Scientists have reconstructed North American drought history over the last 1,000 years and found that the drought of 1934 was the driest and most widespread of the last millennium.
Using a tree-ring-based drought record from the years 1000 to 2005 and modern records, scientists from NASA and Lamont-Doherty Earth Observatory found the 1934 drought was 30 per cent more severe than the runner-up drought (in 1580) and extended across 71.6 per cent of western North America.
For comparison, the average extent of the 2012 drought was 59.7 per cent.
"It was the worst by a large margin, falling pretty far outside the normal range of variability that we see in the record," said climate scientist Ben Cook at NASA's Goddard Institute for Space Studies in New York.
Two sets of conditions led to the severity and extent of the 1934 drought. First, a high-pressure system in winter sat over the west coast of the US and turned away wet weather - a pattern similar to that which occurred in the winter of 2013-14.
Second, the spring of 1934 saw dust storms, caused by poor land management practices, suppress rainfall.
"In combination then, these two different phenomena managed to bring almost the entire nation into a drought at that time," said co-author Richard Seager, professor at the Lamont-Doherty Earth Observatory of Columbia University in New York.
"The fact that it was the worst of the millennium was probably in part because of the human role," Seager said.
According to the recent Fifth Assessment Report of the Intergovernmental Panel on Climate Change, climate change is likely to make droughts in North America worse, and the southwest in particular is expected to become significantly drier as are summers in the central plains.
"We want to understand droughts of the past to understand to what extent climate change might make it more or less likely that those events occur in the future," Cook said.
The abnormal high-pressure system is one lesson from the past that informs scientists' understanding of the current severe drought in California and the western US.
"What you saw during this last winter and during 1934, because of this high pressure in the atmosphere, is that all the wintertime storms that would normally come into places like California instead got steered much, much farther north," Cook said.
"It's these wintertime storms that provide most of the moisture in California. So without getting that rainfall it led to a pretty severe drought," Cook said.
The study was published in the journal Geophysical Research Letters.
