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The angle of sunlight is the main reason the poles are cold and the equator warm. Above about 66°30′ latitude, Earth's tilt gives at least one day a year of continuous daylight and one of continuous darkness; at the North Pole the Sun is below the horizon from about 21 September to 21 March.

Albedo is a unitless number from 0 to 1 that measures how well a surface reflects solar energy: 0 absorbs everything, 1 reflects everything. White sea ice reflects far more sunlight than the darker ocean. When ice melts, the exposed water absorbs more energy and warms, melting more ice: a positive feedback. NSIDC pages say the Arctic is warming about two to three times faster than the planet as a whole.

Sea ice forms as frazil crystals, then nilas or pancake ice; it expels salt, making the water below colder, saltier and denser so that it sinks. Ice that survives the summer can thicken to about 3–4 m. From 1979 to 2020 the Arctic summer minimum fell by 13.4% per decade relative to the 1981–2010 average, with a record low of 3.41 million km² in 2012. Antarctic sea ice varies a lot from year to year, with a record high in 2014 and record lows in 2023.

Experiment: Albedo and heating: white versus black

1. Wrap one beaker in white paper and the other in black paper, including a paper lid with a hole for the thermometer.
2. Add 100 g (100 ml) of water at room temperature to each and record the starting temperatures.
3. Place both beakers the same distance (for example 30 cm) from the lamp, side by side so they receive the same light.
4. Record both temperatures every 2 minutes for 20 minutes.
5. Plot temperature against time and find the heating rate (°C/min) for each from the gradient.
6. Calculate the energy absorbed using Q = m c ΔT with c = 4.18 J/(g·°C), and discuss the uncertainty (thermometer resolution, lamp position, heat loss to the air).

Surface	Start temperature (°C)	After 20 min (°C)	Rise ΔT (°C)	Rate (°C/min)	Energy absorbed Q (J)
White	Prediction
Black	Prediction

Questions

1. In the albedo practical, the white beaker warmed from 20.0 °C to 23.5 °C and the black beaker from 20.0 °C to 29.0 °C in 20 minutes. Each held 100 g of water. Calculate each heating rate and the energy absorbed (c = 4.18 J/(g·°C)).

2. Using NSIDC's figures, calculate the gigatons per millimetre of sea-level rise from the Antarctic data (150 Gt, 0.4 mm) and from the Greenland data (270 Gt, 0.8 mm). Why are the two answers not the same?

3. Draw a flow diagram of the ice-albedo feedback and explain why it is called positive.

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4. NSIDC describes Arctic sea ice as declining but Antarctic sea ice as highly variable with little overall trend. Suggest why scientists are more cautious about the Antarctic trend.

Sources for every fact on this sheet: echoesof.science/school/polar-regions/sources/

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