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article imageHuman influence on global droughts goes back 100 years

By Karen Graham     Jun 3, 2019 in Science
Human-generated greenhouse gases and atmospheric particles were affecting global drought risk as far back as the early 20th century, according to a study from NASA’s Goddard Institute for Space Studies (GISS) in New York City.
In the study, published in the journal Nature, scientists, for the first time have linked human activities with patterns of drought around the world - providing historical evidence connecting human-generated emissions and drought at near-global scales, lending credibility to forward-looking models that predict such a connection.
The study team used predictive data and real-world soil moisture data to look for human influences on global drought patterns in the 20th century. What was interesting was that the models predicted a global pattern of regional drying and wetting characteristic of the climate response to greenhouse gases.
Based on this prediction, it followed that a "fingerprint" should emerge in the early 1900s and increase over time as emissions increased. Using tree ring atlases, rain and temperature measurements and satellite data, the researchers found that this real-world data began to align with the fingerprint within the first half of the 20th century.
Farmers growing corn crops may have to change to less moisture-loving crops  like wheat.
Farmers growing corn crops may have to change to less moisture-loving crops, like wheat.
MIT News Office
“The study is the first to emphasize that, in addition to direct changes in regional and global precipitation and temperature, human activities also have an impact on large-scale droughts”, says Lawrence Livermore National Laboratory researcher (US) and co-author of the study, Paul Durack, reports The Costa Rica News.
According to this new research, the "fingerprint" is likely to grow stronger over the next few decades, potentially leading to severe human consequences.
Searching for human fingerprints
Researchers used the Palmer Drought Severity Index, or PDSI. The PDSI is a measurement of dryness based on recent precipitation and temperature. The index, developed in the 1960s by meteorologist Wayne Palmer, has proven most effective in determining long-term drought, a matter of several months, making it very useful for research on climate change in the past.
Using Dendrochronology  or tree-ring dating  we can get a snapshot of climate change.
Using Dendrochronology, or tree-ring dating, we can get a snapshot of climate change.
National Science Foundation
The team also used drought atlases - maps of drought areas that go back hundreds of years - calculated from tree rings. The thickness of the tree rings indicates wet and dry years across their lifespan, providing an ancient record to supplement written and recorded data.
“These records go back centuries,” said lead author Kate Marvel, an associate research scientist at GISS and Columbia University. “We have a comprehensive picture of global drought conditions that stretch back way into history, and they are amazingly high quality.”
The fingerprint changed briefly
For a period of time, between 1950 and 1975, the researchers found that the atmosphere became cooler and wetter. The authors believe that this break is due to the explosion in the use of industrial aerosols or particles in the atmosphere. This was during a time before the passage of air quality legislation.
The GISS Surface Temperature Analysis (GISTEMP) data product.
The GISS Surface Temperature Analysis (GISTEMP) data product.
NASA’s Scientific Visualization Studio/Kathryn Mersmann
Industry was expelling vast quantities of smoke, soot, sulfur dioxide, and other particles into the atmosphere. Researchers believe the pollutants blocked sunlight and counteracted greenhouse gases’ warming effects during this period. Aerosols are harder to model than greenhouse gases, however, so while they are the most likely culprit, scientists say that more research is needed to prove this hypothesis.
It is interesting that after 1975, as pollution declined, global drought patterns have been trending back closer and closer to the "fingerprint." The main takeaway from this study is that it provides evidence that human activities could continue to influence droughts in the future.
“If you look at the fingerprint, you can say, ‘Is it getting dry in the areas it should be getting drier? Is it getting wetter in the areas it should be getting wetter?’” Marvel said. “It’s climate detective work, like an actual fingerprint at a crime scene is a unique pattern.”
More about anthropogenic climate change, Greenhouse gas emissions, global droughts, last 100 years, tree rings
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