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.

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.

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.

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