NASA Discovers the Worst North American Drought in History

Scientists may have just pinned down the worst drought in North American history. They've examined the last 1,000 years and found that the drought of 1934 was the driest and most widespread of the last millennium.

In order to pin down the severity of the drought, the scientists used a tree-ring-based drought record from the years 1000 to 2005 in addition to modern records. This revealed that the 1934 drought was 30 percent more severe than the runner-up drought, which occurred in 1580. In fact, this drought extended across 71.6 percent of western North America. For comparison, the 2012 drought extended just 59.7 percent.

So what caused the 1934 drought? There were two sets of conditions in play. First, a high-pressure system in winter sat over the west coast of the United States and turned away wet weather; it's a pattern that's similar to the one the occurred in the winter of 2013 to 2014. In addition, the spring of 1934 saw dust storms that were caused by poor land management practices and helped suppress rainfall.

"In combination then, these two different phenomena managed to bring almost the entire nation into a drought at that time," said Richard Seager, co-author of the new study, in a news release. "The fact that it was the worst of the millennium was probably in part because of the human role."

Yet it's not just the past that we have to worry about. According to the recent Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), climate change is likely to make droughts in North America worse. In fact, the abnormal high-pressure system seen in the past may reveal a bit more about the current severe drought in California and the western United States.

This type of high-pressure system is part of normal variation in the atmosphere. Whether or not it will appear in a given year is difficult to predict. Yet a comparison of weather data to models looking at La Niña effects showed that the rain-blocking high-pressure system in 1934 overrode the effects of La Niña for the western states, which then dried out areas.

"We found that a lot of the drying that occurred in the spring time occurred downwind from where the dust storms originated, suggesting that it's actually the dust in the atmosphere that's driving at least some of the drying in the spring and really allowing this drought event to spread upwards into the central plains," said Ben Cook, climate scientist.

The findings reveal the importance of paying attention to the changing climate. Agricultural producers in particular should adapt accordingly and look to the lessons of the past.

The findings are published in the journal Geophysical Research Letters.