EXPERT REACTION: Are people to blame for drought?

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Greenhouse gas emissions and the release of aerosol pollutants have been linked to patterns of drought over the 20th century, suggesting human activities may have been affecting drought for at least the last 100 years, according to US researchers and an Aussie now based in the US. They found drought increased between 1900 and 1949, lessened between 1950 and 1975, and has accelerated since then. They say this pattern is consistent with increasing levels of greenhouse gas emissions in both the first half of the 20th century and at the end of the 20th century. The reversal of this trend from 1950 to 1975 coincides with an increase in the production of aerosols, which have been shown to affect rainfall and alter cloud cover.

Journal/conference: Nature

DOI: 10.1038/s41586-019-1149-8

Organisation/s: NASA Goddard Institute for Space Studies, New York

Funder: K.M. and C.J.W.B. were supported by the US Department of Energy Biological and Environmental Research Grant DE-SC0014423. K.M., B.I.C. and A.P.W. were supported for this work by the NASA Modeling, Analysis, and Prediction program (NASA 80NSSC17K0265). J.E.S. was supported in part by US National Science Foundation (NSF) grants AGS-1243204 and AGS-1602581; J.E.S. and A.P.W. were further supported by NSF grant OISE- 1743738 and A.P.W. was supported by NSF grant AGS-1703029. P.J.D. was supported through PCDMI SFA funding from the DOE Regional and Global Model Analysis Program. Work at LLNL was performed under the auspice

Media Release

From: Springer Nature

Climate science: Linking human activity and drought trends

Evidence of how humans may have influenced global drought conditions over the 20th century is presented in this week’s Nature. A drought-severity record reveals three distinct trends, and suggests that greenhouse gasses and, potentially, aerosols generated by human activity may affect drought risk.

Climate change driven by human activity is thought to alter the global hydroclimate, which determines the risk of persistent droughts or increased rainfall. Attempts to understand the effects of human activity on global drought risk have been complicated by regional differences in hydroclimate variability and a lack of detailed observational data.

To address these issues, Kate Marvel and colleagues analysed ‘drought atlases’ derived from tree-ring data, which provide a picture of regional changes in soil moisture, combined with other climate models and observations, to identify potential drivers of soil moisture changes. Their records indicate that drought increased between 1900 and 1949, lessened between 1950 and 1975, and has accelerated since then.

The drying trend in the first half of the 20th century corresponds to increasing levels of greenhouse gas emissions. The reversal of this trend from 1950 to 1975 coincides with an increase in the production of aerosols, which have been shown to affect rainfall and alter cloud cover. However, the association between aerosol concentrations and drought risk requires further investigation, the authors note. The increased drought towards the end of the 20th century seems to be linked to greenhouse gas emissions, although the underlying cause remains to be determined and the link is not statistically robust.

These findings are consistent with a possible connection between human activity and drought risk from as early as the beginning of the 20th century


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Expert Reaction

These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.

Paul Durack is a Research Scientist at the Lawrence Livermore National Laboratory in the US and is a Visiting Scientist at CSIRO

The study is the first to highlight that in addition to direct changes to global and regional temperature and rainfall, global-scale droughts have now also been found to be impacted by human activities. This is potentially bad news for Australia, and similar climate regions such as California in the US. These regions have experienced devastating recent droughts, and if the model projected changes continue, such droughts will become more commonplace into the future.

Last updated: 01 May 2019 3:19pm
Declared conflicts of interest:
None declared.
Professor John Quiggin is an Australian Laureate Fellow in Economics at the University of Queensland

This research adds to the body of evidence suggesting that climate change, driven by increased emissions of carbon dioxide and other greenhouse gases, is likely to increase the frequency and severity of droughts. Australia is particularly vulnerable in this respect, since our most important agricultural region, the Murray-Darling Basin is already characterized by relatively low and highly variable rainfall. The interaction between drought and policy-driven mismanagement has already produced disastrous outcomes. Without a radical change in both climate policy and water management, things will only get worse.

Last updated: 01 May 2019 3:18pm
Declared conflicts of interest:
None declared.
Ivan Hanigan is a Research Fellow at the Rural Clinical School (Northern Rivers) at the University of Sydney

There are several mechanisms through which drought may affect mental health and increase the suicide rate.

First, droughts increase the financial stress on farmers and farming communities (even if partially compensated by drought relief welfare payments). Such difficulty may occur in conjunction with other economic stresses, such as rising interest rates, falling commodity prices, or an unfavourable foreign exchange rate. Reduced rainfall can also depress economic activity in the broader economic system of rural cities and towns. In some regions the entire economy may be affected. Rural downturns can accelerate migration; weakening and stressing social support systems and lessening social interaction. There may also be resistance by receiving communities against migrants. In some cases rural depopulation may pass a tipping point, leading to an ongoing loss of critical services, such as hospitals, schools and doctors.

Second, there can be a great psychological toll following environmental degradation and this may be acute during droughts linked with decisions and actions to sell or kill starving animals or to destroy orchards and vineyards, which in some cases were painstakingly accumulated over generations. Such loss, and even the apprehension of loss, undoubtedly places a burden on the mental health of farmers and their families. This mourning may not be confined to farmers but extend to other sections of the community likely to be impoverished by long-term environmental degradation. The experience of seeing suffering wild plants and animals, or parched urban parks and gardens, and contemplation of their loss is likely to be extremely painful for some individuals.

Last updated: 01 May 2019 3:17pm
Declared conflicts of interest:
None declared.
Dr Andrew King is Climate Extremes Research Fellow at the ARC Centre of Excellence for Climate System Science, University of Melbourne.

This new study finds that the human influence on the climate has altered droughts for a long time, but that in the mid-20th century the influence was harder to detect than before or afterwards as aerosol effects counteracted the effect of greenhouse gas emissions. As we increase greenhouse gas concentrations in the atmosphere and reduce our aerosol emissions, the human fingerprints in droughts are expected to become clearer into the future.

When we looked at heat extremes, we found much the same effect of a clearer human influence in the 1930s and 1940s than in the subsequent two decades as aerosols masked the effect of greenhouse gas emissions. Like in this study, we found an increased human fingerprint for heat extremes more recently that will continue to increase in the next few decades.

Last updated: 01 May 2019 3:16pm
Declared conflicts of interest:
None declared.
Professor Steven Sherwood is ARC Laureate Fellow at the ARC Centre for Climate System Science and UNSW Climate Change Research Centre

This important study shows that, in the aggregate, lack of rainfall is increasing globally in a way that matches expectations from human-caused global warming but not natural variation. The authors show you can’t tell this just looking at one region at a time, but only by looking at global patterns.  This reinforces previous findings that relative humidity on land has globally become drier over recent decades, also a predicted consequence of global warming.  It is also in a similar vein to work a few years ago showing a global trend toward more extreme rainfall, which likewise usually cannot be reliably detected in specific regions.  These results all show that global warming is altering weather patterns to make rainfall more extreme and less frequent.  Eventually, if warming continues, these changes will become more obvious even region by region.

Last updated: 01 May 2019 3:15pm
Declared conflicts of interest:
None declared.

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