestation and biodiversity loss, and thus undermining the environmental benefits of organic practices5. Here we use a comprehensive meta-analysis to examine the relative yield performance of organic and conventional farming systems globally. Our analysis of available data shows that, overall, organic yields are typically lower than conventional yields. But these yield differences are highly contextual, depending on system and site characteristics, and range from 5% lower organic yields (rain-fed legumes and perennials on weak-acidic to weak-alkaline soils), 13% lower yields (when best organic practices are used), to 34% lower yields (when the conventional and organic systems are most comparable). Under certain conditions—that is, with good management practices, particular crop types and growing conditions—organic systems can thus nearly match conventional yields, whereas under others it at present cannot. To establish organic agriculture as an important tool in sustainable food production, the factors limiting organic yields need to be more fully understood, alongside assessments of the many social, environmental and economic benefits of organic farming systems.

Copyright Nature

And some interesting agent-based modeling from Nature Climate Change:

Emerging migration flows in a changing climate in dryland Africa
Dominic R. Kniveton, Christopher D. Smith & Richard Black

Fears of the movement of large numbers of people as a result of changes in the environment were first voiced in the 1980s (ref. 1). Nearly thirty years later the numbers likely to migrate as a result of the impacts of climate change are still, at best, guesswork2. Owing to the high prevalence of rainfed agriculture, many livelihoods in sub-Saharan African drylands are particularly vulnerable to changes in climate. One commonly adopted response strategy used by populations to deal with the resulting livelihood stress is migration. Here, we use an agent-based model developed around the theory of planned behaviour to explore how climate and demographic change, defined by the ENSEMBLES project3 and the United Nations Statistics Division of the Department of Economic and Social Affairs4, combine to influence migration within and from Burkina Faso. The emergent migration patterns modelled support framing the nexus of climate change and migration as a complex adaptive system5. Using this conceptual framework, we show that the extent of climate-change-related migration is likely to be highly nonlinear and the extent of this nonlinearity is dependent on population growth; therefore supporting migration policy interventions based on both demographic and climate change adaptation.

Migration as adaptation to drought

Labor market integration and long-run health effects of local environmental shocks: Evidence from South Africa
Taryn Dinkelman

Abstract: Geographic, institutional and political features of developing economies often create significant barriers to the spatial integration of labor markets. In this paper, I ask whether constraints on spatial mobility prevent families from using labor migration to protect infant health from local shocks to the environment, with potential long-run consequences for health human capital accumulation. Working in the context ofapartheid-era limits on free labor migration that affected different parts of South Africa to different degrees and relying on quasi-experimental variation in local drought conditions, I use Census data to construct migration histories and show that adult outmigration from the oldest homeland areas – those areas least integrated with the national labor market – was substantially lower than outmigration from other rural areas during drought years. Then, using a nationally representative dataset of South African women born between 1940 and 1988 and a difference-in-difference-in-differences research design, I show that women born in these oldest homeland areas and exposed to drought in infancy are significantly shorter in adulthood. These results highlight an important source of welfare gain from spatially integrating labor markets where local environmental shocks are prevalent.

Americans do care about their climate... they just might not realize it

Climate Change, Crop Yields, and Internal Migration in the United States
Shuaizhang Feng, Michael Oppenheimer, Wolfram Schlenker

We investigate the link between agricultural productivity and net migration in the United States using a county-level panel for the most recent period of 1970-2009. In rural counties of the Corn Belt, we find a statistically significant relationship between changes in net outmigration and climate-driven changes in crop yields, with an estimated semi-elasticity of about -0.17, i.e., a 1% decrease in yields leads to a 0.17% net reduction of the population through migration. This effect is primarily driven by young adults. We do not detect a response for senior citizens, nor for the general population in eastern counties outside the Corn Belt. Applying this semi-elasticity to predicted yield changes under the B2 scenario of the Hadley III model, we project that, holding other factors constant, climate change would on average induce 3.7% of the adult population (ages 15-59) to leave rural counties of the Corn Belt in the medium term (2020-2049) compared to the 1960-1989 baseline, with the possibility of a much larger migration response in the long term (2077-2099). Since there is uncertainty about future warming, we also present projections for a range of uniform climate change scenarios in temperature or precipitation.

Orbital forcing, the green Sahara, human migration and the rise of civilization in the Nile Valley

In a recent talk, Peter B. deMenocal was showing results from this 2006 paper in Science. If you're interested in this stuff, I also recommend looking at Peter's work.  [More paleo-climate/social papers here.]

Climate-Controlled Holocene Occupation in the Sahara: Motor of Africa’s Evolution
Rudolph Kuper and Stefan Kropelin

Abstract: Radiocarbon data from 150 archaeological excavations in the now hyper-arid Eastern Sahara of Egypt, Sudan, Libya, and Chad reveal close links between climatic variations and prehistoric occupation during the past 12,000 years. Synoptic multiple-indicator views for major time slices demonstrate the transition from initial settlement after the sudden onset of humid conditions at 8500 B.C.E. to the exodus resulting from gradual desiccation since 5300 B.C.E. Southward shifting of the desert margin helped trigger the emergence of pharaonic civilization along the Nile, influenced the spread of pastoralism throughout the continent, and affects sub-Saharan Africa to the present day.

In some of my work with colleagues (notably Jesse), we've been finding evidence that it's very costly for populations to adapt to their climate, even in the long run (see here, here, here, and here).  This implies that populations may will endure large welfare losses to the climate without employing adaptation (eg. migrating away).  What I find interesting about the figure above is that even after an abrupt drying event, it takes populations ~1000 years to completely abandon a location.  Presumably, over the course of that millenium, the abruptly dried climate exacted a substantial welfare toll on the population.  I think this general idea has real implications for how we think about human responses to climatic changes. In general, it's assumed that adaptation is relatively cheap so that we adjust quickly to reduce welfare losses to the climate.  However, if the costs of adaptation are high, the adjustments will be slow and the welfare losses will be large. (A more detailed discussion is in this paper.)

Evidence of a link between migration and malaria

Agricultural Policy, Migration, and Malaria in the 1930s United States

Alan Barreca, Price V. Fishback, and Shawn Kantor 

Abstract: The Agricultural Adjustment Act (AAA) caused a population shift in the United States in the 1930s. Evaluating the effects of the AAA on the incidence of malaria can therefore offer important lessons regarding the broader consequences of demographic changes. Using a quasi-first difference model and a robust set of controls, we find a negative association between AAA expenditures and malaria death rates at the county level. Further, we find the AAA caused relatively low-income groups to migrate from counties with high-risk malaria ecologies. These results suggest that the AAA-induced migration played an important role in the reduction of malaria.