Though there have been many extreme droughts over the last 1,200 years, only three other events have been of similar magnitude, all during periods of "megadroughts".
In 2009, an important analysis of lake-bed sediments in Ghana revealed a millenniums-long record of extreme droughts in West Africa, some a century long.
This year's reports of record melting of the earth's ice sheets and extreme droughts have given a new urgency to Bartholomew's messages about the degrading natural world.
The concern that continues to grow in the wake of the terrifying fires, extreme droughts, and devastating storm surges people are living through.
There will be more extreme droughts, with the length of droughts increasing by between 5% and 20%, depending on how quickly greenhouse gases are cut.
"Showing that the most extreme droughts in the Southwest have predominantly occurred during warmer intervals in our past raises important questions about what we should expect in an even warmer future," Mr. Routson said.
Australia is currently experiencing an El-Nino weather pattern, a phenomenon associated with extreme droughts, storms and floods, which is expected to become one of the strongest on record, according to the UN weather agency.
Evidence of the human influence on climate is with us already, Dr. Trenberth noted, in terms of a general trend of more severe storms, extreme droughts, heat waves and wildfires.
In extreme droughts, the plan would continue a provision announced last fall allowing river flows below the current minimum of 5,000 cubic feet a second at the Jim Woodruff Dam near the Florida-Georgia line.
While that extraordinary drought occurred during a cool spell in the Northern Hemisphere called the "little ice age," other sustained extreme droughts appear to have hit West Africa at points when the world was relatively warm over all, they reported.
Richard Seager, a climate scientist at the Lamont Doherty Earth Observatory of Columbia University who has studied past extreme droughts in other dry areas, including the American Southwest, described the century-scale droughts revealed in the lake mud as "startling".
