Weather: A hot, dry, summer can be costly

Sunday, August 10, 2008

Recent studies suggest that the droughts of the past decade may have been at least as bad as those of the 1930s, one reason driving a new, co-ordinated national research program into these punishing weather events

by HENRY HENGEVELD

The seasonal forecast for this summer, issued by Environment Canada at the end of May, projected a warm summer across the country. Only the Prairies were expected to have above normal precipitation to help offset the increased water stress this heat is likely to generate. To date, this prediction has not rung true. Still, in the long term, droughts have taken a toll.

Although the 1930s are often referred to as the decade of the great Canadian drought, recent studies suggest that the droughts of the past decade may have been at least as bad, particularly for Western Canada. Furthermore, climate model studies repeatedly project that, as the world gets warmer, the risk of severe droughts in central continental regions is likely to get even worse. 

These prospects have rekindled an interest in the Canadian research community in better understanding  Canadian droughts. Indicative of this has been a recent issue of the Canadian Meteorological and Oceanographic Society (CMOS) Bulletin that devoted most of its pages to the topic.

One of the reasons for this renewed interest is that droughts can have a punishing impact on both the environment and the economy. Archeological and historical records from all parts of the world indicate that they can cause far more harm to ecosystems, economies, social dynamics – and farmers – than almost any other weather disaster. That is because adequate and timely water supply is critical to the well-being of living things – and to many of our social and economic activities. 

Canadian droughts are no exception. The Prairie drought of 1980, for example, cost the Canadian economy an estimated $5.8 billion, while the cross-country drought of 2001/02 came in at $5 billion. The only other type of disaster which has come close in economic significance was the 1998 freezing rain event that cost us $5.4 billion.

However, getting a good handle on drought statistics isn't a simple task. Such events are complex interactions of variability in precipitation, loss of moisture due to evaporation and other factors, and hence are difficult to define. In one of the drought-related articles published in the recent CMOS Bulletin, Environment Canada climatologist Barrie Bonsal provides an overview of the characteristics of a Canadian drought and some related stats. He describes a drought as "a prolonged period of abnormally dry weather that depletes water resources for human and environmental needs." Such events can be localized and of short duration, or large-scale and lasting many years.

Bonsal notes that those occurring in Ontario and Quebec tend to be the former, while those in Western Canada the latter. In some cases, timing is such that its impact on vegetation and water resources is minimal. In others, it can occur at a critical point in plant growth cycles such that even a drought of short duration can have disastrous implications for crops or natural ecosystems. 

Over the decades, climatologists have developed a variety of ways to track and document droughts. One of the simplest is just to look at extended periods of anomalously low precipitation. However, this does not allow for consideration of other aspects of the water cycle which affect water resources, such as evaporation, or the state of water resources prior to the onset of the precipitation anomaly.

One approach that does is the Palmer Drought Severity Index, or the PDSI. Developed by American hydrologist Wayne Palmer in the mid-1960s, the PDSI is now one of the most commonly used indexes for regional drought monitoring and study around the world. 

Bonsal's analysis of drought frequencies and severity across Canada, using this index, underscores the significance of the droughts in recent years relative to others of the past century. While the analysis reveals that there is considerable decade-to-decade variability and little evidence of a long-term trend in any part of the country, the cross-Canada extent of the drought period that began in the late 1990s and lasted until the mid 2000s is particularly unusual. 

Also unsettling is the evidence from paleoclimate data sources on drought behavior during the past 400 years ago – particularly the period before instrumental weather data became available. Such very long-term drought behavior can be derived from natural indicators of local water stress, such as tree rings or remains of tiny fresh water creatures called diatoms, found in lake sediments.

Such studies, for example, show that droughts in southern Saskatchewan were more frequent and more severe in the 18th and 19th centuries than during the past century, including that of the 1930s. They also show that clusters of severe drought years in that region reappear about every 20 to 25 years. 

Such clustering is consistent with other studies into drought processes. Droughts usually begin with abnormal circulation patterns in the upper atmosphere which block the flow of normal weather systems across a given region. In Canada, such anomalies are often associated with persistent shifts in tropical Pacific Ocean currents that can last for months and even years. 

Once a drought is initiated, local atmospheric-surface feedbacks kick in which can also help cause the drought to persist. For example, drier surfaces evaporate less water vapour into the atmosphere, reducing cloud cover and potential convective rain showers.  Furthermore, less evaporation means warmer soils (since evaporation cools surfaces) and hence higher air temperatures. 

However, despite the importance of drought to our well-being, there has never been a co-ordinated drought research program in Canada which effectively integrates the work of the many scientific disciplines involved. That has recently led the Canadian Foundation for Climate and Atmospheric Research to launch the Drought Research Initiative, a multi-year project bringing together university and government researchers from the atmospheric, hydrological and land surface sciences to 'better understand the physical characteristics of and processes influencing Canadian Prairie droughts, and to contribute to their better prediction….".  More on that some other time.

Meanwhile, let's hope that the forecast for a hot summer doesn't bring another serious drought to Ontario! BF

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