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Supercomputers, simulations, and the new science of extreme weather attribution

In 2017, Hurricane Harvey dumped 60 inches of rain on Nederland, Texas. That was over the course of a few days. Notoriously rainy Seattle gets about 38 inches a year. The storm caused over $125 billion worth of damage, according to the National Ocean and Atmospheric Administration. Was it just a bizarre event, or was it caused by climate change?

In the past, climate scientists have been hesitant to say any particular weather event, no matter how wild, was due to the effects of global warming, greenhouse gases, and other human causes. But Dr. Friederike Otto and the World Weather Attribution team studied Harvey and determined that climate change made the rainfall more intense, causing between 12% and 22% more water to drop on Houston and its surrounding area.

It’s a relatively new science, determining “whether and to what extent anthropogenic — so human-induced — climate change alters the likelihood and intensity of extreme weather events,” Otto told Digital Trends. In her new book, Angry Weather: Heat Waves, Floods, Storms, and the New Science of Climate Change, she explains how the World Weather Attribution project began and how its attribution reports can be used to help people sue greenhouse gas emitters.

She builds models without additional, human-contributed greenhouse gases in the atmosphere. Then she runs simulations over and over to see the likelihood of a particular drought or flood in the more pristine environment, versus today’s climate. If climate change made a heatwave three times more likely, then people can start looking at companies or institutions that caused the greenhouse gases of that event and try to hold them accountable. Though one such lawsuit was already dismissed, Otto thinks attribution reports are a crucial piece in getting governments to start changing their policies to create more carbon-neutral societies.

This interview has been lightly edited for length and clarity.

Digital Trends: Can we start with the difference between climate and weather because, for a lot of people, I think it’s confusing. 

Otto: So ultimately climate is average weather. So if you average the weather over a longer time, you end up with climate. If the climate changes because of either what we have now — that we have more greenhouse gases in the atmosphere, so we have a warmer atmosphere overall — that, of course, can also affect the weather. And it can do so in basically two ways. So we have a warmer atmosphere overall. That means that from warming alone, we see an increase in heatwaves and the likelihood of heatwaves overall and a decrease in the likelihood of cold waves on a global average.

Also, a warmer atmosphere can hold more water vapor. So that water vapor needs to get out of the atmosphere. Again, it does that as rainfall. So on a global average, we also have an increase in extreme rainfall. But apart from this so-called thermodynamic effect — so this warming effect alone — there’s a second effect and that is what we call the dynamic effect. And because we have changed the composition of the atmosphere, that also affects the atmospheric circulation — so how weather systems develop, where they develop, and how they move. And this effect can either be in the same direction; so, from the warming alone, you see an increase in heatwaves. But you might also get more high-pressure systems that lead to heatwaves developing. And so you would have an overall stronger increase in extreme heat in a certain place and season.

Doug Hurley/NASA

But of course, these two effects can also work against each other. So you might expect more extreme rainfall from the warming alone, but if you don’t get low-pressure systems bringing in water in the area, then you will have no change in rainfall. Or if [you lack dynamic] winds, you would have even a decrease in extreme rainfall. And because this second effect is very different from place to place, and from season to season, we have to look at individual locations and individual times of the year to figure out what climate change actually means today, on the scales where people live and where decisions are being made.

Can you talk a little bit about how you build that model of a world without climate change?

Yes. So I think what is important to say is that we don’t use just one model. We use several different ways to simulate a world that might have been without man-made climate change. And one method of doing that is just looking into past observations of weather and comparing that with observations of weather events today, and to see what the difference is in the real world. And of course, that is not a clean attribution to global climate change because other things than climate change have changed as well over the course of the last hundred years. So land-use changes have to have occurred, and so on. The observations are crucially important, on the one hand, to sort of have a ground-truthing of the models to evaluate whether the models are actually capable of simulating the type of event we are interested in. But, of course, because the observations have all the other changes as well. If we want to just have the attribution to man-made climate change, we need to use climate models.

