Global warming likely to yield more heavy rain

A U of S scientist predicts extreme rain events will increase in frequency and severity if global temperatures keep rising

A University of Saskatchewan study shows an increase in the frequency of heavy rain events in North America.

The research by assistant engineering professor Simon Papalexiou, in affiliation with the university’s Global Institute for Water Security, looked at weather stations all over the world in a 50-year period from 1964 to 2013.

“We focus on the period 1964 to 2013 because it’s a half century where global warming during that period accelerated,” said Papalexiou.

“And we wanted to investigate if it also had changes in the frequency of heavy rain.”

Using a model that expects an average of one extreme rain event per year per station, Papalexiou’s study has shown that there has been 8.4 percent more heavy rain events in North America in the last 10 years of the study compared to minus six percent in the first 10 years.

Although the data shows an increase in heavy rain events, Papalexiou said that due to the high variability of rain and the complexity of the phenomenon, his findings shouldn’t be used to try to explain current weather or predict what could happen in specific regions in the future.

“You cannot perform this global analysis or draw conclusions in one or two lookout stations. The variability of precipitation is very large,” he said.

“I wouldn’t attempt to narrow it down. The value of this study is it verifies very clearly that indeed there is a change in the frequency of big rain.”

However, what Papalexiou can predict is that if the global temperature keeps rising, extreme rain events will increase in frequency as well as severity. This is supported by the Clausius-Clapeyron equation, which states that for every degree of temperature in the atmosphere, the atmosphere can hold seven percent more moisture.

This could turn into a problem for Canada, which warms at more than double the rate of the rest of the world, according to a study done by Environment and Climate Change Canada.

“We have to realize that if global warming goes on with the same rate, heavy rain will become more frequent and more dangerous, even in regions that are not in danger right now,” Papalexiou said.

“This will be clear. The physics are there, the data verifies this in the historical period, and we have no excuse to say that we didn’t expect that.”

Papalexiou defines an extreme event as rainfall that “has an element of surprise,” or anything that has one percent or less probability of happening.

For example, a 50 millimetre rainfall in a 24-hour period in Saskatoon could potentially be included in the study as one of the 50 biggest rainfalls in the area in the past 50 years. However, Papalexiou stressed that heavy rain in one place won’t always be considered heavy in another.

“There is always the element of relevance,” Papalexiou said.

“If, for example, you are living in India and you have monsoons all the time, you will not be surprised there by the heavy rain here in Saskatoon.”

Papalexiou said strategies are needed to combat the increase in extreme rain events, such as identifying areas that are vulnerable to floods, developing green infrastructure to deal with that and being more aware of the natural behaviours of water.

“We as humans forget; the water remembers. It wants to follow gravity, it wants to go to the same place it used to go for the last billions of years,” Papalexiou said.

“If we get in its way, this means problems for us, so we have to respect a little more the natural hydrology to try to keep the paths of water clear and not modify it.”

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