School › Polar regions › Ages 15–16
Polar ice, albedo and sea level
Measure how surface colour changes heating, follow the ice-albedo feedback, turn gigatons of melted ice into millimetres of sea-level rise, and compare how scientists describe changes at the two poles.
Read
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.[2][4][3]
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.[11][7][2][1]
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.[7][2]
Ice sheets lie on land, while sea ice floats. Global sea-level rise comes mainly from meltwater from glaciers and ice sheets and from seawater expanding as it warms. If fully melted, the Antarctic ice sheet would raise sea level by about 60 m and Greenland's by about 7.4 m. Satellites measured losses of about 150 gigatons a year from Antarctica (about 0.4 mm of sea-level rise a year) and about 270 gigatons a year from Greenland (about 0.8 mm a year) between 2002 and 2023.[6]
Antarctica is a polar desert: its snowfall equals only about 150 mm of rain a year, yet its ice sheet, up to 4.8 km thick, holds about 90% of the world's surface fresh water. The record low temperature of −89 °C was measured at Vostok.[5]
Polar ecosystems depend on ice. Algae under sea ice feed krill, a major food of penguins. Polar bears hunt seals from the ice and store fat during the winter hunting season, so earlier melt shortens the time they have to feed. Emperor penguins reduce heat loss by huddling at about 21 birds per square metre at −43 °C.[9][8][10]
Try it: Albedo and heating: white versus black
You need
- Two identical small beakers or cans
- White paper and black paper, sticky tape
- Measuring cylinder and water
- Two thermometers or temperature probes
- Desk lamp with a filament or halogen bulb (or strong sunlight)
- Ruler
- Stopwatch
- Optional: two equal ice cubes on white and black card
Safety
The lamp gets hot: do not touch the bulb or shade, and keep paper at least 20 cm from it. Keep water away from the lamp's plug and cable, and switch off at the socket before moving it. Use lab thermometers carefully.
What to do
- Wrap one beaker in white paper and the other in black paper, including a paper lid with a hole for the thermometer.
- Add 100 g (100 ml) of water at room temperature to each and record the starting temperatures.
- Place both beakers the same distance (for example 30 cm) from the lamp, side by side so they receive the same light.
- Record both temperatures every 2 minutes for 20 minutes.
- Plot temperature against time and find the heating rate (°C/min) for each from the gradient.
- 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).
What happens, and why?
The white surface has a higher albedo: it reflects more of the light, so the water inside absorbs less energy and warms more slowly. The black surface absorbs more and heats faster. In the same way, white sea ice reflects far more sunlight than dark ocean water; when ice melts, more energy is absorbed and more ice melts.[11][7]
Play: Land ice or sea ice?
Does each fact describe land ice (ice sheets and glaciers) or sea ice (frozen seawater)? Sort them.
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Quiz
Other ages: Ages 6–7 · Ages 10–11