The big idea: Earth's climate has always changed, long before humans.
The planet stays at a steady temperature when incoming solar energy balances the energy radiated back to space. Anything that nudges that energy balance changes the climate.
Natural causes can push it either way — some warm the planet, some cool it.
Key terms
- Global energy balance -- the balance between solar energy coming in and energy radiated back to space.
- Solar output -- how much energy the Sun gives off; it rises and falls in cycles.
- Global dimming -- a fall in sunlight reaching the ground because particles in the air block or reflect it.
- Feedback loop -- a change that triggers further change, either amplifying it (positive) or damping it (negative).
| Natural driver | What it does | Effect on temperature |
|---|---|---|
| Solar output (sunspot cycle) | Energy from the Sun rises and falls slightly | Warmer when output is high |
| Earth's orbit (Milankovitch) | Slow changes in orbit, tilt and wobble over 1000s of years | Drives ice ages and warm periods |
| Large volcanic eruption | Throws ash and sulfate aerosols high into the atmosphere | Cooling for 1-3 years (global dimming) |
| Ocean cycles (e.g. El Nino) | Move heat between ocean and atmosphere | Warmer or cooler over a few years |
Natural vs human: These are the natural causes. The human (anthropogenic) causes -- burning fossil fuels, deforestation -- are the next micro.
In the exam the key skill is being able to separate the two and weigh them up.
The amount of solar radiation Earth receives is not fixed. Two natural things change it: the Sun's own output, and changes in Earth's orbit around the Sun.
Why incoming solar radiation varies
- Sunspot cycle -- the Sun's energy output rises and falls on an ~11-year cycle, so a more active Sun warms Earth slightly.
- Orbital changes (Milankovitch cycles) -- over tens of thousands of years Earth's orbit shape, axial tilt and wobble shift, changing how much sunlight reaches each part of the planet and driving ice ages.
- Distance from the Sun -- a more stretched orbit changes how far Earth sits from the Sun, altering the energy received.
How this is tested: Paper 2 Q2 has asked students to explain two reasons why incoming solar radiation can vary [4], and to explain how a change in the Sun's output alters the energy balance [2].
Name the driver, then give the mechanism -- how it changes the energy reaching Earth.
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A natural cooling effect: A large volcanic eruption throws ash and tiny sulfate aerosols high into the upper atmosphere.
These particles spread around the globe and reflect sunlight back to space, so less reaches the ground. This is global dimming, and it cools the planet for 1-3 years until the particles settle out.
The chain of cause and effect
- Eruption injects ash and sulfate aerosols into the atmosphere.
- Particles spread worldwide and stay aloft for months.
- Sunlight is reflected back to space, so less reaches the surface.
- Energy balance shifts -- incoming energy drops, so the planet cools temporarily.
Mount Pinatubo, 1991: When Mount Pinatubo (Philippines) erupted in 1991, it pumped about 20 million tonnes of sulfur dioxide into the upper atmosphere.
Global average temperatures fell by roughly 0.5 degrees C for about a year -- a clear, measured example of natural cooling and global dimming.
How this is tested: Once a change starts, feedback loops can amplify it.
Paper 2 Q2 has asked students to describe and explain two positive feedback loops [6] and to outline how extreme warming drives a feedback loop [2]. The extended [10] essay asks how far energy-balance shifts are natural rather than human -- a For/Against/Judgement answer.
| Natural driver | What it does | Effect on temperature |
|---|---|---|
| Solar output (sunspot cycle) | Energy from the Sun rises and falls slightly | Warmer when output is high |
| Earth's orbit (Milankovitch) | Slow changes in orbit, tilt and wobble over 1000s of years | Drives ice ages and warm periods |
| Large volcanic eruption | Throws ash and sulfate aerosols high into the atmosphere | Cooling for 1-3 years (global dimming) |
| Ocean cycles (e.g. El Nino) | Move heat between ocean and atmosphere | Warmer or cooler over a few years |
Two key positive feedback loops
- Ice-albedo feedback -- warming melts bright, reflective ice; the darker land or ocean revealed absorbs more heat; that warms the area further, melting more ice.
- Permafrost-methane feedback -- warming thaws frozen ground (permafrost), releasing trapped methane (a strong greenhouse gas); that traps more heat, causing more thawing.