Climate change

Maps showing predicted increases in temperature and precipitation for Toronto Region over the 21st Century under the high GHG emission scenario. Adapted from TRCA (2025). Click on the image to go to TRCA's Watershed and Ecosystems Reporting Hub and explore future climate trends.

Overview

Climate change refers to long-term alterations in temperature, precipitation, wind patterns, and other aspects of the Earth’s climate system, largely driven by human activities. The dominant cause is the increased concentration of greenhouse gases, primarily from fossil fuel combustion, industrial processes, and land-use change (IPCC, 2021).

Impacts of climate change include:

• Changing rainfall patterns lead to more frequent extreme precipitation events which increase the risk of flooding (Foster et al., 2011)
• Increased temperature worsens heatwaves, which are compounded with the urban heat island effect and increased drought length and intensity (Foster et al., 2011)
• Disrupts groundwater recharge by altering the amount of soil infiltration (Wu et al., 2020)
• Changes in nutrient loading to waterbodies such as the Great Lakes (Fong et al., 2025; Robertson et al., 2016)
• Biodiversity loss and changes in species distribution (United Nations, 2025)
• Health impacts such as heat related illness during heatwaves (PPG, 2023a)
• Economic impacts such as higher cooling costs and post-flood repairs (PPG, 2023b)

• 

  As municipalities grapple with these new climate realities, they are rethinking how to manage stormwater. The Island Lake Reservoir, located near Orangeville, experienced low water levels after a prolonged drought. This was problematic for both the lake ecosystem and for the downstream WWTP which relies on reservoir discharges to dilute treated effluent (Tu et al., 2017)

• 

  Radar tracking of the August 10, 2012 extreme rainfall event. The Lake Ontario nearshore experienced sustained intensities approaching 200 mm/hr, while the southern portion of Peel Region had no measurable precipitation. (Source: Risk Sciences International)

Climate change challenges for grey infrastructure & SWM

Click on the image to read a presentation on Urban Water Policies and Practices for a Changing Climate (2024).

Traditional grey infrastructure focused on conveying stormwater to end-of-pipe storage facilities is not resilient to climate change. Urban areas should be retrofitted with green infrastructure which uses a distributed network of stormwater features to filter and infiltrate runoff at the source. Image adapted from Pennsylvania Wilds (2021) .

Stormwater dynamics are strongly influenced by land use and rainfall patterns, making them vulnerable to both climate change and urbanization. Canadian cities are experiencing climate change impacts at rates often exceeding the global average, as Canada is warming nearly twice as fast as the rest of the world (Bush & Lemmen, 2019). Traditional stormwater systems that focus on efficiently conveying stormwater were designed based on historical climate conditions. However, urbanization and climate change have intensified surface runoff by way of more frequent and intensive rainfall occurrences and increased impermeable land cover. This in turn creates challenges for urban drainage systems that lack sufficient adaptive capacity (Wang et al., 2023). Given these interconnected pressures, green infrastructure is increasingly essential for reducing flood risk, safeguarding water quality, and strengthening urban resilience. LID builds climate resilience by supplementing large end-of-pipe ponds with a distributed network of smaller-scale stormwater features throughout the catchment to retain and treat runoff as close to the source as possible (Canada in a Changing Climate, 2018).

Storm Events During varying Development Phases & SWM Controls

Histogram of rainfall during a typical storm event overlaid with hydrographs showing:
•pre-development (blue) peak discharge is low and creates the least amount of runoff
•post-development without SWM facilities (red) discharge responds quickly to rainfall and has high peak flow
•post-development with an end-of-pipe SWM pond (green) discharge reaches the same peak flow as pre-development conditions but produces more runoff The checkered yellow space between the blue and green lines shows the difference between the runoff volumes - this area is where LID can be used to reduce runoff by storing and infiltrating rainfall. Image credit: Daniel Filippi.]]}}

Stormwater as a valuable resource

Stormwater is increasingly seen as a resource rather than waste: harvesting excess runoff can reduce stream degradation while providing water for urban populations (Walsh et al., 2012). Over time, management has shifted from end-of-pipe disposal to decentralized LID strategies that infiltrate, store, and reuse stormwater (Fletcher et al., 2014). With climate change expected to reduce precipitation, stormwater will likely be a relatively reliable resource for cities (Walsh et al., 2016). This view is reflected in policies such as:

• 

  City of Toronto’s Official Plan (2019) describes stormwater as a valuable resource which should be managed where it falls.

