Three Storms, A Bucket, A Stress Test: 10 Years Later, What Tropical Storm Irene Says About Climate Change In N.H. Now
Nearly ten years before this weekend’s Tropical Storm Henri, New England was preparing for a far more severe hurricane-turned-tropical storm: Irene.
The 2011 storm brought record-breaking floodwaters to places like Plymouth, damaged homes and farms and destroyed infrastructure in the White Mountain National Forest, just downstream from the Hubbard Brook Experimental Forest in North Woodstock. Researchers there were analyzing the storm as part of a decades-long study of a changing ecosystem and its effects on people.
NHPR’s Annie Ropeik spoke with Lindsey Rustad and Peter Groffman, two top researchers at Hubbard Brook, about what they learned from Irene and how New Hampshire can become more resilient in the face of increasing extreme weather brought on by climate change.
This interview has been edited for length and clarity.
Take us back to 10 years ago, when Tropical Storm Irene rolled through New England. What was going on in the Hubbard Brook Experimental Forest?
Lindsey Rustad: I think the most important thing about Irene, it was really a tale of three storms. So you almost have to go back to the beginning of that August. It was kind of a typical August, and soils were relatively dry and they could have absorbed about five inches of water.
So I think of [the soils] as a bucket, and the bucket was, oh, I don't know, a quarter full. But then we had a big storm – the first one was on Aug. 14 to 16 and we got a lot of water and we started to fill up that bucket. And then we got a second storm on Aug. 21 to 22, and we filled that bucket up almost to the top.
So when Irene came [a couple of days later], we got about five inches of rain at Hubbard Brook and the soil water bucket was already at the top, so all of that rain just kind of flowed straight through the soils into the stream and caused just massive flooding. So it wasn't just Irene, it was the events that led up to Irene that ended up with the extreme flooding that we saw.
Peter Groffman: I love the flooding analogy. At Hubbard Brook, when the bucket fills with water, it then transmits the water downstream where it can really cause mischief in the bigger streams. So the real kind of physical degradation of streams and forests occurred farther downstream where you had a whole bunch of upstream areas contributing a bunch of water. And that's when things really started to come apart.
How did Irene compare to past storms that have hit Hubbard Brook, and to Tropical Storm Henri this past weekend?
Rustad: Irene was the 12th biggest rainstorm in [Hubbard Brook's] history – it was about five inches of rain. Our largest was seven inches back in 1973. And it led to the ninth greatest streamflow in our history. So it was by no means the largest storm that we've seen. And again, it just points to – it's not just the storm, but it's the events preceding the storm that really influenced what we saw.
During and after Irene, we really had to go out to our instruments, collect our data, and it took about a month for us to put all of our data together. Now we have real-time data. And during the storm yesterday, I was able to check on our instruments and see if it was raining or what the streamflow was. And I was able to tell yesterday in near-real-time that, nope, we really were not getting a big rainstorm at Hubbard Brook. We did not get much rain from Henri yesterday.
Groffman: The other thing with Irene is we did not have large numbers of trees falling. We did not have a major erosion of the size of the stream channel. It was a large flood, but you didn't get the physical degradation of the system, either of the stream banks or of the forest itself, that you saw farther downstream or that we get from windstorms, that you might expect to get from a hurricane.
Rustad: Though as you pointed out, further down in the White Mountain National Forest, it did take out bridges and roadways. Really, these storms are a tale of wind and rain and then the combinations. If we go back in time to the Great Hurricane of 1938 [which entered New England as a Category 3 storm], there was a lot of wind, and that just flattened large areas of the forest. And in fact, a lot of [Hubbard Brook's tree] stands date back to the hurricane of 1938. Irene was a really big water event, but there was not a lot of wind associated with it.
