Weather: Why we should worry about a few more millimetres of rain

Thursday, June 4, 2009

As our wet days get wetter, we should be concerned that even a small increase in precipitation risks exceeding critical thresholds and causing our protection systems to collapse

by HENRY HENGEVELD

The showers began in the wee hours of the morning of April 3, then settled into a steady rain that continued until the end of the day. By then, much of central Ontario had been soaked with up to 45 millimetres of rain – almost two-thirds of the total monthly rainfall April normally brings. Accompanying winds gusted to speeds as high as 70 kilometres per hour. It was one very dull, wet and blustery day!

Despite the soaking, the rains that day only made a minor dent in the record books. It was, indeed, the wettest April 3 on record at Pearson Airport, far exceeding the previous record of 20.8 mm set in 1950. Most other stations in central Ontario had experienced wetter ones at some point during the past century, yet it was one more reminder that our weather world is changing.

Several years ago, Canadian climate researchers Lucie Vincent and Eva Mekis undertook a thorough analysis of trends in a variety of climate extremes at 214 climate stations across Canada between 1950 and 2003. Each of these stations had first been carefully scrutinized and corrected for possible sources of data errors. 

One of the parameters they examined was heavy precipitation. Not surprisingly, the sporadic nature of heavy precipitation events means that the related trends can vary a lot from one station to the next. However, taking the mean for all of the analyzed stations, there are now on average almost two more days per year with heavy precipitation (exceeding 10 mm a day) than there were 50 years ago.

Furthermore, while about four per cent of the stations showed a statistically significant decrease in the number of days with this much precipitation, almost 10 per cent showed a significant increase. The average frequency of the more extreme once-every-20-year precipitation events also rose. However, while 13 of the stations reported increases that were statistically significant, the mean rise across all stations was not. 

Last year, Canadian climatologists teamed up with colleagues in the United States and Mexico to report on average trends in climate extremes for the same time period, but on the larger North American scale. They found, among other things, that the average amount of precipitation on days when precipitation occurred increased by
a bit more than 0.2 mm, also suggesting wet days are becoming wetter right across the continent. The once-in-20-year extreme precipitation events are several millimetres more intense now than a half-century ago. The maximum rainfall over five-day periods also rose by about the same amount. Much of this increase has been during the past few decades.

A variety of other studies indicate that this trend is likely a harbinger of more to come. Climate model projections persistently indicate that, as the world warms, the number of days per year with precipitation is likely to decrease. This also increases the likelihood of summer droughts.     

However, when precipitation does occur, it will, on average, be more intense. That also makes intuitive sense. As the surface warms, more water is evaporated, thus increasing the loading of moisture in the atmosphere. While a warmer atmosphere can also hold more moisture, most of the increase in water that goes up also comes down again – as precipitation. 

Most animal and plant communities, whether in the wild or domesticated, have largely adapted to historical extremes in weather. So has our social infrastructure. In North America, for example, we have built dikes and dams, dug drainage ditches and installed storm sewers to protect us from floods associated with extreme weather. We have learned how to reduce soil erosion from heavy rains and developed emergency response infrastructure to deal with the consequences of floods when they do occur. So why worry about a few more millimetres of rain?

First, while a rise in precipitation intensity by a few millimetres doesn't sound very alarming, these values represent averages over many stations, hiding the risks of much higher increases at the regional scale.

Secondly, extreme events of this kind approach the tolerance thresholds of the systems we have put into place to protect ourselves. Even a small increase in the magnitude of these extremes could be enough to exceed critical thresholds and cause our protection systems to collapse. Consider what a few more millimetres of river rise would have done to Grand Forks, North Dakota, this spring!

On the greenhouse gas emission reduction front, somewhat like Nero in ages past, our emperors around the world continue to fiddle while fossil fuels burn. Hence, while emission reductions may eventually actually happen, they will likely have little effect on the magnitude of risks due to climate change within the next 50 years (although they can make a large difference in the world our grandchildren inherit).

We are already largely committed, and hence it makes sense for current generations to anticipate and prepare for the changes in weather that they are likely to experience in the coming decades. 

Perhaps it's time to clean out those ditches and, where appropriate, build up the dikes just a bit higher! BF

Henry Hengeveld is Emeritus Associate, Science Assessment and Integration Branch/ACSD/MSC, Environment Canada