JEFFREY BROWN: The storm in Joplin was preceded by a series of tornadoes this spring that has brought devastation to the South and Midwest. All together, it's been the deadliest season since 1950, with more than 520 people killed so far.
All of this has led to many questions about what's behind this - what is happening this year.
Judy Woodruff explores the science.
JUDY WOODRUFF: So, how unusual are the tornadoes and other weather we have been seeing in the U.S. this spring, and what might explain it?
For that, we turn to two weather specialists. Jeff Masters is co-founder and director of meteorology of the Weather Underground, a weather-tracking website. Katharine Hayhoe is a professor and climate scientist at Texas Tech University. She was a member of the review team that studied the work of the U.N. Panel on Climate Change.
And we thank you both for being with us.
Jeff Masters, let me begin with you.
You have been studying meteorology for over 30 years. Just how much more severe are the storms, the tornadoes, we have been seeing this year than in the past?
JEFF MASTERS, Weather Underground: We have never seen a year like this before.
It started off in mid-April, on the 14th. We had a swarm of tornadoes hit the South and then the Southeast. Over a three-day period, we had 162 tornadoes. And that was an all-time record for most tornadoes in a tornado outbreak. We have only ever seen one outbreak similar. Back in 1974, we had 148 tornadoes in one outbreak.
So, that outbreak was followed just two weeks later by the most incredible outbreak we have ever seen: 362 tornadoes in a four-day span, including the Tuscaloosa-Birmingham tornado, which was probably the most damaging tornado of all time - so, more than double the previous record, which was set just two weeks before that.
And on top of this, these storms dump tremendous amounts of rainfall, the heaviest rains ever recorded in April over the Ohio and Mississippi Valleys, which helped contribute to the Mississippi River floods we're seeing.
And then, after those two outbreaks...
I'm sorry. Go ahead, Judy.
JUDY WOODRUFF: I was just going to say, you said there's been a string of these severe incidents, the death toll higher than it's been in a very long time.
Is there a consensus among meteorologists about why this is happening?
JEFF MASTERS: As far as the death toll goes, we just got unlucky. We do see incredibly violent tornadoes fairly regularly. Every few years, we see an EF-5 tornado with 200-mile-an-hour winds. But if one of those storms happens to track over a populated area, then we see some of these incredible death tolls.
JUDY WOODRUFF: Katharine Hayhoe, let me bring you in here.
There's been a lot of question about - questioning about whether the severe weather, these records that are set have a connection to climate change. How do you see that, as somebody who had studied climate change?
KATHARINE HAYHOE, Texas Tech University: Whenever we see a season like we're having right now, it's a natural part of being human to say, is there a pattern to it?
And so, of course, that's what we're asking right now: Is there a pattern to all of the weird weather that we have been seeing this spring? Unfortunately, at least for those of us who want a pattern immediately, we can't tie any one event or even one season to climate change.
Climate is the average statistics of weather over at least 30 years. But what we can do is, we can add this season to the books, and we can start looking at whether we see any trends in heavy rainfall events, in droughts and in tornadoes.
When we do that, we do see trends in some things. We see trends in heat wave frequency and severity in many places around the world. We also see increases in heavy rainfall events across the entire U.S., especially in the Midwest and the Northeast.
But when we look at the tornado record, we don't see any conclusive trends in tornado numbers or severity yet. So, we don't know if, as Jeff said himself, if this is a fluke this year or if it's the beginning of a new trend. It's too early to say.
JUDY WOODRUFF: Jeff Masters, what about that possible connection to climate change? Is it too early to say?
JEFF MASTERS: Well, absolutely.
And we have a problem with the tornado record. It's very hard to measure tornadoes. We can't put wind measurement instruments into them. So we have to indirectly infer their strength by if they happen to hit a building and knock it down. Then you can say, well, this tornado probably had 200-mile-an-hour winds - so, very tough to measure and very tough to figure out if tornadoes are changing with time. And our measurements only go back about 60 years, which really isn't long enough to see if there's a climate trend or not.
JUDY WOODRUFF: Katharine Hayhoe, what about all the flooding we have seen in the Midwest, the South, the Mississippi River, and that entire basin? A connection there? Questions are also being raised about that, you know, the warm air, excess moisture. Possible connection to climate change or not?
KATHARINE HAYHOE: Again, we can't tell what's happening in the Mississippi Basin specifically to climate change.
One event or one region is not enough. But we can look around the whole U.S., and we can look around the whole world. And when we do that we see that we have experienced a significant increase in heavy rainfall events that often do lead to flooding.
This is happening not just here in the U.S., but around the world. Not only that, but that increase has been connected, quite definitely, to climate change. In other words, we wouldn't be seeing an increase as big as we have over the last 50, 100 years if it wasn't for climate change.
JUDY WOODRUFF: So, draw that line for us. What is the connection?
KATHARINE HAYHOE: As it gets warmer, the air can hold more water vapor. So whenever a storm comes through, there's more water available to that storm, whether it's rainfall in the summer or even snowfall in the winter.
We're also seeing shifts in our weather patterns and circulation patterns. So, some places that are already quite dry are getting dryer. Other places that are already quite wet are getting wetter. And some places can even experience increases in heavy rainfall events and droughts at the same time, because if a lot of the water vapor comes down in a few storms then you have a longer dry period in between before you get the next one.
JUDY WOODRUFF: Jeff Masters, as somebody, again, who watches this closely, what would you add to that? And what are you most curious about as you look at these patterns of severe storms and unusually heavy flooding?
JEFF MASTERS: The thing to think about with flooding is that, yes precipitation has increased by 5 percent or more over the last 50 years in the U.S., but flood heights are getting higher not just because of higher precipitation. They're also getting higher because of human-caused changes to floodplains.
We're draining more floodplains. We're putting more water behind levees. We're engineering the levee systems so that we can improve navigation. But those little improvements we do cause the flood heights to go even higher.
So, we need to give the rivers room to expand. We need to have more of these safeguards where you can let the river out through some of these spillways, like we have had to do this year. That was a very smart idea, to open these three spillways that helped the Mississippi go out and not be such at high flood levels.
So, we need to give the river more room. We have got to stop developing our floodplains and putting people in harm's way.
JUDY WOODRUFF: And on that note, we thank you both, Jeff Masters and Katharine Hayhoe.
We appreciate it.
JEFF MASTERS: You're welcome.
KATHARINE HAYHOE: Thank you, Judy.