Will extreme precipitation intensify in the Northwest under global warming?

The answer appears to be yes.

In several of the past blogs I have attempted to damp down some of the hype and exaggeration regarding the past and current impacts of human-induced increases in greenhouse gases here in the Northwest.

But global warming is a serious issue for mankind and the impacts are going to be substantial even here in the Pacific Northwest, particularly during the second half of this century.

One major impact that is relatively certain:  a substantial increase in the heaviest precipitation.

My group at the UW has worked on this issue intensively during the past few years, with several papers in the peer-review literature.   Much of this work has been done by one of my graduate students, Michael Warner, who completed his Ph.D. yesterday (see his picture below!).  Let me tell you some of his results.

The first thing to keep in mind is that the heaviest precipitation events west of and over the Cascade crest are inevitably associated with atmospheric rivers, relatively narrow currents of warm, moist air streaming out of the tropics or subtropics (see image).     And when these currents moisture are forced to rise by our regional mountains, they can dump huge amounts of precipitation.  Like a foot or more in a day.

So the future of  heavy precipitation in our region really depends on what will happen to atmosphere rivers.  Will they get strong or weaker, change structure, move northward or southward under global global warming?

Those are the kinds of issues that Mike Warner, my group, and others have dealt with.

A very good measure of the ability of an atmospheric river to produce heavy precipitation in our region is the rate at which moisture is moved toward our coast.   Makes sense right?  More moisture moving in, means more rain.  We have a fancy term for such moisture transport:  IVT, Integrated Vapor Transport.  Mention this at a cocktail party and your friends will be impressed.

In one part of Mike's Ph.D. thesis he examined how such moisture transport changes in time along a line of points right off the West Coast (see graphic).

To estimate the future, he made use of ten of the latest global climate runs (CMIP-5) made by groups all over the world.  Below are some of the results from our recently accepted peer-reviewed paper on this topic.  The left panel shows you the contemporary situation (1970-1999) and the right side is the future prediction from the climate models (which assume that mankind will continue to inject greenhouse gases into the atmosphere at the current rate...a good bet).

These figures show how IVT varies with latitude during the winter season.  The bottom lines represent the winter average precipitation.  The top lines represent the top 1% of precipitation events (these are the atmospheric rivers).  The green lines are based on observations (from the National Weather Service analysis), while the other lines are based on the ten models, with the darkest line being the mean of the ten models.  Blue lines contemporary period, red lines the future.  You can see the variation among the climate model by the spread of the light lines.

So what does this tell us?   For the contemporary period, the climate models do a pretty good job in duplicating the winter average and extreme precipitation...an important check on the climate models. If they differed from reality, we might not trust their ability to predict the future!

Now, let's look at the right panel, which shows the future (and the contemporary value are plotted as well with blue lines).  The most important thing is that there is a big increase of water vapor transport in the future (26-30%) for extreme events.

The implication is that our strongest atmospheric rivers, like the ones that occurred in November 2006 or November 2009, could be nearly a third stronger by the end of this century.  And precipitation should scale with this moisture transport, so we can expect proportionally more rain.  And clearly more flooding, landslides, and the other impacts of heavy precipitation.

One interesting finding of Mike's research is that the winds--the jet stream--hardly change by the end of the century.   And the structure of the atmospheric rivers doesn't change.  This is shown by this figure where you can see the contemporary (lines) and future (colors) structure for atmospheric rivers influencing one point on the coast...they are virtually the same.

So in the future we will have atmospheric rivers that are very much like today's, except for one thing:  this will have much more moisture, much more moisture transport, and thus precipitation over us.   

Why will the future atmospheric rivers have more moisture?  That's easy.  Because temperatures will be warmer and the atmosphere will have more moisture in it since the amount of water vapor air can "hold" is directly related to temperature.  Warmer air can hold much more water vapor.

Are there uncertainties in such future predictions?  Of course.  Perhaps the models are too aggressive with the warming, in that case the impacts shown above would be delayed years or decades.

But we can be very confident that increasing greenhouse gases will warm the world and that this will lead to more water vapor in the atmosphere.  And that will enhance our atmospheric rivers.  You can bank on this.

If atmospheric rivers are going to intensity and precipitation is going to get more extreme under global warming, we need to lessen the numbers of homes and businesses in the flood plains of local rivers.  This Arby's wa not selling many roast-beef sandwiches that day.

If we are not going to stop global warming...and some warming is virtually inevitable at this point...we need to think about how we will adapt.  In this case, we should plan for heavier precipitation, ensuring our dams can handle the extra load and making sure people are out of harm's way (like moving folks away from rivers).  Improved long-range forecasting will be a big aid in strategically lowering dam levels and getting vulnerable folks to safe locations.  We can deal with these changes, but it will take some planning and investment in infrastructure.

Dr.  Michael Warner, whose research is described above.