Minnesota Public Radio ‘Climate Cast’ meteorologist Paul Huttner recently discussed on his weekly broadcast how a warming planet could make ‘megafloods’ more likely, and examined how new FEMA flood maps will significantly expand flood zones.

An edited transcript of “Climate Cast” initially broadcast Tuesday, Feb. 5:

Kerri Miller, host of MPR News’ “The Daily Circuit” : I spotted this interesting article in Scientific American about how atmospheric rivers are driving more intense rainstorms and they predict that our changing climate will bring more flooding because of it. Make the connection for us.

Paul Huttner: It fits with the overall pattern of climate change. First of all, these atmospheric rivers — or what we might call the Pineapple Express, you can think of it as a fire hose — these moisture plumes that plow into the west coast of the U.S. bring heavy rainfall to California. We know that. Here’s the good part of it: they produce 50 percent of California’s annual rainfall. That’s a good thing.

Katie Jungers of Moose Lake, Minn. surveys a flood last June. ‘Megafloods’ like the one that hit northeastern Minnesota last summer could become more common as the planet warms. (Derek Montgomery for MPR)

But what’s not such a good thing is that every once in a while these atmospheric rivers, which are like a fire hose that kind of snakes around, it becomes stationary and it just blasts California with what we call a megaflood. These events have happened historically. In fact, there was a huge megaflood in 1861-1862, 43 days of rain that flooded the entire Central Valley in California. It killed thousands of people. The concern is that it’s due to happen again. They seem to happen every couple hundred of years.

Here’s the connection with climate change: more water vapor in the atmosphere (about 4 percent increase) may make these storms a little more frequent. We’re due for another one. The concern is this could happen in the near future in California.

Miller: They’re not actually rivers in the atmosphere, but they are supersaturated waves of air? What are they?

Huttner: They are sort of rivers in the atmosphere. The jet stream is the river of air in the atmosphere, we use that analogy, that guides storms. The jet stream focuses these moisture plumes so it really is sort of an atmospheric river. These are the things that occasionally spray California with these heavy rainfalls. The concern here with this sort of nightmare scenario, what they call California’s other big one, is that one of these will happen again and there’s historical precedent that it’s happened in the past.

Miller: Is it right to say that scientists never really understood how these rivers acted and what the effect was until 10 or 15 years ago?

Huttner: That’s right. Some of the diagnostic tools are very powerful that we have to animate moisture plumes in the atmosphere. The advent of satellites since the ’60s and ’70s has really opened a whole new world of research and really opened our eyes to be able to see these things. So you can actually see these plumes as they move throughout the Pacific and make their way into the west coast of the United States. That technology has increased greatly in the last 20 or 30 years.

Miller: Do we get the remnants sometimes of those storms?

Huttner: We do. In fact one of the big megastorms we had a couple of winters ago was a remnant of one of these atmospheric rivers. The climate change component is that the severity and the frequency of these could increase. If one of these occurred today, they’re talking about six million people in the path in California. Remember in that 1861-1862 event, Sacramento was under 10 feet of water. The whole Central Valley turned into an inland lake that was something like 300 miles long and 20 miles wide. So this kind of an event, if and when it occurs, could make Sandy or Katrina look like a walk in the park.

Miller: I want to make the connection here with these rivers and the moisture they carry and the flooding. I know that we’ve talked with Mike Dettinger. He’s the guy that puts it all together, and he describes why this is something we have to be concerned about.

Mike Dettinger (audio clip): If you took all the water vapor that’s being conducted along these atmospheric rivers and condensed it into liquid water, it would be the same as essentially seven to 15 Mississippi Rivers worth of water. Whether you believe in climate change or not, we know from sediments on the ground that in California and frankly up and down the west coast there are storms out there that happened in the past. They’re natural occurrences, they happen about every couple hundred years and we frankly should expect them to happen again. My look into the climate change projections suggests that, if anything, the odds of the really big storms showing up may actually increase.

Miller: The concern there then is that these large floods will happen more often than we’re accustomed to because of the way the climate is changing.

Huttner: We’ve talked about the documented increase in excessive rainfall events in the Midwest and Northeast. This is another angle on that. Yes, they’ve been naturally occurring events in the past, but the infrastructure of California has changed completely since the last time one of these events occurred. Climate change does make the extremes more extreme. It does produce more of these extreme rainfall events, and if something on this scale happens in the state of California, the economic catastrophe we’re talking about, maybe 25 percent of all buildings damaged.

These numbers come from a study, a simulation that they did called ARkStorm (Atmospheric River 1000). Some of the assumptions they made — and they even used a little weaker scenario than the historical storm of 1862 — are just remarkable if you think about that in terms of $725 billion in damage. That is just an incredible economic catastrophe potentially for the state of California and the country.

Miller: Question for you from James in St. Paul:

James (on phone): I’ve been listening to your discussion. One of the things I want to add about California is they have a large number of dykes and levees in and around San Francisco and Sacramento that are really in danger of collapse as they sit, never mind any pressure going forward. That makes me wonder why we concentrate so often on how to reduce or stop climate change, rather than how do we move forward and deal with the fact that it’s here and we’re stuck with some real consequences.

Huttner: Absolutely, James. That is well said. This ARkStorm study said that the kind of flood they would experience would just overwhelm the flood protection in California. They’re just designed for a 500-year storm. What he’s talking about is adaptation. In my opinion, that’s the reality of where we are right now with climate change in the U.S. The mitigation part — reducing some of the greenhouse gases — is a tougher road than adaptation. We learned this from the Duluth flood last year. I talked to Mayor Don Ness up there and they are adapting their storm water runoff for this increase in excessive rainfall events. James hit the nail on the head. Adaptation is really where we’re at right now with climate change.

Miller: But the other part of adaptation is not living in places that are going to — as FEMA can predict — clearly be in danger of being flood zones. So far, we don’t see a lot of retreat from these places, do we?

Huttner: No, we don’t. In fact, that’s timely as well because FEMA is releasing new flood zone maps in 2013, and they’re expanding these 10- and 100-year flood zones as these maps come out. That puts more property owners in an area that will have to buy flood insurance and it’s going to get more costly. In fact, [Governor] Chris Christie is requiring new building standards now in New Jersey. Folks are going to have elevate their homes or face much higher insurance premiums. This is the new normal with climate change, and it’s something we’re seeing happen on an annual basis.

Miller: Governor Christie and others have also suggested that at some point those insurance premiums for living in a place that’s vulnerable might just get so expensive that that’s the way to try to get people to pull back from those areas.

Huttner: It’s called disincentivizing living in these storm-vulnerable areas. It’s something the insurance industry is already reacting to. It’s something we’re going to see more of. There’s even a concept where we establish climate-safe communities in safer areas.

Editor’s note:  MPR News’ weekly “Climate Cast” and The Yale Forum have a content-sharing partnership under which edited transcripts of meteorologist Paul Huttner’s new weekly broadcast are frequently shared with Yale Forum audiences. The program airs weekly at 9:50 Central time, and is archived online.