Xubin Zeng, University of Arizona
Extreme rainfall and flooding have left traces of destruction in communities around the world this summer. The latest was in Tennessee, where preliminary data shows a shattering record 17 inches of rain fell within 24 hours, turning streams into rivers that inundated hundreds of homes and killed at least 18 people.
Many people ask: was it climate change? Answering this question is not that simple.
There has always been extreme weather, but man-made global warming can increase the frequency and severity of extreme weather. For example, research shows that human activities such as burning fossil fuels unequivocally warm the planet, and we know from fundamental physics that hot air can hold more moisture.
Ten years ago, scientists were unable to confidently link an individual weather event to climate change, even though the broader trends in climate change were clear. Today attribution studies can show whether extreme events have been affected by climate change and whether they can be explained by natural variability alone. With the rapid advancements in research and the increase in computing power, the attribution of extreme events has become a new and booming branch of climate science.
The latest attribution study, published on August 23, 2021, looked at rainfall from the European storm that killed more than 220 people in floods that swept through Germany, Belgium, Luxembourg and the Netherlands in July. 2021.
A team of climatologists from the World Weather Attribution group analyzed the record-breaking storm, dubbed Bernd, focusing on two of the worst affected areas. Their analysis found that human-induced climate change made a storm of this severity between 1.2 and 9 times more likely than it would have been in a world that was 1.2 degrees Celsius colder (2, 1 F). The planet has warmed by just over 1 Â° C since the start of the industrial age.
Similar studies have yet to be conducted on the Tennessee storm, but they likely will.
So how do scientists understand this? As an atmospheric scientist, I have participated in attribution studies. Here’s how the process works:
How do attribution studies work?
Attribution studies generally have four stages.
The first step is to define the magnitude and frequency of the event based on the observational data. For example, the July rainfall in Germany and Belgium largely broke records. Scientists have determined that in the current climate, a storm like this would occur on average every 400 years across the region.
The second step is to use computers to run climate models and compare the results of those models with observational data. To be confident in the results of a climate model, the model must be able to realistically simulate such extreme events in the past and accurately represent the physical factors that contribute to these events.
The third step is to set the baseline environment without climate change – essentially creating a virtual world of Earth as it would be if no human activity had warmed the planet. Then run the same climate models again.
The differences between the second and third stages represent the impact of human-caused climate change. The final step is to quantify these differences in the magnitude and frequency of the extreme event, using statistical methods.
For example, we analyzed how Hurricane Harvey in August 2017 and a unique weather model interacted with each other to produce the record-breaking Texas rainstorm. Two attribution studies found that man-made climate change increased the likelihood of such an event by about a factor of three and increased Harvey’s precipitation by 15%.
Another study determined that the extreme heat of western North America in late June 2021 would have been virtually impossible without man-made climate change.
What is the quality of attribution studies?
The accuracy of attribution studies is affected by the uncertainties associated with each of the above four steps.
Certain types of events lend themselves better to attribution studies than others. For example, among long-term measurements, temperature data is the most reliable. We understand better how human-caused climate change affects heat waves than other extreme events. Climate models are also generally adept at simulating heat waves.
Even for heat waves, the impact of man-made climate change on the magnitude and frequency could be very different, as in the case of the extraordinary heat wave in western Russia in 2010. Climate change was found to have had minimal impact on magnitude but significant impact on frequency.
There may also be legitimate differences in the methods underlying different attribution studies.
However, people can make decisions for the future without knowing everything for sure. Even when planning a backyard barbecue, it is not necessary to have all the weather information.
Read more: Water cycle intensifies as climate warms, IPCC report warns, meaning more intense storms and flooding
[Get our best science, health and technology stories. Sign up for The Conversation’s science newsletter.]
Xubin Zeng, professor of atmospheric sciences and director of the Center for Climate Dynamics and Hydrometeorology, University of Arizona
This article is republished from The Conversation under a Creative Commons license. Read the original article.