By the ESYS 500 team

The climate of the St. Lawrence River Basin has experienced multiple changes in the 49-year period from 1970-2018. Consistent with broader global impacts associated with human-caused climate change, altered patterns in river ice cover, temperature, and precipitation have resulted in trends towards shorter and more mild winters, as well as shifts in the timing of seasonal weather events. These trends have accelerated significantly over the last 30 years.

Winters have become milder since 1970. Average winter temperatures in the region have risen 0.28ºC per decade, and average and maximum ice extent in the St. Lawrence has declined by 1-3% per decade. However, total amount of snow and rain has not changed. Winters have also become shorter since 1970: on average each decade has seen 5 fewer days of below-0ºC temperatures, these freezing temperatures begin later in the year, and the amount of time the river is frozen has decreased by nearly 10 days per decade.

Reading this plot bottom to top, we can see how winter unfolds: first, it snows (blue), followed by temperatures falling consistently below 0ºC (red). A couple weeks later the river freezes (yellow). The trend lines in temperature and ice cover show how these events are shifting later in the year.

At the same time, the Montreal-Lake Ontario section of the St. Lawrence Seaway has closed later in the winter and opened earlier in the spring, meaning ships can pass through for longer portions of the year. Yet this has not translated into an increase in overall shipping activity in the region: the total number of ship transits declined by over one-half since 1970, while amount of cargo tonnage passing through the section of the Seaway declined by over one-third.  

When looking at these regional climate trends over the last 30 years (1989-2018), winters are becoming milder and shorter at a much faster rate. Since 1989, average winter temperatures have risen 0.95ºC per decade, below 0ºC temperatures set in 9 days later each decade, and the first snowfall occurs 5 days later per decade. The amount of time the St. Lawrence is frozen has decreased by over 18 days per decade, or almost two days shorter per year. The biggest shift has happened at the end of the winter, with ice break-up happening over 10 days earlier per decade.

Based on the 49-year trend, an ice-free winter in the St. Lawrence is probable around the year 2078. If recent trends continue, an ice-free winter is probable as early as 2043.