Extreme precipitation is expected to become more intense, frequent with warming

Across the continental United States, massive and often devastating precipitation events – the kind climate scientists have long called “centenary storms” – could become three times more likely and 20% more severe by 2079, led research projects from UCLA.

This is what would happen in a scenario where greenhouse gas emissions continue to increase at a rapid pace, what the paper calls a high-warming scenario. Extreme rain events, so-called centennial storms, are likely to occur once every 33 years.

The paper, published in the American Geophysical Union’s Earth’s Future journal, notes that warming has a more profound effect on both the severity and frequency of extreme precipitation events than it does on common precipitation events.

The findings have serious implications for how we prepare for the future, said UCLA climate scientist Daniel Swain.

“The five-year floods, the 10-year floods, are not the ones that cause enormous amounts of damage and disruption to society,” said Swain, who is also a fellow at the Nature Conservancy. “This happens when floods of 50 or 100 years occur, the kind of low probability but high consequence events.”

For example, the occurrence of historical rain events such as the one that caused the Great California flood of 1862 or the flooding of Houston from Hurricane Harvey in 2017 is increasing much faster than that of smaller events occurring every about ten years.

The paper predicts extreme increases in rainfall for the entire continental United States, but some areas are expected to see larger relative increases than others, including the west coast and hurricane-prone southeast.

The document also delves into the consequences of these extreme rainfall events: the increase in the number of floods and the number of people who would be exposed to them.

Combining climate, water physics, and population patterns, the paper also predicts that, under a high-warming scenario, increasing extreme rainfall alone would put up to 12 million more people at risk of exposure to damage. and destruction from catastrophic floods: 29.5% more people facing this risk today.

The paper also made projections using other scenarios that combine the effects of warming and projected population growth. For example, high warming juxtaposed with high population growth would increase the number of people exposed to the risk of so-called 100-year floods by about 50 million in the continental United States.

And even in the absence of climate change – at least some of which are inevitable in the next 30 years – medium or large population growth would expose 20 or 34 million more to such floods, respectively, highlighting the importance of demographic factors in driving the growing risk. .

The combination of the factors would exacerbate changes in some regions that have hitherto been outside flood zones and are sparsely populated because, thanks to climate change and population growth, those areas are likely to be within floodplains and have a higher population density in the future. This “hot spot effect” could put up to 5.5 million more people at risk of devastating floods than warming or population growth alone.

“There is a huge difference between the best and worst case scenarios,” Swain said. “People’s exposure to flooding in a hot climate is sure to increase. It could increase by a rather manageable amount or by a really huge amount, and that depends on both the climate trajectory we take and the US demographics.”

Previously, projections for extreme rainfall events were based on limited historical records dating back just 100 years. For the new study, the researchers used a modeling technique to create more plausible pasts and futures, substantially increasing the amount of data available 40 times what was available from history alone.

“We don’t just have one 100-year event that we can pull off the historical record; we have many rare and really serious events that we can pull off to give us a better idea of ​​how they are likely to change,” said Swain, who is a member of the UCLA Institute of the Environment and Sustainability.

Importantly, the authors write, the risk of flooding in the United States will increase significantly over the next 30 years, even with moderate warming, which means a temperature increase of 1.5 to 2.5 degrees Celsius (from 2.7 to 4.5 degrees Fahrenheit) globally. This is projected to expose more than 20 million more people to a 100-year flood within the next 30 years.

Even the term “100 years of flooding” is probably already a misnomer, Swain said. With global temperatures already rising by around 1.2 degrees Celsius (around 2.1 degrees Fahrenheit) over the past century, the term is fast becoming obsolete.

James Done, co-author of the article and climate scientist at the National Center for Atmospheric Research, said more work is needed to understand exactly why extreme events are increasing faster than less extreme ones.

“It’s not just because of a change in flood distribution,” Done said. “There is something else that is reshaping the most extreme of very dangerous rain events.”

The predicted precipitation changes are already starting, he added. And the nation’s infrastructure, from flood control canals to the heavy concrete urban project that slowly drains, was not designed for the scenarios that now seem likely.