• 

  York Region’s Official Plan (2022) calls for the use of innovative and integrative stormwater practices such as rainwater harvesting to combat the effects of climate change .

• 

  MECP (2022) acknowledges that LID BMPs have an important role in mitigating effects of climate change.

• 

  City of Vancouver’s Climate Adaptation Strategy (2024) includes the investment of $41M into green infrastructure renewal and upgrades to combat extreme rainfall.

Adapting to climate change using LID

LID can help offset climate change impacts in urban environments (IISD, 2021) .

Research highlights LID, such as bioretention, infiltration, and rainwater harvesting, as a critical strategy for building climate-resilient cities. Studies show that LID can reduce runoff volumes during extreme events, decrease peak flows, reduce overflow of combined sewers, and promote groundwater recharge:

• Modelled green roofs, permeable pavements, and bioretention cells were shown to increase resilience against combined sewer overflows during heavy future rainfall, helping prevent untreated sewage from being released (Rodriguez et al., 2024).
• LID with controlled outlets outperformed traditional grey infrastructure to decrease peak flow reduction and overflow volume which helps minimize nuisance flooding and erosive flows. LID was more resilient to increased flow intensity from climate change than grey infrastructure (Lucas and Sample, 2015).
• Vogel et al. (2015) documented that the use of LID in New York and Minnesota have the potential to reduce flood risks under climate change scenarios.
• Infiltration-based LID practices promoted groundwater recharge during climate change-adjusted design storms more effectively than conventional SWM (Barbu et al., 2009).
• Vegetated LID features, such as green roofs and treed bioswales, provide natural cooling that helps regulate urban temperatures and reduce energy demand and costs (ECONorthwest, 2007).

Although LID BMPs are primarily designed to manage water quantity and quality, they also provide additional co-benefits for climate adaptation:

• Gill et al. (2007) demonstrated that increasing urban greenspace by 10% by installing green roofs could offset projected urban heat increases until the 2080s.
• New York City’s Green Infrastructure Plan, launched in 2010, uses green roofs, rain gardens, and permeable pavements to reduce combined sewer overflows and improve water quality. By 2020, over 5,000 projects were managing more than 760 million liters of stormwater annually. Beyond stormwater benefits, these projects also improve air quality, support biodiversity, and create green spaces in underserved communities (Montazeri, 2024).
• LID practices contribute to climate change mitigation by sequestering carbon (Haque et al., 2025).

A variety of LID features distributed across a catchment addresses climate vulnerabilities more effectively than traditional grey infrastructure. Image adapted from the CT Stormwater Quality Manual (2025)

A stormwater retrofit in Scarborough captures roof runoff, directing it toward enhancing green space, irrigating landscaped areas, and infiltrating stormwater. This system diverts water from the municipal storm sewer, strengthening watershed resilience to climate change. With aging stormwater infrastructure in the area that no longer meets current standards, this project serves as a showcase for lot-level LID retrofits (STEP, 2019).

These stormwater benefits and additional ecosystem services position LID as essential tools for cities facing intensifying climate change impacts.

Watershed scale approaches to LID

Watershed-scale implementation of LID is more effective than isolated measures, as single installations rarely address all climate risks. Combining multiple BMPs that are tailored to a site’s vulnerabilities in a treatment train approach creates broader urban climate resilience.

• Ayalew et al. (2015) demonstrated that distributed stormwater retention ponds provide more widespread flood control benefits across a catchment than a single large pond at the outlet .
• Hydrological models show that combinations of rain barrels and rain gardens reduce runoff more than either LID alone (Neupane et al., 2021).

Retrofitting

Urban areas are constantly changing, and activities such as sewer upgrades and building renovations create opportunities to retrofit LID BMPs. Coordinating climate adaptation measures with these activities can help lower overall project implementation costs (Voskamp et al., 2015).

• Jarden et al., 2015 demonstrated that catchment-scale retrofits, such as a combination of street-connected bioretention cells (with and without underdrains), rain gardens, and rain barrels, can significantly reduce peak flows.
• A cistern/irrigation system and soakaway ponds/infiltration trench retrofitted into an industrial-commercial lot in Toronto reduced runoff volume from their roof drainage areas by 64% (~650 m² roof area) and 89% (~204 m² roof area), respectively (STEP, 2019).

