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Suicides Linked to Warming Temperature increases are associated with decreases in rainfall and ensuing crop yields. A recent study estimates that such warming over the last 30 years is responsible for 59,300 extra suicides in India, and proposes a new perspective to contextualize the linked effects of global warming in affected agricultural communities.

Steven Stack

I

n India, a nation of 1.2 billion people, there were approximately 258,075 suicides in 2012. This is more than a quarter of the recorded 804,000 for that year worldwide1. India’s rate of 21.1 (the standard for reporting suicide rates is the number of suicides per every 100,000 people in a population) places it eleventh among the 172 nations with estimated suicide rates1. Since 1980, that rate has doubled. Carleton analysed data from India for up to 32 states and territories2. Much of this analysis is focused on the period 1980–2013. At the heart of the paper she postulates a causal chain that is confined to the growing season. First, global climate change is responsible for excessive increases in temperature. Second, the increased temperatures result in lower agricultural production; yields per acre decline with high temperatures. This link is soundly documented. The crop failures, in turn, are presumed to promote economic strain. It is suggested that the ensuing elements of economic hard times include loss of farms through foreclosures and bankruptcies, pressures of heavy debt, unemployed farm workers, lower agricultural wages, lower incomes for farmers, and inflation of prices for agricultural products. These financial strains then contribute to rising suicide rates. Overall an increase of 1 °C in a single day’s temperature correlates with 70 suicides on average according to Carleton’s analysis, but only during India’s growing season and only for temperatures above 20 °C. While the data is clear, the causal link between temperature and suicides is less certain. First there are no direct measures of the economic strains created by yield failures and the analysis only presumes they exist and thus drive the suicide rates. Further, it is unclear whether those experiencing the financial strains are actually the ones who account for rising suicide rates. These twin limitations suggest useful points of departure for future work.

Although there is little doubt that some of the excess suicides involve distressed farmers, data on who actually dies by suicide in rural India are needed as a check on this assertion. Available evidence suggests that only between 5–11% of rural suicides involve persons undergoing financial stress3,4. Such strains are not leading contributors to suicide, according to a review of 35 studies based in India5. Future work carefully documenting the disproportional presence of economic strain in cases of rural suicide is needed. The link between temperature and suicide is assumed to rely on crop failures and economic strain in the Carleton study. While this may be the case in India, research on temperature and suicide rates over long time periods elsewhere suggests caution. A recent paper analysed data for each of 15 Asian cities in Japan, South Korea, and Thailand. These urban locations (such as Tokyo, Seoul, and Taipei) have urban labour markets that are relatively free from devastations resulting from increases in temperature on crop yields, and the ensuing economic strains felt by farm owners and workers. Nevertheless, long-term changes in temperatures in the urban labour markets were associated with increases in suicide rates6. Given the urban locations of these investigations, it is likely that the link between temperature increase and suicide rates is due to other intervening conditions, not crop failures. One such condition, correlated with global warming, might be cultural change. Rapid cultural change in neighbouring Asian countries (in China and South Korea, for example) has been linked to high and increasing suicide rates. For example, declines in the vitality of the institutions of family and religion, and ensuing cultural ambivalence between the values of Confucianism and modernism, have been linked to the rising suicide rates in South Korea as well as high rates

in rural China7,8. Suicide rates in South Korea quadrupled from 7.9 in 1989 to 31.0 in 20097, even more than the doubling in India cited by Carleton. The cultural clash between the values of Confucian culture, which stress the subordination of women, and the culture of modernism, which emphasizes gender equality, has been documented as a leading cause of suicide among rural Chinese women8. The analysis in Carleton’s study would be strengthened if cultural conflict and change could be controlled for in accounting for competing contributors to rural suicides in India, should the necessary data become available in the future. Several other limitations might be corrected through further analysis of the existing and new data. While the study gathered the extensive data available on crop yields and suicide rates, an analysis of the link between these two phenomena is not reported. The analysis would be greatly strengthened if it could be demonstrated that low crop yields predict suicide rates. If this is not the case, however, the framework that assumes crop failures cause economic strain that, in turn, predicts suicide would be weakened. Since the data had already been collected, failure to perform such an analysis would be a missed opportunity. A secondary part of the analysis presents very convincing evidence that as rainfall increases the suicide rate decreases. The presumed causal factor is that rainfall increases crop yields, thus reducing economic strain. However, it is not clear which factor, temperature or rainfall, is more important in predicting suicide rates. An increase of 1 °C in temperature increases the suicide rate by only 0.008 per 100,000 or 3.5% in standardized units. A 1 cm increase in rainfall decreases the suicide rate by 0.8per 100,000 or a reduction of 7% (but no corresponding change in standardized units is reported).

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news & views In any event, the reported changes in the suicide rate are not overwhelming. Other factors are clearly involved in the variation in suicide rates. Additional data collecting would also strengthen Carleton’s arguments by attempting to account for the 9% drop in suicide rates in India from 23.3 in 2000 to 21.1 in 20121,9. This seems contrary to the author’s notion of a long-term link between global warming and suicide rates. Also, data on insecticides, a key method of suicide in Asia, need to be marshalled to clarify the links found by Carleton. For example, the spread of highly lethal types of insecticides has been associated with suicide risk in other nations, such as China10. These trends in the use and availability of highly lethal insecticides may run parallel with the rise in temperatures, lower rainfall, and crop

failures. Crop shortfalls might provide an incentive for increasing pesticide use to increase yields. If so, suicide rates may increase, in part, by simply increasing the availability of a highly lethal method of suicide. Carleton is to be congratulated on a new theoretical perspective on global climate change and suicide. It is the first such argument, and makes a valuable contribution to the growing literature on suicide in low- and middle-income countries11. This new perspective needs to be tested elsewhere in regions afflicted by drought, high temperatures, and agricultural short falls. ❐ Steven Stack

Department of Criminal Justice, Wayne State University, Detroit, Michigan 48202, USA. e-mail: [email protected]

Published: xx xx xxxx

DOI: 10.1038/s41477-017-0029-1 References

1. Carleton, T. Proc. Natl Acad. Sci. USA 114, 8746–8751 (2017). 2. Preventing Suicide: A Global Imperative (World Health Organization, 2014). 3. Ebenezer, J. A. & Joge, V. Ind. J. Psychol. Med 38, 567–570 (2016). 4. Manoranjitham, S. S. et al. Brit. J. Psychiat. 196, 26–30 (2010). 5. Rane, A. & Nadkarni, A. Shanghai Archiv. Psychiat 26, 69–80 (2014). 6. Kim, Y. et al. Environ. Health Perspect. 124, 75–80 (2016). 7. Park, B. C. B. in Suicide and Culture: Understanding the Context (eds Colucci, E & Lester, D.) 237–267 (Hogrefe, Cambridge, MA, 2011). 8. Zhang, J. & Zhao, S. Asian J. Psychiat 6, 510–514 (2013). 9. Patel, V. et al. Lancet 379, 2343–2351 (2012). 10. Kong, Y. & Zhang, J. Psychiat. Res. 179, 217–221 (2010). 11. Bantjes, J. et al. Glob. Ment. Health 3, e32 (2016).

Competing interests

The authors declare no competing financial interests.

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Suicides Linked to Warming.

Temperature increases are associated with decreases in rainfall and ensuing crop yields. A recent study estimates that such warming over the last 30 y...
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