Saturday, October 4, 2014

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.




21 comments:

Christopher Herndon said...

Global Warming? nonexistent. It's called climate change and it has been going on since the earth was created. It makes no sense to be saying "global warming" when only LOCAL warming is occuring in some areas, and local COOLING in many others.

Unknown said...

I've lived in Seattle since 1982. My unscientific observation is that in the last 10-15 years the incidence of hot, dry periods in the summer have increased, and winters have shifted from extended periods of grey drizzle and relatively short squalls, to short periods of extended, very heavy rains.

I've also heard from old-timers that it used to get cold enough in the winter that Greenlake would freeze and people would skate on it. When was the last time THAT happened?

Unscientific, but seems like a trend to me, including the northward incidence of birds not commonly seen here, like the scrub jay (former limit northern Oregon) and the brown pelican.

Jack Bloss said...

Christopher, what credentials do you have that allow you to make that claim?

Weatherfreak said...

Cliff, you seem to spend a lot of blog time talking about AGW and how it is going to be most evident 50 years from now. Everything with AGW seems to be "down the road" or "by mid century". I am not a blind skeptic but do have serious questions about the actual scientific proof regarding how a trace gas, absolutely essential for life on earth, is going to in the end destroy us! At least if you listen to some on the left... Would love to have you blog sometime about exactly how increased CO2 will warm the planet and whether you believe natural variables could play a role in limiting the warming. You seem to be very adamant and I would love to know exactly why, minus the spin we get in the news by those that have no clue about climate science.

Michael Snyder said...

Christopher Herndon-

The planet is warming as a whole, hence global warming.

Bonus- Each time the climate has changed in the past , it has changed for a reason.

Guess what that reason is this time around?

Trisha Long said...

Congrats to Dr Warner! What an accomplishment!

JewelyaZ said...

Congrats, Dr. Warner, and thanks very much, Cliff, for highlighting the excellent work of your students. It is much appreciated.

Once again, I'm glad we live on a (so-far) stable hilltop. :-)

Christopher Herndon said...

Jack Bloss and Michael Snyder-

First, I should ask you, what evidence do you have to suggest the climate as a WHOLE is warming? Don't trust NOAA statistics as they are measured only at certain sites typically around cities with more thermal mass than rural areas. Also, think about how easily these statistics could be "fixed" to suit a global warming, excuse me "Climate Change" agenda.

To prove my point?

Antarctic sea ice is at near record levels and arctic sea ice is above normal as well.

Besides, I don't worry much because we all know God is in control and He will take care of the earth's climate.

Fixed Carbon said...

Nice! Science. Clearly presented.

Cliff Mass said...

Chris Herndon,
The term global warming is not precise... you are right. Although the planetary temperature will warm, it will not be uniform--some places will warm more rapidly then others.
That the earth will warm when you add greenhouse gases is beyond debate. It is basic physics. If you put on more blankets you warm...it is as basic as that. Most of the warming is ahead of us and a good year or period for the Arctic or Antarctic--or a two-decade pause in warming--- does not mean that the earth won't warm as greenhouse gases increase.

...cliff mass

Cliff Mass said...

Chris Herndon,
The term global warming is not precise... you are right. Although the planetary temperature will warm, it will not be uniform--some places will warm more rapidly then others.
That the earth will warm when you add greenhouse gases is beyond debate. It is basic physics. If you put on more blankets you warm...it is as basic as that. Most of the warming is ahead of us and a good year or period for the Arctic or Antarctic--or a two-decade pause in warming--- does not mean that the earth won't warm as greenhouse gases increase.

...cliff mass

codetalker said...
This comment has been removed by the author.
codetalker said...

Leading Climate Scientist Defects: No Longer Believes in the 'Consensus'

http://www.breitbart.com/Breitbart-London/2014/05/08/Leading-climate-scientist-defects-no-longer-believes-in-the-consensus

Patrick Wright said...

Christopher Herndon,
Regarding your comment concerning the location of NOAA sites. Have a look at the Mauna Loa site (one of our best long-term records of atmospheric CO2), as well as Trinidad Head, American Samoa, Barrow Alaska, South Pole Station, and Summit Station, Greenland. All of these sites are specifically located to NOT be near urban centers. They are situated as to sample "background" atmospheric composition, often transported long distances over oceans or ice sheets before reaching the observatory.

Jack Bloss said...

Christopher, I'm necessarily trying to disprove you. I don't really have to means to do so. I'm not knowledgeable enough but I'm curious as to whether you are either. Amen to God by the way. I agree with your last statement. But first off NOAA weather gauges are usually at airport runways away from the city. They also have them at buoys and on many outlying areas around the state. I visited one and let me just say it's nowhere near a city. Many in fact are in the mountains and coastal areas. Lets just say they get a good spread of data. What little I know about the Antarctic ice is that the ozone is disappearing there, strangely, and causing cooler temperatures to form there. At the same time though, the ocean temperatures have been increasing faster there than almost anywhere in the world. So what causes increased ice. Well with less ozone there is less heat being trapped in the stratosphere. Also winds are becoming more cyclonic causing ice to move around more causing something called polynyas Which are open areas of water. More polynyas causing more freezing. Also, although the ocean there is heating it is heating below the surface due a number of recent changes that could even be attributed to global warming or climate change. To make assure that I am speaking with little prior knowledge here is an article that I got most of this info from http://www.skepticalscience.com/increasing-Antarctic-Southern-sea-ice-intermediate.htm. The areas that are dropping in temp are few compared to the rising. And most temperature gauges are not in cities but outlying areas or on runways. Peace be with you.

Dan McShane said...

Thanks for the new information and model work update on atmospheric river projections. Important stuff for policy. Great work Dr. Warner and the UW team.

Placeholder said...

Cliff, as you know, the issue with the global warming hypothesis has to do with the feedback estimates.

There is now overwhelming evidence that the AGW believers have vastly overestimated the feedbacks. As a result, the increases in global temperatures predicted 20 to 25 years ago simply have not occurred.

In fact, the question is now whether or not, given that the feedbacks were so radically overestimated, other natural processes greatly outweigh any change in CO2.

As for your grad student's work, I'd ask this: Did he, or you, backtest it?

Cliff Mass said...

Placeholder,
What is your basis for saying that feedbacks were overestimated? Give me a few papers....

The bigger issue is natural variability versus the greenhouse gas signal.

What do you mean by "backtest."? If you mean comparing the models against contemporary values...we did that and the climate models did well...cliff

Lorenzo said...

Seriously, forecasters can only predict measurable precipitation yes/no probabalistically tomorrow, but know with certainty how much it will rain half a century in the future?

I could be convinced, but it will take proof stronger than "because the Koch Brothers pour money on science deniers you ignorant knuckle dragger" or "some guy with a computer and a PhD says so, therefore shut up".

Rob said...

Weatherfreak said... "I am not a blind skeptic but do have serious questions about the actual scientific proof regarding how a trace gas, absolutely essential for life on earth, is going to in the end destroy us!"

Clearly you are not a skeptic, otherwise you'd have already read the research and understood these issues.

P Aronson said...

Tell it, Cliff!