When days of heavy rain in late May caused deadly river flooding in France and Germany, Geert Jan van Oldenborgh got to work. Geert Jan is not an emergency responder or a disaster manager, but a climate researcher with the Royal Netherlands Meteorological Institute. With several colleagues around the world, he took on the task of answering a question about the floods, one that arises these days whenever extreme weather occurs: Is climate change to blame?
For years, most meteorologists and climate scientists would answer that question with a disclaimer, one that was repeated so often it became like a mantra: It is not possible to attribute individual weather events like storms, heat waves or droughts to climate change. But increasingly over the past decade, researchers have been trying to do just that, aided by better computer models, more weather data and, above all, improved understanding of the science of a changing climate.
Attribution studies, as they are called, can take many months, in large part because of the time needed to run computer models. Now scientists like Geert Jan, who is part of a group called World Weather Attribution, are trying to do such studies much more quickly, as close to the event as possible. “Scientific teams are taking on the challenge of doing this kind of analysis rather rapidly,” said Peter A Stott, who leads the climate attribution group at the Met Office, Britain’s weather agency.
Sound scientific analysis
The goal is to get sound scientific analysis to the public to help counter misinformation, deliberate or otherwise, about an event. “It’s worthwhile to give the best scientific evidence at the time, rather than not saying anything and letting others say things that are not related to what really happened,” said Friederike Otto, a researcher at the University of Oxford who works with Geert Jan.
In the case of the European floods, World Weather Attribution, which is coordinated by Climate Central, a climate-change research organisation based in Princeton, USA, released a report less than two weeks after the Seine and other rivers overtopped their banks. The group concluded that climate change had made the French flooding more likely, but could not draw a conclusion about the flooding in Germany. “In the French case, we had five almost independent measures, and they all agreed. With Germany, we only had two, and they disagreed,” Geert Jan said.
Studies have shown that the effects of climate change are occurring on a broad scale. The National Climate Assessment, for instance, notes that heavy downpours have increased across most of the United States in the last 25 years.
But analysing individual events is problematic, largely for two reasons: weather is naturally variable, even without climate change, and global warming may be only one of several factors influencing a particular event. Since reliable data is required, studies are also less likely to be undertaken for events in countries that lack much data-collecting infrastructure, or where governments do not share data widely.
Over and over again
Attribution studies usually involve running climate models many times over. Because no model is a perfect representation of reality, varying them slightly for each run and averaging the results give scientists more confidence in their accuracy. One set of runs simulates the climate as it actually is, incorporating the effects of greenhouse gas emissions, while the other set simulates the climate as if that human influence had never happened. Researchers then compare historical data, as well as data from the actual event, such as rainfall or temperatures, to the different model results to assess any climate-change impact. The analysis is usually given in terms of probabilities, or increased or reduced likelihood, that climate change had an effect.
Rather than running models after an event, researchers like Geert Jan and Friederike shorten the process by using models that have already been run. “The only way we can do this rapid attribution is by precooking everything that we can,” Geert Jan said. The process starts with emails among members of the group, usually followed by a Skype session to discuss whether a specific event is worthy of study.
Among the models they use is one that Friederike’s group at Oxford, the Environmental Change Institute, runs regularly, using the personal computers of a large number of volunteers, a project called climateprediction.net. While the model is a global one, regional results can be extracted and used for the rapid analysis.
Geert Jan said that the European flooding analysis was a good illustration of the need for transparency, because while the researchers were confident in their findings for France, they acknowledged that they could not draw conclusions regarding Germany. “We have to make sure we’re open-minded enough to conclude that, although we’ve spent a lot of time on this, we can’t conclude anything,” he said.