On a February morning in Ottawa it might be −28 °C; a week later it might be +5 °C and raining. Neither day tells you much about whether Ottawa is getting warmer over the decades. That is the first big idea of this unit: weather is what the atmosphere is doing right now, while climate is the pattern of weather a place experiences over many years. Climate change is a change in that long-term pattern, so it can only be seen by looking at decades of data, not at one cold snap or one heat wave.
Weather versus climate
- Conditions of the atmosphere at a particular place and time
- Changes over minutes, hours and days
- Described by temperature, precipitation, humidity, wind, cloud cover and air pressure today
- Forecast by meteorologists a few days ahead
- Example: "It is snowing in Sudbury this afternoon."
- The average weather and its usual range at a place over a long period
- Normally calculated over 30 years (a climate normal)
- Described by average temperatures, total precipitation and how they vary through the year
- Studied and projected by climatologists over decades
- Example: "Sudbury has cold, snowy winters and warm summers."
Earth's climate system
The climate system is everything that stores, moves or exchanges energy, water and carbon at Earth's surface. Energy from the Sun drives it. The parts are usually called spheres: the atmosphere (the layer of air), the hydrosphere (all the water: oceans, lakes, rivers, groundwater and water vapour), the cryosphere (frozen water: sea ice, glaciers, ice sheets, snow and permafrost; sometimes counted as part of the hydrosphere), the lithosphere (rock, soil and landforms) and the biosphere (all living things). A change in one sphere ripples into the others.
| Component | What it includes | How it affects climate |
|---|---|---|
| Atmosphere | Nitrogen, oxygen, water vapour, carbon dioxide and other gases | Traps heat (greenhouse effect), carries heat and moisture around the globe as wind and weather systems |
| Hydrosphere | Oceans, lakes, rivers, groundwater, water vapour | Stores enormous amounts of heat, moves it with currents, supplies water vapour; a major heat sink |
| Cryosphere | Sea ice, glaciers, ice sheets, snow, permafrost | Reflects sunlight (high albedo); stores fresh water; frozen soils lock away carbon |
| Lithosphere | Rocks, soil, mountains, continents | Mountains steer winds and rain; volcanoes release gases and particles; rock weathering slowly removes CO₂ |
| Biosphere | Plants, animals, microorganisms, people | Photosynthesis removes CO₂; respiration, decay and human activity release it; forests affect humidity and albedo |
- Cycles: matter and energy move around the climate system in loops, such as the water cycle and the carbon cycle, so a substance that leaves one sphere ends up in another.
- Heat sink: a part of the system that absorbs and stores a large amount of thermal energy without warming much; the oceans are the biggest heat sink on Earth.
- Albedo: the fraction of incoming sunlight a surface reflects, from 0 (absorbs everything) to 1 (reflects everything). Fresh snow is high; open ocean is low.
- Anthropogenic: caused by human activity, as in anthropogenic greenhouse gas emissions.
What sets the climate of a place
- Latitude: places nearer the equator get more direct sunlight all year, so they are warmer on average; places near the poles get slanted light and long winter nights.
- Nearness to large bodies of water: water heats and cools slowly, so coastal and lakeside places have milder winters and cooler summers than inland places at the same latitude.
- Ocean currents: warm currents warm nearby coasts; cold currents cool them.
- Prevailing winds and air masses: over most of Canada, winds blow from west to east, carrying the influence of whatever the air passed over.
- Elevation: air gets cooler as you climb, by roughly 6 °C per kilometre on average, so mountain towns are colder.
- Relief (mountain barriers): air rising over mountains cools and drops rain on the windward side, leaving a dry rain shadow on the other side.
Classifying climates: Köppen and Canada's ecozones
The most widely used climate classification was devised by Wladimir Köppen, who published it around 1900 and kept refining it into the 1930s. It sorts climates mainly by monthly average temperature and precipitation, which also match the kinds of vegetation that can grow. Each climate gets a code of letters; the first letter is the main group. At this level you only need the main groups and a feel for where Canadian cities fit.
| Köppen group | Main rule (simplified) | Canadian or world example |
|---|---|---|
| A: Tropical | Every month averages 18 °C or warmer | Singapore, the Amazon basin (none in Canada) |
| B: Dry | More water could evaporate than falls as precipitation | Parts of the southern Prairies near the semi-arid limit; the Sahara |
| C: Temperate | Coldest month between about −3 °C and 18 °C | Vancouver and Victoria (mild, wet winters) |
| D: Continental | Coldest month below about −3 °C, warmest above 10 °C | Toronto, Ottawa, Winnipeg, Thunder Bay, Moosonee |
| E: Polar | Warmest month below 10 °C | Iqaluit, Resolute (tundra) |
Canada also maps its land into ecozones: large regions with similar climate, landforms, soils, plants and animals. There are 15 terrestrial ecozones, each divided into smaller ecoregions and then ecodistricts. Ontario spans three ecozones: the Mixedwood Plains in the south (the warmest, most densely populated and most farmed), the Boreal Shield across the middle (Canadian Shield rock, lakes and boreal forest) and the Hudson Plains along Hudson Bay and James Bay (flat, wet peatlands and permafrost in places). Classifying climates this way lets you compare regions and predict how climate change will affect each one.