Tailoring LID features to climate change

During Hurricane Hazel (a devastating event in 1954 where 81 lives were lost), the two-hour maximum precipitation was 91 mm and the total amount of rainfall was 285 mm over nearly two days (Toronto Star, 2013). Conventional municipal drainage systems could not carry stormwater away fast enough; roads and highways were overcome with floodwater closing major transportation corridors, GO Train passengers were stranded, and power outages and basement flooding were widespread with property damage of more than $1 billion (Upadhyaya et al., 2014). Hurricane Hazel served as a catalyst for change, revealing the urgent need for an overhaul of flood management practices (TRCA, 2024). Hurricane Hazel was larger than the 100-year design storm (Institute for Catastrophic Loss Reduction, 2000)

Canadian cities typically experience three types of rainfall (City of Vancouver, 2025):

1. light showers
2. rainstorms
3. extreme storms

Most precipitation events fall into the first category. However, climate change is intensifying storm severity, shifting more events into the second and third categories. One study reported increases in the intensity of 20- and 50-year return period winter precipitation events across the western United States. Another study projected rising intensity of annual maximum precipitation in Canada, with the greatest increases expected in Ontario, the Prairies, and Southern Quebec (Guinard et al., 2015). As these storm events grow more frequent and intense, much of our infrastructure faces conditions it was never designed to withstand (Government of Ontario, 2012). To remain effective, the design of LID features must account for these changing precipitation patterns. Go to Understanding rainfall statistics to learn more about rain event distributions.

Intensity-Duration-Frequency Curves

Reading IDF Curves: A precipitation event with a duration of 12 minutes, and intensity of 5 inches/hour is predicted to occur once every 5 years (Bentley, N.D)

IDF curves may shift under future climate scenarios. In this example, the future 50-year storm is predicted to have a higher rate of rainfall than historical storms of the same return period. LID practices should be designed to accommodate this shift. Image credit: ECCC (2025)

Intensity–Duration–Frequency (IDF) curves are a standard tool in hydrology that describe the statistical relationship between rainfall intensity, storm duration, and frequency of occurrence (return period) to determine the likelihood of extreme rainfall events (Martel et al., 2021).

• Intensity: the rate of rainfall (mm/hr)
• Duration: the length of the rainfall event (minutes to days)
• Frequency: how often a given storm is expected to occur (e.g., a 10-year storm has a 10% chance of occurring in any given year)

IDF curves are developed from long-term rainfall records and models and are widely used to estimate design storms for stormwater management. IDF curves are crucial for sizing LID BMPs because they:

1. Define design storms: LIDs must be able to manage runoff from specific return-period events (e.g., 2-year or 10-year storms). IDF curves provide the rainfall intensity and depth to use for those design events.
2. Determine runoff volumes: By applying rainfall depth (from IDF data) to a given catchment, engineers estimate the stormwater volume that LIDs need to capture, infiltrate, or detain (USDA, 2021).
3. Assess peak flows: LID facilities are often designed to reduce peak discharge. IDF curves help simulate storm hydrographs and assess how much peak flow reduction is required (City of Toronto, 2006) .
4. Account for climate change: Historical data is no longer an accurate predictor of future extreme rainfall (CSA, 2025). Since IDF curves are based on historical data, climate change-scaled IDFs can be used to design LIDs that are resilient under future rainfall conditions (ECCC, 2025).

Estimating future rainfall

When designing infrastructure with a service life longer than 10 years, future IDF curves must account for climate change impacts on the frequency and intensity of extreme rainfall. Future precipitation patterns are estimated to account for climate change impacts.

The atmosphere’s capacity to hold moisture, and thus the occurrence of extreme rainfall, increases with warming. The relationship between increases in rainfall extremes and temperature is called rainfall–temperature scaling. Future rainfall intensities can be estimated via rainfall-temperature scaling using the following equation (CSA, 2025):

$I_{fut,D,RT}=I_{ref,D,RT}\times (CC_{adj})^{\Delta T}$

$I_{fut,D,RT}$

the median estimate of the projected future rainfall intensity of duration D and return period RT

$I_{ref,D,RT}$

historical rainfall intensity, obtained from IDF data produced by Environment and Climate Change Canada using stations that are representative of the project site location and have at least 10 years of complete and validated observations. Refer to CSA 231:25 for details on calculating from alternative sources of rainfall observations, if required.

