(CNN) Human activity has probably had an impact on the world's risk of drought since the start of the 20th century, according to a new study, which also predicts that droughts related to climate change will get much worse.
This could come at a great cost. Each drought costs the United States about $9.5 billion, according to government statistics. It is the second most costly weather disaster, behind tropical cyclones. Droughts can drive up the cost of food, threaten drinking water, increase the risk of wildfires, cause mass migrations and even hurt people's health.
The research, published Wednesday in the journal Nature, finds that greenhouse gases generated by power plants, farming, cars, trains and human activities in general have influenced the risk of droughts.
The researchers found that droughts increased between 1900 and 1949, lessened between 1950 and 1975 and have been accelerating since.
Each of these periods seems to correspond with human activities. The drying trend at the start of the 20th century seems to be related to the increase in greenhouse gas emissions, according to the authors.
The period when droughts were fewer coincides with increased production of aerosols. Earlier studies have found that aerosols can affect rainfall and change cloud cover, but scientists caution that connection needs more research.
The authors of the new study also need more research to directly link the increase in drought toward the end of the 20th century with increased production of greenhouse gases. They believe that there is a link but want more evidence.
"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," study co-author Paul Durack, a research scientist at the Lawrence Livermore National Laboratory, told the Australian Science Media Centre. "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."
Researchers have had a hard time gauging how much effect human activity has had on droughts; some years, one region will get a drought, but another region will be hit in other years, complicating the records. Also, these records aren't nearly as detailed as scientists would like in order to draw large conclusions.
The researchers on the new study figured out an interesting workaround: They used modern models in combination with records from trees.
Trees are great weather monitors. The concentric circles inside can be used to tell how old a tree is. Scientists can also look at those rings and determine what the weather was like in a particular year.
If the line is wider, the year was warm. Trees don't grow as much in cold and dry time periods, so those years' rings would be skinnier. If the tree is stressed by the weather, like in a drought, it may not grow much at all.
With climate change and the modern increase in greenhouse gas emissions, the authors don't paint a happy future and instead see one that will experience many more droughts.
"The human consequences of this, particularly drying over large parts of North America and Eurasia, are likely to be severe," the study concludes.
John Quiggin, a fellow at the University of Queensland who has worked on climate science issues, would agree.
"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," Quiggin, who was not involved in the new research, said in a statement to the Australian Science Media Centre. "Without a radical change in both climate policy and water management, things will only get worse."