Sims that took a year to run show US current path risks five-fold jump in extreme rainfall, intensity increases of 70%, so a storm dropping 5 cm of water today would likely dump 9 cm by 2100

“Think about the most intense thunderstorm last summer. In the future climate in some places in the United States, you’ll get as many as five of those [each summer],” says lead study author Andreas Prein, a scientist at the National Center for Atmospheric Research in Boulder, Colorado. This pattern follows the increasing storm frequency and severity that scientists have seen over the past 50 years, a trend that is likely to continue.

(Dec 5 2016, Space Daily) Large swathes of the United States could see a five-fold jump in the frequency of extreme downpours by century’s end due to climate change, according to a study released Monday.  Moreover, the intensity of those bouts could increase by about 70 percent, researchers reported in the journal Nature Climate Change.

That would mean that a storm dropping five centimetres (two inches) of water today would likely dump nine centimetres (3.5 inches) by 2100 if global warming continues unabated.  “These are huge increases,” said lead author Andreas Prein, a scientist at the US National Center for Atmospheric Research (NCAR) in Boulder, Colorado.

The most intense thunderstorm in a given season today would on average happen five times during the equivalent period in the future, with each storm as or more intense, he explained in a statement.

Summertime downpours are predicted to multiply across the continental United States, but the highly-populated Gulf and Atlantic coasts, along with much of the Southwest, would be hit especially hard.

More frequent and intense storms “would cause major challenges for existing infrastructure systems” hammered by flooding, landslides and debris flows, the researchers concluded.

The long-term link between global warming and wetter weather is well established: when the atmosphere warms, it is able to hold more water, causing heavier rains.  But not all regions in the US, or elsewhere, will become wetter with global warming — indeed, climate change is set to enhance droughts in some areas as well.

But even in part of the US forecast to become drier on average, such as the Midwest, the occasional summer downpour could also be more extreme.

The study presumes a “business-as-usual” scenario in which average global temperatures rise a devastating five degrees Celsius (nine degrees Fahrenheit) over pre-industrial levels.

With 1 C (1.8 F) of warming so far, the world has already seen an upsurge in extreme weather, including droughts, superstorms, heatwaves and coastal flooding boosted by rising seas.

The study, using a new dataset based on weather records between 2000 and 2013, achieved for the first time a resolution fine enough to simulate individual storms.  The simulations required a full year of computer time to run, with the calculations performed at the NCAR-Wyoming Supercomputing Center.

Popular Science 5 Dec 2016:  Harder, Wetter, Faster, Stronger:  Our Climate Change Path is Likely to Quadruple Severe Rainstorms in Just Decades By Peter Hess December 5, 2016

map showing increase in extreme rainstorms

Courtesy of Andreas Prein, National Center for Atmospheric Research

A hard rain’s a-gonna fall

This map, produced by researchers at the National Center for Atmospheric Research in Boulder, Colorado, shows how much more often severe rainstorms are predicted to occur around the United States by the end of the 21st century as compared to the period from 2000-2013.  The “breadbasket” of the world and the US is also vulnerable to drought.

Warmer air holds more moisture. This simple physics principle could become a problem through the rest of the 21st century. As global climates get hotter and hotter, more moisture will be extracted from the soil (same operating principle as a clothes dryer).  Atmospheric scientists predict that extreme weather events will become stronger and more frequent. In a paper published today in Nature Climate Change, researchers used climate data from 2000 to 2013 to predict that certain regions of the United States will see four times as many extreme rainstorms by the end of this century, and that rainstorms will increase in intensity by up to 70 percent.

“Think about the most intense thunderstorm last summer. In the future climate in some places in the United States, you’ll get as many as five of those [each summer],” says lead study author Andreas Prein, a scientist at the National Center for Atmospheric Research in Boulder, Colorado. This pattern follows the increasing storm frequency and severity that scientists have seen over the past 50 years, Prein tells Popular Science, and he and his colleagues at NCAR think this trend is likely to continue.

In addition to the number of storms increasing, Prein and his colleagues have predicted that the intensity of storms will increase by as much as 70 percent in some parts of the country, with coastal regions being most affected. That means that a rainstorm that dumped three inches of water in 2000 would drop more than five inches in 2100. And while those coastal regions present extreme cases, they are not total outliers. “On average it’s still 35 to 40 percent,” Prein says, “which is still a very large amount of water.” 