$CC_{adj}$

the rainfall-temperature scaling factor (typically 1.07 unless otherwise justified)

$\Delta T$

the projected change in local temperature (°C) between the reference and future time periods. The following procedure should be used to obtain $\Delta T$:

1. Select climate data source – Use a reputable, bias-corrected, high-resolution data source for Canada (e.g., climatedata.ca) covering at least 1950–2100
2. Define reference period – Must overlap with the historical records used for IDF curves
3. Define future period – Same length as the reference, with anticipated end of service life falling in the middle of the future projection period when possible
4. Choose emissions scenarios – Use both a representative and a high scenario from the latest Coupled Model Intercomparison Project (CMIP) phase
5. Select climate change scenarios – scenarios that correspond with emissions scenarios from step 4 should be chosen from the data source in step 1, ensuring they represent variability among global models and account for uncertainty
6. Calculate projected temperature change – Determine mean annual temperature change between reference and future periods using climate change scenarios
7. Summarize results – Report both the median and 75th percentile of projected temperature changes from all climate change scenarios for use in future IDF curve calculations.

Risk thresholds are a useful tool when prioritizing climate change impact management efforts. Low impact and low frequency events may not require management planning, while high impact and high frequency events require adaptation strategies (MECP, 2022)

To read about IDF curves and best practices, learn how to integrate climate change into IDF curves, and download climate change-scaled IDF data, visit:

• https://climatedata.ca/interactive/idf-curves-101/
• https://climatedata.ca/resource/idf-data-and-climate-change/
• https://climatedata.ca/resource/best-practices-for-using-idf-curves/
• https://www.csagroup.org/store/product/2705526/?srsltid=AfmBOooZzvRihuxPEkbonAPD3jtG2dFxd5Y-lAFFop5O8Yq4e9ZQggd8

Climate risk and uncertainty

Risk

Historical (grey) global surface temperature compared to a range of projected future surface temperatures based on five emission scenarios (from very low to very high). Not only is the state of future emissions uncertain, but there is also uncertainty associated with each scenario – indicated by the red and blue shaded areas (City of Toronto, 2024)

Risk is typically defined as a combination of the likelihood of failure and the consequences of that failure. More detailed frameworks also consider hazard (what could trigger damage), exposure (what is at stake), vulnerability (sensitivity to hazards), and resulting consequences. In the context of climate change, risk is often expressed as (CSA, 2024):

Risk = (Probability of Climate Event) × (Probability of Asset Failure Given Event) × (Consequences: a function of exposure and vulnerability)

Not all risks can be fully managed; therefore, management efforts are typically prioritized toward impacts that exceed established thresholds of severity and frequency. (MECP, 2022)

Uncertainty

Climate change is inherently characterized by uncertainty, particularly in projecting local impacts. This uncertainty poses challenges for engineers, making risk management a critical tool for prioritizing vulnerabilities and selecting effective risk reduction strategies. Uncertainty surrounding climate impacts underscores the need to build resilience through LID practices (MECP, 2022). The ”no regrets” strategy refers to actions, such as LID, that have positive benefits for people and the environment, regardless of how climate change unfolds.

Planning for climate change in LID design

Building climate change resiliency into a project is not a reactive process and should be undertaken during early project phase. Climate planning should first identify system vulnerabilities across a wide range of possible future climates, then evaluate the probability and severity of impacts, and finally design adaptation strategies for impacts that exceed the threshold level of risk. A four-step process can be applied to guide resilient LID design (MECP, 2022):

1. Identifying climate change considerations
2. Evaluating risk caused by climate change parameters
3. Climate change impact management planning
4. Monitoring and adaptive management

To learn more about each step and access climate assessment templates, scroll through the PDF below which displays chapter 6.8 of the Draft LID Stormwater Management Guidance Manual, written by MECP in 2022. You can download (downward facing arrow on the top righthand side) and print (Printer emoticon on top right hand side) the PDF.

Climate planning at different scales

TRCA’s regulatory flood plain map (TRCA, 2025). As directed by the Province, the regulatory flood event standard applicable within TRCA’s jurisdiction is the greater of Hurricane Hazel or the 100-year storm (TRCA, 2025).

Site scale

Individual sites can be assessed for climate change risk by evaluating stormwater systems for components vulnerable to projected changes in precipitation patterns. This includes considering increased frequency or intensity of system responses beyond design expectations, such as pipe surcharging, nuisance flooding, overtopping of storage facilities, erosion, failure to meet water quality or temperature targets, impacts on sensitive wetlands, and interactions with wells and septic systems (MECP, 2022).

For sites with an existing stormwater management plan, it can be valuable to assess how much resiliency was incorporated into the system at the time of design by evaluating these thresholds. For instance, pipes may have additional capacity beyond the design return period depending on their installed size (MECP 2022).

