“As (Helene) advanced through the stages of (low pressure to) tropical storm to category 5 hurricane, it was sitting overwater in the Gulf of Mexico that was anomalously warm. And it’s possible to estimate the likelihood ofthat warm water and — along the track of the hurricane or the track of the storm — and ask, ‘is it more likely to have been that warm water along that track as a result of climate change?’

“That’s one of the kinds of studies that is possible now, given the data record that wehave and the kinds of modeling that we can do. And so, we know that the water temperature alongthat track was 1.2, almost 1.3 degrees Celsius warmer than it would have beenwithout the effects of climate change.

“That area just has warmer temperatures as a result of climate change overall than the global mean.”

HP: There’s ample data to support that the planet is indeed warming, and there’s a link to stronger storms?

“We are able to estimate the amount of warming that’s happened globally, and over time in that region as well. When we take that analysis and then look at the track that the hurricane followed, you can say that the temperature along that track was almost 1.3 degrees Celsius warmer than it would have been otherwisegiven the same track, say, pre-industrial.

HP: And that is based ona bunch of sophisticated modeling that all comes together with that conclusion? And that modeling is based on straight temperature data? 

Yes. What modeling studies can add on top of that is an estimate of how much more likely it is that water was warmer as a result of climate change.

“And another way to look at it is, how certain are we of that? How much more likely is it that the water temperatures would be that warm as the storm is making this particular track at this particular time of year?

“The answer is, at the time that it turned from a low-pressure system into a tropicalstorm, it was 500 times more likely due to climate change to have those warm temperatures at that place at that time. And then, as it tracked north toward land, it was still on the order of about 200 times more likely that it would be experiencing those conditions as a result of climate change.

So, that just gives you some sense of how certain we are that the warm temperature anomaly is not just a random internal variability of the climate system, but rather something that’s caused by climate change.”

HP: At the time it turned from a low-pressure system into a tropical storm, it was 500 times more likely the waters were so warm due to climate change? Did I get that right?

“That’s right. So, what was more likely is that the water temperatures along that track would be that warm as a result of human-caused climate change.

“So, what that tells us is that in one out of 500 simulated approaches to trying to estimate what would happen with and without climate change, one out of 500 times you might see a storm like that … with that warm of water temperature at this time of year without climate change.

“And 499 times it’s going to be because of climate change. So, it just begins to put some probability bounds on not just the temperatures, but how likely is that temperature actually to be connected to human-caused climate change.”

HP: And again, please provide a really broad brush of that methodology, like a five-second elevator spiel.

“The broad brush is, this is based on evaluating models that can predict both the track of a storm like that and what the water temperature would look like with and without the additional warming caused by greenhouse gases in the atmosphere.

“The temperature part of it comes from the observations. And this likelihood assessment comes from looking at how models behave with and without climate forcing.”

HP: And to what extent is ORNL’s giant, super-fast computer used?

“In fact, for this (WWA) study — and I was sort of disappointed to see this, honestly, they picked some of the models from a recent modeling analysis called CORDEX, and our results were not in that analysis. And so, the E3SM Energy Exascale System Model was not part of that analysis. There are various parts of this analysis.

“Some they used 10 or more models. Some they had six models. But they had a pretty good collection of models from around the global community for this particular study.”

Rainfall extremes

“Given the amount of water that fell, it was three times more likely to have happened because of climate change. It could have happened without climate change, (but) if you didn’t have climate change, (it would be) three times less likely to have that kind of rainfall event.

“And in terms of the amount of water itself, what they can say is that it’s around 20 percent more intense. So, for that particular event, there was — and that’s a harder thing to estimate, so there’s bigger uncertainties associated with that — about 20 percent more intense rainfall because of climate change.

A water rescue under way in Swannanoa, N.C. during flooding of the Swannanoa River east of Asheville on Sept. 27, 2024.  Bill Rhodes Photo

“When you compare that to previous storms, going back to Katrina and Irma, and a collection of storms — it has ranged from a low of 4 or 5 percent for storms Katrina and Florence in terms of the rainfall amplification, that 20 percent number.

“Ian, in 2022, was estimated to be 22 percent more intense. And Harvey was up to — maybe up to, like, 38 percent more. So this is on the high end of that intensity metric for this particular storm.” 

HP: So, right now, modeling indicates that the intensity of the rainfall was 20 percent more?

“You could summarize that as 20 percent more rainfall over a given period of time ... 48-hour totals at the coast; three-day totals inland.”

The inland data “was from was northern Georgia and northern South Carolina and all of western North Carolina into western Virginia, parts of West Virginia, Kentucky, eastern Tennessee.

“That inland region was three times more likely to be hitting those rainfall totals.” 

Storm intensity increasing

“We know that, based on historical records and modeling, just kind of globally, there are more storms that have this higher intensity. They’re not — there’s not a global increase in the number of storms overall, but more of them end up in this higher-intensity category.

“And we do know that there is a (global) increase in extreme rainfall associated with storms in general.

“We know that these North Atlantic storms are moving more slowly (and thus able to deposit more rainfall on a given area). That’s part of the pattern. There’s more rapid intensification, (which) we saw in both of these recent storms. And the maximum intensity is also increasing.”

HP: So there is still a vocal cohort who, for political reasons or whatever, still try to sow disinformation and call climate change a hoax. It’s like arguing whether the sky is blue or why the sky is blue. What should I tell people to point out the fact that science is your friend?

“I would just say that this is not inherently a political issue. These are just facts, facts and interpretations of facts to the best ability of a range of experts from a range of disciplines.

“My approach is to just kind of put myself out there in terms of talking to groups and just lay it out the way it is and answer questions and just be honest about what we know and what we don’t know. I think that’s a critical piece of this: to not claim that we know things better than we do.

“But when we know things as well as we do, then we shouldn’t hesitate to say so. This is an example of a case where’s a lot of certainty in some of these conclusions, and we shouldn’t shy away from that.”