The heat budget of the Earth and albedo

Detail

What "heat budget" means

• Heat budget of the Earth: the balance between the solar energy the Earth receives and the energy it sends back to space.
• Insolation (incoming solar radiation) arrives as short waves.
• The Earth sends energy back as long waves (terrestrial radiation: heat given off by the warm ground, sea and air).

• Over a long period, energy in = energy out. So the Earth as a whole neither keeps warming nor keeps cooling.

• 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 by the land, ocean and atmosphere [2].

• NCERT makes the arithmetic simple by calling the insolation at the top of the atmosphere 100 units (Class 11, Fig. 8.2).

The 100-unit budget, step by step (Fig. 8.2)

Step 1: Incoming 100 units are split three ways

Flow	Units	What happens
Reflected to space (albedo)	35	Sent back before it can heat anything
– from cloud tops	27	The biggest reflector
– from snow and ice-covered areas	2
– scattered back by the air (gas molecules, dust)	6	Part of the 35, though NCERT does not list it separately
Absorbed by the atmosphere	14	Mostly by water vapour, ozone and dust
Absorbed by the Earth's surface	51	The surface gets more than the air
Total absorbed	65	14 + 51

Step 2: The surface sends back its 51 units as terrestrial (long-wave) radiation

• 17 units go straight out to space. They pass through "windows" in the atmosphere where gases do not absorb.
• 34 units are taken up by the atmosphere:
• 6 absorbed directly as long-wave radiation
• 9 by convection and turbulence (convection: warm air rises and carries heat up; turbulence: swirling, mixing air)
• 19 as latent heat of condensation. Latent heat is "hidden" heat. Water uses it to evaporate. The heat is released into the air when the vapour condenses into cloud droplets.

Step 3: The atmosphere sends its energy to space

• The atmosphere holds 14 (absorbed from the sun) + 34 (received from the surface) = 48 units. It radiates all 48 to space.

Step 4: The balance

• Total outgoing long-wave = 17 (from the surface) + 48 (from the atmosphere) = 65 units.
• This equals the 65 units absorbed. The books balance.
• Key lesson: the atmosphere is heated mainly from below, by the Earth's surface (34 units), not directly by the sun (14 units).
• Exam practice: redraw Fig. 8.2 from memory. Draw three boxes (space, atmosphere, surface) and arrows carrying 35, 14, 51, 17, 6, 9, 19 and 48. Check that the 65 in equals the 65 out.

Albedo

• Albedo: the share of incoming solar radiation that a surface or planet reflects without being heated by it. It is written as a per cent (or as a fraction from 0 to 1).

• High albedo means a bright surface that reflects a lot. Low albedo means a dark surface that absorbs a lot.

• The Earth's albedo: NASA puts it at about 30%, measured by the CERES satellite sensors (Clouds and the Earth's Radiant Energy System) [2][3]. (NCERT: 35%.)

• Albedo matters because it largely sets the planet's average surface temperature [3].

Albedo of common surfaces (approximate)

Surface	Albedo	Where you see it
Fresh snow	up to 80–90%	Greater Himalaya, Antarctica, Greenland
Sea ice	50–70%	Arctic Ocean, Southern Ocean
Desert sand	about 40%	Thar (Rajasthan), Sahara
Grassland	about 25%	Steppes, Prairies, Deccan grasslands
Forest	10–20%	Western Ghats, Amazon, Congo basin
Open ocean	under 10%	Indian Ocean, Pacific (high-sun hours)

• Two extreme cases [3]:
• An Earth covered fully in ice would have an albedo of about 0.84 (84% reflected).

• An Earth covered fully in dark green forest would have an albedo of about 0.14.

• Ranking trap: fresh snow > sea ice > desert sand > grassland > forest > open ocean.

Ice-albedo feedback and Arctic amplification

• Ice-albedo feedback: a self-strengthening loop (a positive feedback).

• Bold chain → warming melts snow and sea ice

  • The melting uncovers darker land and dark ocean water underneath.
  • Dark surfaces absorb more sunlight, so the region heats up further.
  • More heat melts more ice, and the loop repeats.

• Arctic amplification: the Arctic warms faster than the rest of the planet.

• Over 1979–2021 (43 years), the Arctic warmed nearly four times faster than the globe. Older studies said "two to three times" [5].

• The loss of sea ice, through the ice-albedo feedback, is a key cause. Others are the moisture and lapse rate feedbacks (lapse rate: the fall in temperature with height) and heat carried in by the ocean and the air [5].

• Link: climate-change. In India, the same logic applies to shrinking snow cover in the Himalaya.

Latitudinal heat balance (Fig. 8.3)

• Latitudinal heat balance: the way net radiation (energy in minus energy out) changes from the equator to the poles.
• Between about 40°N and 40°S: surplus. More energy comes in than goes out, because the sun is high in the sky all year.

• Poleward of 40° (towards the poles): deficit. More energy goes out than comes in, because the sun's rays are slanting and there is long winter darkness.

