Albedo

Topic: Solar Radiation, Heat Balance and Temperature · NCERT: Class 11, Ch 8 "Solar Radiation, Heat Balance and Temperature"

Meaning

Albedo is the share of incoming solar radiation that a surface or a planet reflects back without being heated by it. It is written as a per cent, or as a fraction from 0 to 1. The Earth's albedo is about 30% [2][3]. NCERT gives 35%.

Albedo matters because it largely sets the planet's average surface temperature [3]. If albedo changes, the heat balance changes too. When ice melts, for example, albedo falls and the Earth warms more.

Explanation

How albedo works

• Sunlight reaches a surface → part of it bounces back → that part never heats anything
• A bright surface (high albedo) reflects a lot and stays cooler.

• A dark surface (low albedo) absorbs a lot and gets warmer.

• Sunlight comes in as short waves. This is insolation (incoming solar radiation).

• Reflected sunlight leaves as the same short waves. It is not changed into heat.
• Only the absorbed part heats the ground, sea and air. That heat later goes back out as long waves, called terrestrial radiation (heat given off by the warm ground, sea and air).

Albedo in the Earth's heat budget (NCERT Fig. 8.2)

• The heat budget is the balance between the solar energy the Earth receives and the energy it sends back to space.
• NCERT calls the insolation at the top of the atmosphere 100 units:
• 35 units are reflected to space. This is the Earth's albedo.
  • 27 from cloud tops. These are the biggest reflector.
  • 2 from snow and ice-covered areas.
  • 6 scattered back by the air (gas molecules, dust). This is part of the 35, but NCERT does not list it separately.

• 14 units are absorbed by the atmosphere and 51 by the surface, so 65 are absorbed in total.

• In real units, about 340 W/sq m of solar energy reaches the top of the atmosphere, averaged over the whole planet. About one-third (roughly 100 W/sq m) is reflected. About 240 W/sq m is absorbed [2].

• NASA measures the Earth's albedo with CERES satellite sensors (Clouds and the Earth's Radiant Energy System). It puts the figure at about 30% [2][3].

Albedo of different surfaces

Surface	Albedo (approximate)
Fresh snow	up to 80–90%
Sea ice	50–70%
Desert sand	about 40%
Grassland	about 25%
Forest	10–20%
Open ocean	under 10%

• Two extreme cases [3]:
• An Earth fully covered in ice would have an albedo of about 0.84.
• An Earth fully covered in dark green forest would have an albedo of about 0.14.

Ice-albedo feedback

• Ice-albedo feedback is a self-strengthening loop, called a positive feedback.
• Warming melts snow and sea ice → albedo falls → more warming
• The melting uncovers darker land and dark ocean water.
• Dark surfaces absorb more sunlight, so the region heats up further.

• More heat melts more ice, and the loop repeats.

• Fewer clouds and less sea ice lower the Earth's albedo. This is one reason the Earth's energy imbalance (more energy coming in than going out) about doubled over 2005–2019 [4].

Where it occurs

• Very high albedo (fresh snow): Greater Himalaya, Antarctica, Greenland.
• High albedo (sea ice): Arctic Ocean, Southern Ocean.
• Medium albedo (desert sand): Thar Desert (Rajasthan), Sahara.
• Medium-low albedo (grassland): Steppes, Prairies, Deccan grasslands.
• Low albedo (forest): Western Ghats, Amazon basin, Congo basin.
• Very low albedo (open ocean): Indian Ocean and Pacific Ocean, during high-sun hours.
• Arctic: sea-ice loss through the ice-albedo feedback is a key cause of Arctic amplification. Over 1979–2021, the Arctic warmed nearly four times faster than the globe [5].
• India: shrinking Himalayan snow cover lowers the albedo and speeds up melting. This affects river flow in the Indus, Ganga and Brahmaputra basins.

Don't confuse with

• Insolation: the total solar radiation that arrives (100 units). Albedo is only the part of it that is reflected back (35 units in NCERT).
• Terrestrial radiation: long-wave heat sent out by the warmed Earth after it absorbs sunlight. Albedo is short-wave sunlight reflected without heating anything.
• Absorption by the atmosphere: the 14 units that the air takes in and that heat it. The 6 units scattered back by the air count under albedo, not absorption.
• Negative feedback: a loop that weakens the original change. The ice-albedo feedback is a positive feedback, because it strengthens warming.

Prelims Hooks

• Earth's albedo: about 30% (NASA/CERES) [2][3]. NCERT gives 35 units out of 100.
• Of the 35 units reflected, cloud tops reflect the most (27). Snow and ice reflect only 2, and the air scatters back 6.
• Albedo from highest to lowest: fresh snow > sea ice > desert sand > grassland > forest > open ocean.
• An all-ice Earth would have an albedo of about 0.84, and an all-forest Earth about 0.14 [3].
• CERES satellite sensors measure the Earth's radiation budget, including albedo [2][3].
• The ice-albedo feedback is a positive feedback. It is a key cause of the Arctic warming nearly 4 times faster than the globe over 1979–2021 [5].

Mains Points

• Albedo as a climate lever (GS-I/GS-III):
• When snow, sea ice and cloud cover shrink, the albedo falls and more sunlight is absorbed.
• Through the ice-albedo feedback, this drives Arctic amplification (nearly 4 times the global rate) [5].

• Fewer clouds and less sea ice helped the Earth's energy imbalance double over 2005–2019 [4].

• Himalayan snow and Indian rivers (GS-I):

• Less Himalayan snow cover means a lower albedo and faster melting.

• This changes river flow in the Indus, Ganga and Brahmaputra basins, which affects water supply, farming and hydropower.

• Land-use change and local heat balance (GS-I/GS-III):

• Deforestation and urban heat islands (cities that stay warmer than the countryside around them) change the local albedo.
• That changes how much sunlight is absorbed, and so it changes local temperatures and the local heat balance.

Related concepts

• Heat budget of the Earth
• Latitudinal heat balance
• Earth's energy imbalance

Read more

• The heat budget of the Earth and albedo