The modeling software used by Prein and his colleagues is extremely high-resolution, so this particular simulation took NCAR’s computers a year to run. One other limitation of the software, Prein says, is that it can’t account for uncertainties like slowing rates of global warming. “With these simulations you just have one realization of the future,” he tells Popular Science. So while these simulations are accurate given climate trends from 2000 to 2013, the future is not sealed.

Prein says that one of the most important ways to tackle this looming problem is to reduce greenhouse gas emissions, which would slow global warming down. Since warmer air can hold more moisture, rainstorm frequency and intensity will increase as temperatures continue their upward trajectory. Slowing down global warming could help combat this.

Assuming we don’t slow the rate of global warming, though, Prein says communities will need to enhance stormwater infrastructure to become more resilient in the face of these super-rainstorms. This would include building larger drainage systems to get water out of cities, as well as enlarging water storage areas. “Our current reservoirs are not built for these kind of storms,” Prein says.

He points to Boulder, Colorado, as an example of a city that is well-equipped for current weather patterns but will struggle under future scenarios. “We had a significant flood in 2013. It didn’t destroy Boulder because we’d built up this resilience. The question is whether we can handle 40 percent increases.” And this issue of climate resilience is not just technological, but also financial. Some cities simply won’t be able to afford the cost of flood-control infrastructure upgrades.

But again, these scenarios are based on the assumption that climate trends will continue as they have since 2000. The researchers’ next step is to incorporate uncertainty into their simulations, allowing them to look at a future in which weather systems have been altered by climate change. As with their newest study, this next one will take a long time to complete. “I guess it will take another year,” says Prein. Hopefully things won’t have gotten too much worse by then.

From The Guardian

When the skies open up and deluge an area, the results can be catastrophic, with roads washed out and homes destroyed by the resulting flash floods. Such extreme downpours are already occurring more often across the US, but a new study finds that as global temperatures rise, storms could dump considerably more rain and skyrocket in frequency.

The study, in the journal Nature Climate Change, suggests that storms that now occur about once a season could happen five times a season by the century’s end, a 400% increase.

And when such storms do occur, they could produce up to 70% more rain. That means that an intense thunderstorm that would today drop about 5cm (2 inches) of rain would drop 9cm in the future.

Such massive amounts of rain occurring more often could put significant strain on infrastructure that already struggles to deal with heavy rainfall, as seen across the country this year in places from Louisiana to West Virginia.

“I think this is one of the most severe consequences of climate change, at least in the US,” said the study’s co-author, Andreas Prein, of the National Center for Atmospheric Research (NCAR) in Boulder, Colorado.

Heavy downpours have already increased across the entire continental US, according to the 2014 National Climate Assessment, mostly notably in the north-east, where they have risen by 71%.

Such an increase in extreme precipitation is expected as temperatures rise due to heat-trapping greenhouse gases accumulating in the atmosphere. One of the basic properties of the atmosphere is that moisture increases with temperature. That means when a storm forms, it has more moisture available to fuel rains.

Several extreme rainfall events have already been made more likely because of warming, analyses have shown.

“We see this in the real climate already. It will only intensify,” Prein said.

The most extreme rains happen in convective storms, or ones that feature rapidly rising air at their core. These storms happen on scales too small for global climate models to capture, though, meaning they can’t provide a detailed look at rainfall trends.

To get down to the level of those storms, the NCAR researchers used a higher-resolution model that captures finer details, but takes much longer to run – in this case, an entire year.

What they found was that if greenhouse gas emissions continue on their current path, the storms that produce the most intense rainfall will increase broadly across the Lower 48.

The Gulf and Atlantic coasts, where oceans provide abundant moisture, would see some of the biggest increases in frequency and rainfall amounts.

But even in the central portions of the country, which are expected to get drier overall as rising temperatures cause more soil evaporation, the most intense storms will drop more rain in the future. Essentially, the region will see fewer of the moderate storms that are a crucial source of water now and more of the intense storms.

That shift has serious implications for agriculture, one of the main economic drivers in the region, as heavy rains can be damaging to crops.

However, if the world, including the US, acts to limit emissions over the coming decades, these increases in rainfall won’t be as large.

“It’s really in our hands to change that if we want to,” Prein said.