• Bold chain → surplus heat is moved poleward

• By winds: the Hadley cell (about 0°–30°), Ferrel cell (about 30°–60°) and polar cell (about 60°–90°) are giant loops of rising and sinking air. Cyclones (large storms around a low-pressure centre) also carry heat and moisture poleward.
• By ocean currents: for example, the Gulf Stream. It is a warm current that starts in the Gulf of Mexico, flows out through the Florida Strait, and continues as the North Atlantic Drift to north-west Europe. This keeps ports in Norway and the UK ice-free in winter.

• Result: the tropics do not keep heating up, and the high latitudes do not freeze permanently.

• This imbalance is the driving force of the general circulation of the atmosphere (link: pressure-winds-circulation) and of ocean currents (link: ocean-movements).

Earth's energy imbalance (beyond NCERT)

• Earth's energy imbalance (EEI): the gap between the solar energy the Earth absorbs and the energy it sends back to space. Today more comes in than goes out, so the planet is gaining heat.
• Size: roughly 0.5–1 W/sq m. The imbalance about doubled over 2005–2019, according to a joint NASA–NOAA study published in Geophysical Research Letters on 15 June 2021 (Loeb and others, 2021) [4].

• It was measured in two separate ways that agree well: CERES satellites (energy in and out at the top of the atmosphere) and the Argo ocean floats (how fast the oceans are warming) [4].

• Causes of the doubling [4]:

• More greenhouse gases (carbon dioxide, methane) from human activity trap outgoing long-wave radiation.
• More water vapour traps further heat.
• Fewer clouds and less sea ice lower the albedo, so more sunlight is absorbed. This is the albedo idea in action.

• A strongly warm phase of the Pacific Decadal Oscillation (PDO) from about 2014 reduced clouds over the ocean. (PDO: a long-lasting pattern of warm or cool sea-surface temperatures in the North Pacific.)

• Where the extra heat goes:

• About 90% goes into the oceans [4][6]. As water warms, it expands (thermal expansion), which raises sea level.
• The rest melts ice and warms the land and air.
• Ocean heat content hit its highest level in the 65-year record in 2024, and rose further in 2025 (preliminary data) [6][7].

• Each year for the past two decades, the ocean has absorbed about 18 times humanity's annual energy use [6][7].

• WMO's State of the Global Climate 2025 included EEI as a key climate indicator for the first time. It reported EEI at its highest in the 65-year record [6][7].

Prelims Hooks

• In NCERT's 100-unit budget, the atmosphere absorbs 14 units directly from the sun, while the surface absorbs 51. The atmosphere is heated mainly from below.
• The largest single flow from the surface into the atmosphere is the latent heat of condensation (19 units). It is larger than convection and turbulence (9) or direct absorption (6).
• Of the 35 units reflected, cloud tops reflect the most (27), far more than snow and ice (2).
• Outgoing radiation: 17 from the surface + 48 from the atmosphere = 65 = units absorbed.
• Earth's albedo: about 30% (NASA/CERES); NCERT gives 35%.
• Albedo, highest to lowest: fresh snow > sea ice > desert sand > grassland > forest > open ocean.
• Surplus zone in Fig. 8.3: about 40°N to 40°S. Poleward of it is a deficit, which is made up by winds and ocean currents (e.g. the Gulf Stream).
• CERES measures the radiation budget from satellites. Argo floats measure ocean heat. Both showed EEI doubled over 2005–2019.
• About 90% of the excess heat is stored in the ocean, not the atmosphere.
• The Arctic warmed nearly 4 times faster than the global average over 1979–2021. A key cause is the ice-albedo feedback (a positive, not negative, feedback).

Mains Points

• Heat transfer and the general circulation (GS-I): the surplus between about 40°N and 40°S and the deficit poleward of it drive the Hadley, Ferrel and polar cells, mid-latitude cyclones and warm currents such as the Gulf Stream. Without this transfer, the tropics would keep heating and the poles would keep cooling. This is the root cause of world wind and ocean-current patterns.
• Albedo as a climate lever (GS-I/GS-III): when snow, sea ice and cloud cover shrink, the albedo falls. More sunlight is then absorbed, and Arctic amplification follows (nearly 4 times the global rate). For India, less Himalayan snow cover means a lower albedo and faster melting. This affects river flow in the Indus, Ganga and Brahmaputra basins. Land-use change (deforestation, urban heat islands) also changes the local albedo and local heat balance.
• Earth's energy imbalance as a climate indicator (GS-III): EEI (about 0.5–1 W/sq m, doubled over 2005–2019) shows global warming directly. About 90% of the heat goes into the oceans. This drives sea-level rise by thermal expansion, marine heatwaves and coral bleaching, and stronger cyclones. These are relevant to India's 11,000-plus km coastline. The way forward: cut greenhouse gas emissions, and expand ocean observation (Argo floats) and satellite radiation monitoring.