And there, climate models are basically the same models that are used to do the weather forecast. They are built on the same principles, and we use them to simulate possible weather in the world we live in today. And so that gives us then an estimate of what type of extreme treatment event it is with climate change. So it might be that heatwave we are interested in is, in today’s climate, a 1-in-10 year event. And because we know very well how many more greenhouse gases have been added into the atmosphere since the beginning of the industrial revolution, we can take these out of the atmosphere in our climate models. And so our climate models are then simulations of the world that might have been without man-made climate change. And then again, we simulate what’s possible weather in that world, but without climate change. So it’s important to say that we don’t simulate what would happen if the industrial revolution would never have happened. The world is in all other aspects exactly the same as it is today. So we have the same sized cities and so on, but we just take the greenhouse gases out of the models’ atmosphere. And that allows us to disentangle what is man-made climate change and what might be other things that have changed, like land-use changes and so on.

You talked about having multiple models, and in the book you talk about having to run the simulations multiple times to take in different factors and other effects. Can you talk a little bit about that, and the role of citizen scientists?

When we do these studies, what we want to know is what is possible weather in the world we live in today. And so if we use observations, that shows us the weather that has actually happened. But from the observations alone, we don’t know if the event that has happened was a 1-in-1,000 year event, because we do not have a thousand years of observational data or whether it was a 1-in-10-year event, because the observations alone don’t allow us to do these kind of statistics. And so, therefore, we can’t run our climate model once or twice because then we would have the same problem as we have with the observation. Well, OK, in this model, this type of event has happened once. But that doesn’t tell us if it’s a rare event or not.

In order to find out what a rare event is and how rare it actually is, we need to run a model several hundreds of times to have lots of possible worlds in today’s climate and in the climate, as it might have been without manmade climate change. And therefore, we have to use as many model simulations and that can be done on supercomputers and in the big modeling centers in the world. So they do run the models 100 times. But that is very expensive and it still doesn’t allow you to really look at the very extreme events. And so what we here in Oxford have done instead is that we ask volunteers to run one simulation, so one possible trajectory of weather with a model on their personal computer. But we ask a lots of different people, so we have many, many thousands of volunteers who do that. And when we then put them all together, we have actually thousands of simulations of what is possible weather and can also look at a very rare event.

In the book, you say that there’s not a lot of hurricane experts; is it just because they are so complicated?

So hurricanes are not necessarily more complicated than other events, but they are just relatively small. So compared to, for example, a heatwave which is usually caused by a large blocking system over a large part of the hemisphere, a hurricane is only kilometers in its diameter. And that means that with the climate models that we have available, we can only use the climate models that have a very high resolution.

best hurricane trackers

In every weather and climate model, you have the world as a grid, so a bit like with the latitude and longitude grid, and you only calculate the weather at the intersections of the grid, so at the knots. And the closer these knots are together, the higher the resolution of the model, so the better you are able to simulate even small-scale events like hurricanes with the model, but also the more expensive the model is to run. And so with today’s technology and computing power, there are only a few models, a few centers in the world that can afford actually to run simulations to look into hurricanes. So that is why it’s much more difficult to get the right data to do these studies, and the same holds for observations because you can’t observe hurricanes, really, with weather stations because they form over the oceans and go over the oceans. And so there are no historic weather stations in the oceans. So we only really have good observations since the beginning of the satellite era. So compared to just extreme rainfall over land, just our databases, it’s much worse.

Can you talk about some of the satellites that are used in the work that you do, and what they’re measuring?

I only use satellite data in a very removed way. They’re not like the rain gauge or the weather station that actually measures the raindrops that fall on the area, but they do measure through the optical properties of clouds. They measure the thickness of the cloud and also the color of the cloud. And then that is then used to estimate rainfall. And so that means that they don’t measure rain directly, but just the likelihood of rain of this cloud, raining right now, but of course, looking from above at the top of the cloud.

And this is not error-free. The satellites have, of course, a much better way to look over the whole country and not just everywhere where a weather station is. They don’t measure rain directly, so in order to then translate these optical properties of the clouds into rainfall, you need to compare what comes out of the satellite with station data. So you have still the problem that the satellite measurements are really good in the areas where you do have good stations on the ground but maybe not so good in the areas where you cannot make these comparisons. So what you can do with satellites very well is to see the tracks and so on but then to estimate from that how much rain actually is falling is still not so straightforward.

Why are we starting to see hurricanes in areas where they weren’t previously common? Was Hurricane Ophelia, which affected Ireland in 2017, an example?

