An atmospheric scientist in Boulder says that half the world’s precipitation falls in just 12 days.

That measurement is part of a new study meant to quantify uneven precipitation and extreme weather events.

Simulations in climate models show a scenario that could be considered conflicting by some. On one side, drier climate with more extreme drought, and on the other side, more extreme storms with heavier precipitation.

This study is a new way to help clarify that conflict.

“Part of my aim with this study was to have something that everybody could kind of get their heads around,” said lead author Angie Pendergrass with the National Center for Atmospheric Research (NCAR).

Pendergrass has an expertise in global precipitation. She said she added all the days of precipitation in a year from 185 reliable weather stations over the course of 16 years (1999-2014) then sorted them from the heaviest days of precipitation to the lightest days, including the days with no precipitation. She then took the median across those different years and different locations.

“What you get out of that is, in the median, half of the precipitation falls in just the heaviest 12 days each year,” Pendergrass said.

Scientists have known for years that global precipitation is uneven, but even Pendergrass was surprised to find just how uneven it really is.

“And one of the reasons to be concerned about the unevenness of precipitation is that it’s kind of a driver for a lot of the impacts of precipitation events, so both on the flooding end and on the drought end,” Pendergrass said.

Pendergrass then ran climate model simulations and found that this uneven precipitation could get more unbalanced in a future climate.

“We found that precipitation will increase slightly on average and that increase will be even more uneven in the future,” Pendergrass said. “Meaning that most of that increased precipitation will fall on what are already the wettest days, and the days which are lighter with get even lighter.”

Her research found that by the year 2100, half the world’s precipitation will fall in just 11 days. Taking account for an anticipated increase in overall precipitation, that means more extreme single storm precipitation events, and more days with lighter or no precipitation.

“The magnitude of that increase is more than most of us expected,” Pendergrass said. “That’s one of the things that really concerns us a lot about climate change.”

She was able to run simulations in 36 of the world's most advanced computer models, and all but one showed this level of unevenness. And she said the one that didn’t was very close, giving her high confidence in her findings.