LEARNING FROM TREES
What wood samples tell us about flooding and drought in Saskatchewan
By David Sauchyn, Director, Prairie Adaptation Research Collaborative
Average weather is a strange concept on the Canadian prairies. Most years, there seems to be either too much or too little water. We have one of the most variable climates on Earth, due to our location in the middle latitudes and our lengthy proximity from the main source of our water: the Pacific Ocean.
In April 2015, the Regina Leader-Post ran a story on farmland affected by the expanding Quill Lakes. The article quoted an impacted landowner, who said: “I don’t know how you plan for something that’s never happened.” This tendency to perceive floods, storms, and drought as ‘unprecedented’ is common. After all, it is human nature to view extreme conditions as out of the norm, given that our general frame of reference — such as a human lifespan — is relatively short.
Data sets from weather and water gauges are typically used to establish a baseline for determining whether a drought, fire, or flood is ‘worst on record.’ Throughout the prairies, these instruments were first installed in the 1880s, although long, continuous records exist at only a few locations. So, the question — and one that my research assistants and I study in the Prairie Adaptation Research Collaborative (PARC) Tree-Ring Lab at the University of Regina — is: Are these records sufficient to conclude that recent weather-related events have never occurred before?
For the answers, we turn to the memory of trees.
PARC provides research and training related to climate variability and change, and adaption planning. Our tree-ring data from the Rocky Mountain and northern plains have enabled us to reconstruct the climate and hydrology of the past millennium.
In our research, we have collected old wood from the treed uplands, valleys, as well as montane and boreal forests of four provinces, two territories, and two U.S. states. Unlike tropical and warm coastal forests where the trees never stop growing, in our climate, tree growth is limited each year by the availability of heat or water. At high elevations in the Rocky Mountains, heat is in short supply, and we can reconstruct growing season temperatures over the past millennium from the precise measurements of the width of the annual tree rings. This type of information, from hundreds of scientists and thousands of locations, is some of the best evidence that the earth has recently warmed at an unusual rate.
While some of our sampling sites are at mountain timberline, the rest (about 200) are from sites where tree growth is limited each year by the availability of water. Canada’s western interior, east of the Rocky Mountains, has a dry climate — too dry for trees over a large area. But, on the margins of the prairie grassland ecosystem, and in some of the valleys and uplands, the trees record the amount of water generated by rain and snowmelt, and stored in the soil, rivers, and lakes. Variations in the annual growth of these trees look very much like the year-to-year fluctuations in measured rainfall, water levels, and river flow. We use the statistical relationship between the tree-ring and instrumental data to reconstruct the climate from before the monitoring of weather and water.
The below chart shows the results of the research using the annual volume of water in the Assiniboine River at Kamsack and ring-width data from bur oak trees growing in the valleys of the Qu’Appelle and Souris Rivers and Pipestone Creek, which flow into the Assiniboine River. The reconstructed volumes of water in the Assiniboine River are shown as annual anomalies — that is, the difference from the normal or average flow. The blue and red bars represent years of above and below-average flow, respectively, from 1790 to 2019.
The first thing to note is that the 2010s is the longest period of above-average water levels, relative to the last 230 years. The longest blue bar is in 1852 — the year of a famous flood that is well-documented. Consecutive blue bars (positive anomalies) dominate the first two decades of the 20th century. In the 1910s and 20s, the population of Saskatchewan exploded, reaching almost one million people by 1931. Moisture conditions were favourable in most years — until the devastating droughts of the 1930s and early 1940s.
The 1860s, meanwhile, were a decade of low water levels. Steamboats were a popular mode of transportation on the Red and Assiniboine rivers during that period and history books describe the difficulty they had navigating the rivers that decade. The 1790s was another dry decade. Geological studies suggest that the sand dune fields of southern Saskatchewan were much more active back then compared to today.
Tree-ring reconstruction for southeastern Saskatchewan is one of many paleoclimate records from throughout the world. Other tree-ring records include ours for the Saskatchewan River basin that extends back to the year 888 A.D. and 10,000 years of tree-ring data from the southwestern U.S. and central Europe. The tree rings capture natural climatic variability before the world’s climate was significantly modified by human activity.
What this all tells us is that, going forward, our climate will be determined by a combination of natural and human factors. A large amount of scientific research suggests the variability of the climate and the intensity of weather events will be amplified by anthropogenic (resulting from influence of humans) climate change, as more water is transferred by evaporation from the warmer oceans into warmer air that can store more moisture. At the same time, precipitation deficits will coincide with higher rates of evaporation from land. Thus, truly exceptional weather events are increasingly likely to occur as the global climate continues to warm.
Dave Sauchyn is the director of PARC, and has been a professor of geography and environmental studies at the University of Regina for more than 35 years. His research focuses on the climate and hydrology of the past millennium, and how knowledge of the past can inform scenarios of future climate and water supply.