When days of heavy rain in late May caused deadly river flooding in France and Germany, Geert Jan van Oldenborgh got to work.

Dr. van Oldenborgh 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.