Some of the more devastating rain events that we have had at Hubbard Brook come in the winter. We're now having shallower snow packs, more exposed soil, and we're getting more rain-on-snow events. And at that point, you get this rain coming, falling on the frozen surface, and that's just cascading in the stream, bringing with it big chunks of ice and gravel and boulders. And those types of events are really changing some of our stream channels.
Irene was a summer tropical cyclone event. And I think the watershed did exactly what it was supposed to do.
What can Hubbard Brook’s response to extreme weather events like Irene teach New Hampshire about how to be more resilient in future storms as the climate changes?
Groffman: We're certainly having more of these extreme events due to climate change. We see extreme heat. We see extreme rainfall. And humans have a great capacity to recognize, OK, we had this event, what are we going to do next time to have less damage and a quicker recovery?
On the ecological side, we spent a lot of time thinking about the state of our forests and their vulnerability to disturbance. So in 1938, there was a lot of rain and wind. You wonder what was the condition of the forest that caused a lot of those trees to blow down. And when Irene came in 2011, we had an older forest, which you might think would create some increased vulnerability to the damage but that's not what we saw, so there was obviously quite a bit of resilience.
This is an active topic for us, but to me, when we think about these extreme climate events, we want to think about: what's the resilience of our natural systems, what's the resilience of our social systems, and what are the linkages between the resilience of the two?
"Forests are providing a very important service by absorbing lots of water... and if we're going to lose those forests, we're going to have to spend a lot more money on storm water controls and things like that."Peter Groffman
Rustad: If you go back to my bucket analogy, these forests have the ability to buffer our downstream ecosystems from a lot of this flooding. So I think part of my message is, it's important to keep forests as forests. Think about that bucket, right? It had that capacity to fill up, fill up, fill up, fill up. And in Irene, you know, eventually, you know, it overflowed over the top. But it would have been a lot worse if those water catchments weren't there.
Groffman: So when we think about our mixed-land use landscape, we just have to recognize that forests are big, deep buckets that absorb a lot of water. And so as we put more roads in, as we put more impervious surfaces in, then we end up with less storage and much faster transport. Water falls on a road and it gets into the stream right away instead of being stored or kind of meandering through the landscape.
So the forests are really providing a very important service by absorbing lots of water. It comes down to being careful about land use change and wetland protection – wetlands are really a big part of these of these buckets, and sometimes they get overlooked in our land-use planning – and if we're going to lose those forests, we're going to have to spend a lot more money on storm water controls, green infrastructure and things like that.
What else do you hope to learn from future extreme weather events at Hubbard Brook?
Groffman: We look forward to future events to see, has the system response changed? I like to use the analogy of a stress test. You go to a physician, you get a stress test for your heart as a way of learning, how healthy are you or how strong is your heart? These large events function as a stress test for our ecosystems. We know how to have a healthy ecosystem response to a stress test, so if we get a different response, we can use that to learn something about the health of the ecosystem.
That's the kind of long-term research that we do at Hubbard Brook. And there's some concern that there might be some erosion of resilience in the system – that the trees, the insects, the birds may be a little more vulnerable to some of these climate disruptions than they were decades ago [due to changes in acid rainfall, warming temperatures and rising greenhouse gas emissions].
Rustad: We are deeply interested in how extreme events are going to impact forests now and forests of the future. Many of us have been involved in the climate change debate really since the mid-1990s, and we've looked at those projections that we're going to see an increase in the frequency and severity of extreme events, but it always seemed like it was in the future. But that future is here. We are now seeing that increase in the frequency and certainly in the intensity.
One of the things that we as ecologists are looking out for are tipping points. So a forest is going to be damaged by, say, an ice storm. But is there a point that that ice storm is so big and so extreme that that forest is not going to respond to what it was before? A northern hardwood forest might be impacted and come back as a fern glade, where the dynamics are dramatically different.
And we are now beginning to be able to attribute the change in extreme events to a changing climate. So as we improve our models of how climate is changing now and into the future, we really need to understand how the forests are going to respond to and recover from the new normal of these extreme events.