Chapter 4 – Climate & Climate Change

Release Date: November 2012


Climate is the meteorological data for a given region over a long time period. The climate of a region is affected by a number of factors including: latitude, terrain, and altitude, as well as large water bodies. Climate within the Credit River Watershed is generally characterized by warm humid summers (average air temperature of 18.7 °C) and cold winters (average air temperature of -4.9 °C). Precipitation is moderate and well-distributed throughout the year with an average of 856 mm of precipitation falling annually. Climate is an important driver of the physical, chemical and biological components of the Credit River Watershed. Therefore, anthropogenic climate change is an important stressor that may transform the system.

Climate Change

Similar to land use/cover change, climate change is a stressor that can affect the natural balance of the Credit River Watershed. Climate change is defined by the Intergovernmental Panel on Climate Change (IPCC) as:

“A change in the state of the climate that can be identified (e.g., by using statistical tests) by changes in the mean and/or the variability of its properties and that persists for an extended period, typically decades or longer. Climate change may be due to natural internal processes or external forcings, or to persistent anthropogenic changes in the composition of the atmosphere or in land use.” (IPCC 2012).

Climate change can greatly influence the watershed’s ecosystems and consequently the components of the Integrated Watershed Monitoring Program (IWMP). For example, an increase in summer temperatures may negatively influence Salmonids which prefer cold water habitats.

Climate change models indicate that southern Ontario can expect to see hotter and drier summers and warmer and wetter winters into the future. These models also indicate that extreme weather events such as floods, snowstorms, wind storms, drought and fire may become more frequent and more severe (Figure 1).

Figure 1: Flooding in Eldorado Park, Brampton, 2007

Changes in air temperature and precipitation patterns are reflected in changes in water temperature, water cycles and biological productivity. These alterations, in turn, affect terrestrial and aquatic ecosystems, and ecosystem responses can be documented through changes in plant and animal community composition. Some species may adapt to climate change while others may alter their range (i.e. where they are located). For example, it is expected that the range and abundance of invasive species will increase in response to climate change.

Climate Monitoring

CVC operates five climate stations within its jurisdiction which collect data on parameters such as rainfall, air temperature and wind speed. With growing interest in climate change, both at global and local scales, CVC will be expanding its network of climate stations over the coming years.

Because the data record for CVC’s climate stations has a duration less than ten years, CVC relies on continuous data collected by Environment Canada for long-term analysis of climate patterns in the watershed. Representative stations with long-term records of climate data were used for each of the watershed’s three physiographic zones (Figure 2). Data from these stations were used to determine average monthly temperatures and monthly precipitation as well as annual statistics since the start of the IWMP (1999). These values were compared to a climate “normal” that is defined as the 30-year average climate from 1971-2000. Please note, however, that due to the shorter length of the climate record from the Georgetown climate station, the Environment Canada climate normal for the middle watershed is based on 20-year (1981-2001) averages.

Figure 2: Climate stations used for monitoring the Credit River Watershed


Climate Status

Climate is the meteorological data for a given region over a long time period. Thus, a single year is not reflective of overall climatic conditions for a region. For this reason, this chapter focuses on long-term climate trends and not the “status” of a single year.

Figure 3: Difference between average annual air temperatures (1971-2011) and normal conditions in accordance with Environment Canada standards for the Upper, Middle and Lower Watershed. Warmer than normal years are red whereas colder than normal years are blue. Years which cross the dashed lines are outside the 95% confidence interval of the climate normal and indicate particularly warm (red) or cold (blue) years.   

Climate Change Trends

Air Temperature

Across the Credit River Watershed there has been a statistically significant increase in annual air temperature from the early 1970’s to 2011, with the most pronounced warming occurring during the period of the IWMP (1999-2011; Figure 3). For example, average annual air temperatures in the Lower Watershed have been on average 1.4°C higher than normal during the IWMP, with every single year reporting temperatures above normal.

Although the warming trend is consistent across the watershed, the magnitude of the warming varies spatially with the Lower Watershed warming at twice the rate as the Upper Watershed from 1971 to 2011 (0.6 vs. 0.3 °C per decade respectively).

Seasonal air temperature followed a similar pattern to annual temperature trends. Average air temperatures over the length of the IWMP (1999-2011) have been higher than normal across the watershed for all seasons with the greatest warming occurring in autumn (September-November; Figure 4 ), where all three climate stations recorded significantly warmer temperatures than normal.

figure 4

Figure 4: Average seasonal air temperature during the IWMP (1999-2011) compared to the temperature normal. The stars indicate statistically significant differences between the temperature normal and the average temperature during the IWMP.



Precipitation patterns have shown greater variability than temperature among the watershed’s three physiographic zones (Figure 5) . Total annual precipitation in the Upper Watershed has significantly increased from 1971 to 2011 whereas the climate station in the Middle Watershed recorded a significant decline in precipitation from 1971-2011. The Lower Watershed, in contrast, showed no discernable pattern in annual precipitation from 1971-2011.

Figure 5: Difference between annual precipitation (1971-2011) and normal conditions for the Upper, Middle and Lower Watersheds. Wetter than normal years are blue whereas drier than normal years are red. Years which cross the dashed lines are outside the 95% confidence interval of the precipitation normal and indicate particularly wet (blue) or dry (red) years.


The seasonal trends in precipitation provide further insight into changing precipitation patterns in the Credit River Watershed. Over the last 13-year period covered by the IWMP (1999-2011), examination of seasonal changes in precipitation have shown that the annual precipitation patterns have largely been driven by greater than normal winter (December-February) precipitation in the Upper Watershed and drier than normal summers (June-August) in the Middle Watershed (Figure 6).

Figure 6: Average seasonal precipitation during the IWMP (1999-2011) compared to normal conditions. The stars indicate statistically significant differences between the precipitation normal and the average temperature during the IWMP.



Patterns consistent with climate change model predictions are beginning to emerge through CVC’s climate monitoring. In particular, predictions for warmer than normal air temperatures are evident in the watershed. The pattern of drier summers and wetter winters predicted by climate models is, however, not as clear across the entire watershed, although average values during the IWMP indicate the Upper Watershed had wetter winters and the Middle Watershed drier summers than normal.

CVC will continue to monitor climate and expand our network of climate stations across the watershed. This effort will improve our understanding of long-term climate trends and support plans for mitigation and adaptation to climate change.


The next chapter will look at groundwater levels and their influence on water quantity. Groundwater is used by thousands of people in the Credit River Watershed and is critical for maintaining the Credit River ecosystem. How are groundwater levels changing in the Credit River Watershed?

Did you know?

Approximately half of the flow in the Credit River comes from groundwater.


EC (Environment Canada). 2012. National Climate Data and Information Archive. (accessed October 11, 2012)

IPCC (Intergovernmental Panel on Climate Change). 2007. IPCC Fourth Assessment Report:Climate Change.

IPCC (Intergovernmental Panel on Climate Change). 2012. Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation.

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