What this lesson teaches
I can use evidence of past climates to explain the natural ways climate varies.
Syllabus K226, Topic 3.2, Key Question 1. What is the natural variability of climate?
Make a guess
A huge volcano erupts and Earth's average temperature drops for a year or two. What most likely caused the cooling?
- Volcanic carbon dioxide trapped less heat.
- Ash stayed in the sky for many years, blocking sunlight.
- Lava cooled the land around the volcano.
- Tiny droplets high in the air reflected sunlight.
Show the answer
Tiny droplets high in the air reflected sunlight.
Sulfur dioxide reaches the stratosphere and forms sulfate aerosols. They reflect sunlight, so less reaches the surface for a short time.
Changes in Earth's orbit, the Sun's output and big volcanic eruptions change climate, each over a different timescale.
Slow changes in Earth's orbit and tilt change how sunlight is shared between latitudes and seasons. They happen over tens of thousands of years or more. These orbital cycles help set the timing of ice ages, with ice and greenhouse-gas feedbacks making their effects bigger. They are different from the yearly seasons caused by Earth's tilt during each orbit.
The Sun's energy output rises and falls slightly over a cycle of about 11 years. Sunspots show this cycle. The spots themselves are dark, but bright regions around them mean the Sun gives out slightly more energy overall when it is more active. What matters for climate is how much energy reaches Earth.
Big explosive eruptions can cool Earth's surface for a year or two. Sulfur dioxide gas reaches the stratosphere and forms tiny droplets called sulfate aerosols. These reflect sunlight away, so less reaches the surface. Ash usually falls out sooner. Volcanoes also release carbon dioxide, which warms, but that is a separate process from the short-lived cooling.
Sort these
Natural or human? Sort each cause of climate change.
Groups: Natural cause, Human cause.
- Slow changes in Earth's orbit and tilt over tens of thousands of years
- The Sun's energy output rises and falls over an 11-year cycle
- Cattle farms and flooded rice fields release methane
- Power stations burn coal and gas to make electricity
- Forests are cleared and burned for farmland
- A big explosive eruption puts sulfate aerosols into the stratosphere
Show the answer
Natural cause
- Slow changes in Earth's orbit and tilt over tens of thousands of years Orbital cycles change how sunlight is shared between latitudes and seasons, with no human part.
- The Sun's energy output rises and falls over an 11-year cycle Sunspot cycles change the energy reaching Earth slightly, and people have no hand in them.
- A big explosive eruption puts sulfate aerosols into the stratosphere The aerosols reflect sunlight and cool the surface for a year or two; this is a natural, short-lived driver.
Human cause
- Cattle farms and flooded rice fields release methane Farming adds methane, a strong greenhouse gas, so this is a human cause even though the gas is natural.
- Power stations burn coal and gas to make electricity Burning fossil fuels releases carbon dioxide, the main long-lived greenhouse gas added by people.
- Forests are cleared and burned for farmland Burning releases stored carbon, and fewer trees are left to take carbon dioxide back in.
A proposed cause must fit the timing, length and pattern of a change. Slow orbital cycles and brief volcanic cooling cannot explain the steady warming of recent decades. That is mainly caused by human greenhouse-gas emissions. Natural ups and downs still affect single years and regions.
- Orbit and tilt
- Change over tens of thousands of years.
- Sunspot cycle
- About 11 years; small changes in the Sun's energy.
- Volcanic aerosols
- Reflect sunlight and cool Earth for a year or two.
Step by step
Orbit and tilt
Slowly move sunlight between seasons and latitudes.
Sun's activity
Small changes in energy reaching Earth.
Volcanic aerosols
Reflect sunlight for a short time.
Worked example: A cause must fit the evidence
If temperatures dip for a few years after a big eruption, volcanic aerosols are a good cause to test. Warming that lasts many decades needs a cause that also lasts decades.
Watch out for this
Climate changed naturally before, so recent warming must be natural.
Past natural change does not explain a new change. Each possible cause must be tested against the evidence.
Check your understanding
How can a large eruption cool Earth for a short time?
- The eruption stops Earth moving around the Sun.
- Aerosols high in the atmosphere reflect sunlight.
- Lava soaks up carbon dioxide from the air around it.
Show the answer
Aerosols high in the atmosphere reflect sunlight.
Yes. Less sunlight reaches the ground.