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A novel forecast looks at the size and fury of the storm with and without human-caused warming

A novel forecast looks at the size and fury of the storm with and without human-caused warming

FLORENCE'S FURY Hurricane Florence, as seen from the International Space Station on September 12, was so big two days before it was expected to roar ashore along the North Carolina coast that astronauts 400 kilometers above the storm had to use a super wide-angle lens to capture all of it.

Even as Hurricane Florence bears down on the Carolinas, bringing fierce winds and heavy rains, one team of scientists has undertaken a different kind of forecast: Understanding the influence of human-caused climate change on a storm that hasn't made landfall yet. Real-time storm forecasts continuously update as new data become available. But what would happen if, from a single starting point - in this case, the state of the atmosphere on September 11 - Florence roared ahead in two parallel worlds: one with and one without the influence of human-caused climate change? Florence would be bigger in today's world than if it occurred in a world with no human-caused warming, climate modeler Kevin Reed of Stony Brook University in New York and colleagues conclude in a study posted on the university's website September 12. And thanks to warmer sea surface temperatures and more available moisture in the air, it would dump 50 percent more rain on some parts of the Carolinas.

The goal of such climate change attribution studies is to determine whether - and by how much - human-driven climate change might have caused a particular extreme event, such as a hurricane, a heat wave or a flood. It's an increasingly high-profile area of research, particularly after three studies last year found that a trio of extreme events in 2016 simply could not have happened without climate change (SN: 1/20/18, p. 6). Until now, such studies have been conducted only when the event is long over. Reed and his colleagues got a jump on that question, conducting the first attribution study for an extreme event that is still in progress. It's not yet clear what role such real-time attribution studies might play in society; they could aid emergency planning, policy making and even climate-related litigation. In the meantime, what this study reveals is that "dangerous climate change is here now," says study coauthor Michael Wehner, a climate scientist at Lawrence Berkeley National Laboratory in California. "The chances and magnitude of dangerous extreme weather have already been significantly increased." Reed talked with Science News about what a forecast attribution study is, how the new study suggests climate change may have altered Florence's rainfall and size, and the future of real-time attribution. His responses are edited for space and clarity.

SN: We've heard many times that scientists can't say whether or not a particular storm was caused by climate change. How is this different? Reed: We're not making a statement about whether this storm is more likely due to climate change; that's not something we can do. We're doing forecasts of an existing storm, looking at the impacts of climate change on this storm that's already occurring. SN: OK, so how does that work? Reed: It's actually relatively simple. What we did was basically take a model that's traditionally used for climate forecasting and changed it to operate like a weather model. We started with the state of the atmosphere as it was on September 11. We run a set of weather forecasts, or ensembles, [that forecast out to] seven days. Then we go back to those initial conditions and remove the signature of climate change. SN: You describe doing that as "a counterfactual forecast of Hurricane Florence if it were to occur in a world without human-induced global warming." How do you create that world? Reed: We use climate simulations created by an international project called C20C+ that model the climate with and without human-created greenhouse gases over the last 100-plus years. That creates a climate change signal that we can remove from our initial conditions of the forecast. SN: So you ran these two scenarios to see how climate change has changed Florence's fate. What did you find? Reed: We see increased rainfall. In the areas of heaviest precipitation, for example, the ensembles with the climate change signal show that rainfall is heavier by more than 50 percent, compared with the simulations without climate change.

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