Earth’s connected systems
Weather, climate and oceans are all linked. The Sun heats Earth unevenly, and the atmosphere and oceans move that heat around the planet. Human activities are now changing this system.
The atmosphere
The atmosphere is a thin layer of gases held by gravity.
| Gas | Percent of dry air |
|---|---|
| Nitrogen (N₂) | 78% |
| Oxygen (O₂) | 21% |
| Argon (Ar) | 0.9% |
| Carbon dioxide (CO₂) | about 0.04% |
| Others | Trace amounts |
Water vapour varies from nearly 0 to about 4%.
Layers
| Layer | Altitude | Features |
|---|---|---|
| Troposphere | 0 to about 12 km | All weather; temperature drops with height |
| Stratosphere | 12–50 km | Ozone layer absorbs UV; jets cruise at its base |
| Mesosphere | 50–85 km | Coldest layer; meteors burn up |
| Thermosphere | 85–600 km | Very hot but thin; auroras; ISS orbits here |
| Exosphere | Above 600 km | Fades into space |
What makes weather
Air masses
An air mass is a huge body of air with similar temperature and humidity.
| Type | Forms over | Characteristics | Effect in Canada |
|---|---|---|---|
| Continental arctic (cA) | Arctic | Extremely cold and dry | Bitter winter cold snaps |
| Continental polar (cP) | Northern Canada | Cold and dry | Clear, cold weather |
| Maritime polar (mP) | North Pacific and Atlantic | Cool and moist | Rain and fog on coasts |
| Maritime tropical (mT) | Gulf of Mexico | Warm and humid | Humid summer heat; thunderstorms |
Fronts
A front is where two air masses meet.
| Front | What happens | Weather |
|---|---|---|
| Cold front | Cold air pushes under warm air, forcing it up quickly | Thunderstorms, then cooler and clearer |
| Warm front | Warm air slides up over cold air gradually | Long, steady rain or snow; then warmer |
| Stationary front | Neither moves much | Days of cloudy, wet weather |
| Occluded front | Cold front overtakes warm front | Complex mix |
Pressure and wind
Wind blows from high pressure to low pressure. Earth’s rotation deflects winds (the Coriolis effect), so in the Northern Hemisphere, winds circle counterclockwise around lows and clockwise around highs.
Global wind belts include the westerlies, which carry most of Canada’s weather from west to east, and the fast jet stream high in the troposphere.
Oceans and climate
Oceans cover about 71% of Earth and store huge amounts of heat.
Ocean currents
Ocean currents move heat around the globe.
- Surface currents are driven by winds. The Gulf Stream carries warm water north, keeping western Europe mild.
- Deep currents are driven by differences in temperature and salinity (thermohaline circulation). Cold, salty water sinks near Greenland and flows south, part of a "global conveyor belt" that takes about 1000 years to complete one loop.
El Niño and La Niña
Every few years, Pacific Ocean temperatures change:
- El Niño: warmer water in the eastern Pacific; often brings milder winters to much of Canada.
- La Niña: cooler water; often brings colder, snowier winters to western Canada.
The greenhouse effect
- Sunlight passes through the atmosphere and warms Earth’s surface.
- The surface gives off heat as infrared radiation.
- Greenhouse gases (water vapour, CO₂, methane, nitrous oxide) absorb some of this heat and send it back down.
Without the natural greenhouse effect, Earth’s average temperature would be about −18 °C instead of about 15 °C.
Human impacts
Climate change
Burning fossil fuels and clearing forests add greenhouse gases faster than nature removes them.
- CO₂ has risen from about 280 ppm before 1750 to over 420 ppm today.
- Global average temperature has risen about 1.2–1.3 °C since the late 1800s, and 2024 was the warmest year on record.
- Canada is warming about twice as fast as the global average, and northern Canada about three times as fast.
Effects of climate change include melting glaciers and Arctic sea ice, rising sea levels, more intense heat waves and wildfires, and shifting ranges for plants and animals.
Other impacts
| Impact | Cause | Response |
|---|---|---|
| Ozone depletion | CFCs from old refrigerants and sprays | The 1987 Montreal Protocol phased them out; the ozone layer is slowly recovering |
| Acid rain | Sulfur and nitrogen oxides | Pollution controls have greatly reduced it |
| Smog | Vehicle and industrial emissions | Cleaner cars and fuels |
| Ocean plastic | Litter and poor waste management | Bans on some single-use plastics |
Common mistakes
- Confusing the greenhouse effect with the ozone hole. They are different problems.
- Thinking the greenhouse effect is bad. The natural effect keeps Earth livable; the problem is the human-caused increase.
- Thinking one cold winter disproves climate change. Climate is a long-term average.
Try it yourself
- Put two thermometers in jars in the sun, one covered with plastic wrap. Compare temperatures after 30 minutes. Explain.
- Track a weather map for a week. Identify fronts and describe the weather as they pass.
- Research how your local area’s average temperature has changed over the past 50 years.
Quick recap
- The atmosphere has layers; weather happens in the troposphere.
- Air masses and fronts create changing weather.
- Ocean currents move heat around the world.
- The greenhouse effect keeps Earth warm; extra greenhouse gases cause climate change.
- Human actions affect the atmosphere and oceans, but international agreements can help.