Hurricanes or tropical cyclones, in general, only form when you have very high sea surface temperatures. That’s why there are never hurricanes in the far north or far south because the ocean temperatures are way too low. But of course, in a warming climate, the areas around the tropics where these high surface temperatures are reached expand because we have overall warming of the globe, and that means that we do see hurricanes where we haven’t seen them before. And so one example that I described in the book, where actually from much further north in the Atlantic than usual, tropical cyclones still have so much energy that they had, still, tropical cyclone strength as far north as Ireland. And also where we have just seen that just last week again is it over the Mediterranean. So we have seen quite a few so-called “medicaines” in recent years, which are tropical cyclone-like storms over the Mediterranean, because we have now at the end of the summer season, these extremely high sea surface temperatures over the Mediterranean.

With the fire season in the Pacific Northwest, people are attributing it to climate change. Is that helpful, or do we need to wait for attribution studies to back up what feels like a clear link?

The meteorological weather conditions that are needed to have wildfires are a combination of high temperatures, low or no rainfall, low humidity, and extreme winds. And from all the studies that have been done so far, we see that there is an extreme increase in the likelihood of very high temperatures. So for high temperatures, climate change really is a game-changer, and that’s no exception in California. Everywhere in the world, the changes in extreme temperature are really orders of magnitude higher than any change you see in rainfall or humidity. Given that, climate change is increasing fire risk almost everywhere in the world, and that is also true for California.

Getty Images

If you want to know how much higher, exactly, you also need to look at all these other components, like wind and drought, which is not so straightforward, especially for wind and also for soil moisture. You have again the problem that there is not very good data. We have done a study, not for California, but we have looked at wildfires in Sweden, and in Australia. So very, very different places but [in] both found that, yes, climate change does exacerbate the likelihood of fire.

But of course, that is only one of the many causes that lead to wildfires. It is important to not just blame everything on climate change, even if climate change does play a role, because that also makes you a bit powerless, locally, if you think, ‘OK, there’s nothing you can do, because only if we if we stabilize temperatures at one degree that we have now, then we might have a chance.’ But there are also many other things, and forest management is certainly playing an important role as well. I feel, always, I’m squished between denial and doom, so it’s really difficult to say, ‘Yes, climate change is happening, but it’s not the only thing that is happening.’ And it’s not that because of climate change, we’re completely powerless to improve our situation. That’s just not the case.

You mention how your role is changing from just figuring out that you can do this kind of work to the next steps, with more targeted attribution and lawsuits over specific weather events. As people decide to rebuild after disasters, are they using your work to inform their decisions?

I think one very important aspect of these kinds of studies is that they link the projections of future climate change, which have been done for a long time, with current experiences in the real world, on the ground. And so basically, it’s bringing the theory back to the ground. While you could, in theory, of course, estimate, ‘OK, What kind of events is my city vulnerable to?’ and then, ‘OK, how can I decrease the vulnerability?’ But also, ‘What hazards are particularly dangerous for my city because of this vulnerability?’ and then look at specifically targeted climate change projections — and if you’re very lucky, you might find them.

But that’s usually not how humans work. We become aware of our vulnerability when something happens, and that opens a window of opportunity to actually change how we deal with different risks. And if you then have information available, how the different aspects of what turns weather into a disaster are changing, then you can actually target your resources to what the future might hold. And that is what ultimately the attribution studies are trying to do.

In the book, you say some of your colleagues were skeptical about your work at first. Have you seen a shift both in their attitude about your work but also maybe a wider shift in the way that people are talking and thinking about climate change?

I think there is definitely a shift in both. So in the scientific community, that’s a huge shift from, ‘Oh, you weirdos, this can’t be done,’ to ‘Oh, actually, this is a really interesting thing to do. Let’s join in.’ So we have many more scientists who now also do attribution studies. And, therefore, also much more of the information is available, which of course also helps to make this information more available outside of the scientific world. So the two things are definitely not disconnected. And I think a lot of the increase in scientists and climate scientists doing this has been driven by a huge demand from the general public or from the media. Because now, every time an extreme event happens, people do ask the question, ‘What’s the role of climate change?’ And before we did this work, the question was always answered with, ‘We can’t do this,’ but now there has been a realization that it’s actually possible.

And so now I think there’s a lot more reporting on the actual scientific evidence also in the general media, so that has definitely changed a lot. And that’s nothing to do with me or my work, but because of the last two years with the Fridays for Future movement, and we are talking today on the Global Climate Strike day. So that has certainly shifted the awareness of a large part of the population to the fact that climate change is actually happening today in our backyards and is not something that will happen at some point to someone else in the future.

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