Climate change and SWM LID BMPs will affect site land use types differently. To view different LID opportunities for each unique land use type, please view the subsequent pages linked below:

• LID opportunities in road-right-of-ways
• LID opportunities in residential types
• LID opportunities in industrial, commercial and multi-residential types
• LID opportunities in public lands

Watershed scale

Climate change is expected to alter overall watershed behavior, including snow accumulation, spring freshet timing, streamflow patterns, evapotranspiration, groundwater recharge, wetland hydroperiods, and the frequency and severity of drought. Hydrologic models can be developed and applied at the watershed or subwatershed scale to assess the impacts of climate change on both groundwater and surface water systems (MECP, 2022).

Municipal and provincial scale

Watershed and sub-watershed plans, official land use plans, master drainage plans, and design standards should consider climate change impacts as they relate to (CSA, 2024):

• identification of natural infrastructure and associated floodplain and hydrologic functions
• delineation of floodplains and associated setbacks for development
• design, modeling, sizing, and placement of minor and major systems
• assessment of a balanced cut and fill approach on a reach basis to provide additional developable area

Any form of rain or snow.

Toronto and Region Conservation Authority

The addition of water to ground water by natural or artificial processes.

The infiltration and movement of surface water into the soil, past the vegetation root zone, to the zone of saturation or water table.

The total mass of a pollutant entering a waterbody over a defined time period.

The net amount of something (e.g. chemical, such as phosphorus), calculated as the product of concentration and volume in a given time. Some BMPs significantly reduce loading of pollutants to the environment by reducing volume more so than concentration.

Surface runoff from at-grade surfaces, resulting from rain or snowmelt events.

A biological community, including humans and their natural environment.

Stormwater Management

Natural vegetation and vegetative technologies in urban settings such as: urban forests; green roofs; green walls; green spaces; rain gardens; bioswales; community gardens; natural and engineered wetlands and stormwater management ponds; and porous pavement systems. These systems are designed to provide multiple benefits, such as moderate temperatures, clean air and water, and improve aesthetics.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

The changing of land cover and land uses from rural to urban; the growth of urban settlements.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

The land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.

refers to the characteristics and functions of a system prior to urban development.

The greatest volume of stream flow occurring during a storm event.

the maximum flow rate occurring during a flood event measured at a given point in a river, street, or pipe system.

A body of water smaller than a lake, often artificially formed.

The practice of intercepting, conveying and storing rainwater for future use. Captured rainwater is typically used for outdoor non-potable water uses such as irrigation, or in the building to flush toilets.

The liquid waste from domestic, commercial and industrial establishments. City of Toronto Wet Weather Flow Management Guidelines November 2006 49

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

Sustainable Technologies Evaluation Program

The drainage area of a river.

An area of land that drains into a river or a lake. The boundary of a watershed is based on the elevation (natural contours) of a landscape.

Relating to the properties, distribution and effects of water on and below the earth’s surface, and in the atmosphere.

Human application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47

A rainfall pattern for use for the design of a hydraulic structure or system. A design storm is a specified amount of rainfall from a storm1 with its spatial and temporal distribution used to estimate a design discharge.

The average time between years when an precipitation event of specified magnitude is met or exceeded.

The rate of rainfall in millimeters per hour.

Refers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer.

Annual spring increase in water levels caused by melting snow, ice, and increased rainfall.

The quantity of water transpired (given off). Retained in plant tissues, and evaporated from plant tissues and surrounding soil surfaces. Quantitatively it is usually expressed in terms of depth of water per unit area during a specified period. e.g. mm/day

The combined loss of water to the atmosphere from land and water surfaces by evaporation and from plants by transpiration.

Increases in groundwater storage by natural conditions or by human activity. See also artificial recharge

The inflow of surface water to a groundwater reservoir or aquifer.

The area adjacent to a stream that is, on average, inundated once a century

Layer of rock or soil that holds or transmits water.

The drainage area of a river.

An area of land that drains into a river or a lake. The boundary of a watershed is based on the elevation (natural contours) of a landscape.

Relating to the properties, distribution and effects of water on and below the earth’s surface, and in the atmosphere.

Low impact development is a stormwater management and land development strategy applied at the parcel and subdivision scale that emphasizes conservation and use of on-site natural features integrated with engineered, small scale hydrologic controls to more closely mimic pre-development hydrologic functions.

A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

Refers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer.

The addition of water to ground water by natural or artificial processes.

The infiltration and movement of surface water into the soil, past the vegetation root zone, to the zone of saturation or water table.

The land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.

Any form of rain or snow.

The greatest volume of stream flow occurring during a storm event.