Climate Change and its Impact on Ecosystem Health

Sun beaming through a forest.

Five-part Series Highlighting the Top Stories of the Watershed Plan

The first phase of the Watershed Plan characterizes the health of the Credit River Watershed and how it has changed from the 1950s to today.  

This newsletter dives into our fourth story: climate change and its impact on ecosystem health.  

Warmer and Wetter Conditions 

Data from long-term weather monitoring stations at Toronto Station (1938 to 2020) and Orangeville Station (1950 to 2020) show that our climate is changing across the Credit River Watershed.

Air Temperature

Since 1938, the average air temperature has increased by 1.8 degrees Celsius (C) at Pearson Station. At Orangeville Station, average temperatures have increased by 1.2 C since 1950. As a result, every season is now warmer than it used to be, but winter is warming faster: 

  • Since 1980, average annual air temperatures have been consistently warmer than normal. 
  • The number of extreme cold days, when temperatures fall below -10 C, is decreasing by 12 days at Pearson and seven days at Orangeville. 
  • The number of consecutive frost-free days, when minimum temperature is above 0 C, has increased by 14 days at Pearson and 25 days at Orangeville. 
A bar graph showing average annual temperatures from 1938 to the present with more warming and record warm stripes after 1990.
Warming stripes show annual average temperatures are increasing.


Conditions are wetter now than they were 25 years ago, especially in the northern part of the watershed: 

  • There has been an increase in the total amount of rain and snow, called precipitation, resulting in overall wetter conditions.  
  • Warmer temperatures have changed how precipitation hits the ground. In winter, 13 per cent more of the precipitation is falling as rain, instead of snow.  
Heavy rain falling on a residential neighbourhood with a large garden separating the opposing lanes.

Ecosystems are Showing the Impacts 

Local ecosystems are responding to changes in air temperature and precipitation patterns.  

Warmer Stream Temperatures

Three brook trout swimming above a gravel stream bed.
Brook trout captured by CVC’s Jon Clayton.

Since 1999, stream temperatures have been warming across the watershed. This includes more days where water temperature is higher than what coldwater fish species, such as brook trout and red-side dace, are adapted to.  

Over this same period, brook trout populations have decreased by 65 per cent in the northern part of the watershed and 30 per cent in the middle watershed. Brook trout have also completely disappeared from the urban areas of Brampton and Mississauga. This is likely a response to the cumulative impacts of urbanization and climate change. 

Increase in Invasive Pests and Disease

A tree trunk with bark missing exposing emerald ash borer feeding canals on the inner bark.
Impacts to ash tree infested by emerald ash borer.

We’ve seen a large increase in the number of diseased trees in forest communities throughout the watershed. 

Over 90 per cent of American beech trees are now infected with beech scale, which is an invasive pest that causes beech bark disease. Scale populations stay low in cold winters, but they thrive in warmer conditions. This allows them to spread and ultimately kill more trees.  

Over half of monitored ash trees have also been infested by the invasive emerald ash borer. Emerald ash borer is an insect that is unable to survive when temperatures dip below -35 C. The decline in the number of extreme cold days means this insect will likely survive over the winter. 

Species are Expanding their Range

A bird with a black-and-white striped back and a red cap and nape.
Red-bellied woodpecker.

Some species are starting to spread northwards throughout the watershed. For example, the population of red-bellied woodpeckers has increased by 65 per cent since 2002. This species is more common in Carolinian parts of the province but is moving north as temperatures warm. 

Other less desirable species have also moved into the watershed, including black-legged ticks which can spread Lyme disease to people and some animals. The Region of Peel is now considered to be in a high-risk area for tick exposure. 

Changing Climate Patterns 

Climate change patterns can have many impacts on watershed residents. These impacts can include: 

  • Increased vulnerability to flooding and erosion hazards due to the increase in rainfall intensity. 
  • Damage to property and infrastructure.  
  • Stress to agricultural systems because of summer droughts and high-intensity storms. 
  • Heat stress due to extreme heat. 

Guiding Conservation Action

Information from Phase 2 of the Watershed Plan will help to guide conservation action. 

We’re using modelling to understand how climate conditions in the future will influence: 

  • Flood risk 
  • Erosion potential

This will provide us with the information we need to identify where conservation action is needed, what areas of the watershed are most at risk, and what parts of the watershed are most likely to provide refuge habitat to our rich diversity of trees, plants, fish and terrestrial wildlife.  

Stay tuned for those results over the coming year.  

To learn more about the Watershed Plan, visit

By Shanice Badior, Coordinator, Watershed Plans and Analytics 

Comments (2)

  1. The temperatures at Pearson and Orangeville have been compromised by the growing heat island effect. The Orangeville sewage treatment plant was closed because the parking lot was next to the Stevenson screen holding the thermometers. A better choice to see true climate change is the Env Can site at Egbert.

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