A Wetter Wet Coast

Floods we get. Drought, we're not so sure.

A new study published in Geophysical Research Letters finds that the entire west coast of North America is in for a lot more rain - and flooding - caused by atmospheric rivers of the sort that's been hammering California.

It's a matter of physics.  Warmer temperatures increase evaporation. A warmer atmosphere holds more water vapour. A warmer, wetter atmosphere is more energized, powerful, and leads to the creation of atmospheric rivers.

From Climate Central:

Days on which atmospheric rivers reach the West Coast each year could increase by a third this century, if greenhouse gas pollution continues to rise sharply, Pacific Northwest National Laboratory researchers concluded after running model simulations.

Currently, the West Coast is likely to receive rain or snow from atmospheric rivers between 25 and 40 days each year, the analysis concluded. By century’s end, that’s expected to rise to between 35 and 55 days annually.

Meanwhile, the number of days each year on which the atmospheric rivers bring “extreme” amounts of rain and snow to the region could increase by more than a quarter.

The good news is that there's not a lot of level land along the coast. Unfortunately the exceptions include estuaries such as the Fraser Valley, including the densely populated Lower Mainland. Already susceptible to sea level rise and storm surge, heavy mountain runoff overwhelming the banks of the Fraser present another major flooding risk.

Does this mean that west coast droughts are solved? Not so much. Today we're seeing a new phenomenon sometimes called "flash droughts" or "hot droughts." These are destructive, seasonal droughts marked by an interruption of precipitation coupled with intense heat waves, conditions that can impair crop growth.

Recent research has suggested that higher temperatures linked to global warming exacerbated the intensity of California’s ongoing drought, by drying out the state. It’s far less clear what effect climate change had on the likelihood that such a drought would occur.

“The role of anthropogenic influences on the lack of precipitation is still an open question,” said Kevin Anchukaitis, a paleoclimatologist and earth systems geographer at the University of Arizona. “Different research groups have come to different conclusions.”

Rising temperatures are expected to accelerate evaporation and lead to drier conditions across the West — producing what scientists call hot droughts.

Anchukaitis said atmospheric rivers don’t necessarily affect the conditions that produce hot droughts.

But the “severity and duration” of droughts, Anchukaitis said, “will depend on a complex interplay between temperature increases, uncertain long-term precipitation trends and the punctuated role of drought-busting atmospheric rivers.”