2 www.georgianbay.ca GBA UPDATE Summer 2023 Continued from page 1 Results of ECCC Study The projected probability of exceedance of the monthly mean lake level differences for Lake Michigan-Huron for 2050 and 2080 relative to historical levels are shown in the following tables. Table 1: Intermediate climate change scenario (metres)1 Michigan-Huron 5% 50% 95% Historical (1961-2000) +0.59 0 -0.71 2050 +0.88 +0.19 -0.57 2080 +0.84 +0.22 -0.40 Table 2: High-emissions climate change scenario (metres)1 Michigan-Huron 5% 50% 95% Historical (1961-2000) +0.59 0.0 -0.71 2050 +0.77 +0.07 -0.69 2080 +1.27 +0.53 -0.25 Note: The tables include 5 th and 95 th percentage exceedance, a variation of percentile obtained by subtracting the percentile scale value from 100 percent. The 50 per cent probability of exceedance represents predicted annual mean values relative to the historical water level data recorded from 1961 to 2000. The findings suggest that average annual water levels are not expected to change very much in the next 60 years, except for the high emissions case towards the end of the century. The 5 per cent and 95 per cent probability of exceedance values provide insight into the likelihood of more extreme water level events. The 95 per cent exceedance is the level that is exceeded in 95 per cent of all years. That means for most years, the water level will be greater than this value. The 5 per cent exceedance is a value that is likely to be exceeded in only one out of every 20 years. Looking at the historical record from 1961-2000 (the green line in both tables), the 5 per cent exceedance was 59 cm (23.2 inches) higher than the long-term annual average, and the 95 per cent exceedance was 71 cm (28 inches) lower than the average. The intermediate climate change scenario shows that the extreme levels are similar for the years 2050 and 2080. However, the high-emissions climate change scenario predicts larger variability by 2080 (table 2). This same pattern is seen in all the Great Lakes: greater water levels change with higheremissions scenarios later in the century. The report also suggests that year-over-year water levels will likely change more rapidly in the future and may exceed long-standing upper and lower water level limits, particularly towards the end of the century. Importantly though, the ECCC study warns the reader not to focus on the extreme water levels that were found for a small number of the simulations, as their statistical significance and physical robustness require additional investigation, which is beyond the scope of the study. It is also necessary to consider the limitations of the CGLRRM used to calculate water levels. This model was developed as a tool for development of the regulation plans and is quite limited in its validated scope. The CGLRRM manual advises that supply sequences outside of historical ranges may result in impractical output values. More precisely, the model is not designed to calculate water levels beyond those occurring in the past 100 years and does not account groundwater transfer between subbasins. It does not account for vertical/lateral movement of lake surface to calculate basin runoff as water levels rise above known values (Kayastha et al. (2022).2 The limitations of the model tend to generate higher water level predictions, and the errors are likely to be greater with increased divergence from maximum historical values. As for other research, there are very few studies that have explored long-term lake level forecasting. The following two refereed journal articles have simulated long-term lake levels for the Upper Great Lakes: Lofgren et al (2002) predicted dry and wet scenarios resulting in mean water level changes ranging from -1.38 to +0.35 m (-4.5 to +1.1 ft) for Lake Michigan-Huron by 2081-2100 relative to 1954-1995.3 Angel and Kunkel (2010) predicted that the full range of water levels for Michigan-Huron is -1.77 to +0.89 m (-5.8 to +2.9 ft) during 2080-2094, relative to 19701999.4 The paper emphasized that there is a large range of uncertainty regarding long-term projections of future lake levels. In balance, including the most recent ECCC study, we have three different studies that are arriving at similar latecentury water level predictions for the Upper Great lakes. Understanding Lake Michigan-Huron Levels The recent State of the Great Lakes 2022 Report published jointly by the Environmental Protection Agency (EPA) and ECCC reported that water levels in Lake Michigan-Huron show no significant overall average change over the last 100 years.5 In addition, despite great variations in lake levels year to year, their peak maximum and minimum water levels have remained relatively constant. Nevertheless, the year-to-year water level variations have been problematic for us all. When the lakes were at their lowest level, people thought water levels would never come back. But the water levels rose with a vengeance, without human intervention, and peaked just six years later. The ferocity with which the water returned between 2013 and 2019 underscores the power of the natural system, making any water level control mechanism that we might consider inept if not counterproductive. Rapid changes from low to high Continues on page 4
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