Heat budget of the Earth

Also called: Heat balance, Energy budget · Topic: Solar Radiation, Heat Balance and Temperature · NCERT: Class 11, Ch 8 "Solar Radiation, Heat Balance and Temperature"

Meaning

The heat budget of the Earth (also called heat balance or energy budget) is the balance between the solar energy the Earth receives as short waves and the energy it sends back to space as long waves. Over a long period, energy in equals energy out, so the Earth as a whole neither keeps warming nor keeps cooling. It explains why the atmosphere is heated mainly from below. It also explains why heat must move from the tropics to the poles, and why today's small imbalance shows global warming directly.

Explanation

How the balance works (NCERT Fig. 8.2, 100-unit model)

• Insolation (incoming solar radiation) at the top of the atmosphere is about 340 W/sq m, averaged over the whole planet [2]. NCERT calls this 100 units to keep the sums simple.
• Step 1: the 100 units split three ways
• 35 units reflected back to space without heating anything (this is the albedo):
  • 27 from cloud tops (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, though NCERT does not list it separately.
• 14 units absorbed by the atmosphere, mostly by water vapour, ozone and dust.
• 51 units absorbed by the Earth's surface.

• Total absorbed = 14 + 51 = 65 units.

• In real units, about one-third (roughly 100 W/sq m) is reflected and about 240 W/sq m is absorbed by land, ocean and atmosphere [2].

Where the surface's 51 units go

• The surface gives off its heat as terrestrial radiation (long-wave heat given off by the warm ground, sea and air).
• 17 units go straight to space. They pass through "windows" in the atmosphere where gases do not absorb.
• 34 units go into 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 ("hidden" heat. Water takes it in when it evaporates and gives it to the air when the vapour condenses into cloud droplets.)

• The atmosphere is heated mainly from below

• It gets 34 units from the surface.
• It gets only 14 units directly from the sun.

The books balance

• The atmosphere holds 14 + 34 = 48 units and radiates all 48 to space.
• Total outgoing long-wave = 17 (surface) + 48 (atmosphere) = 65 units.
• 65 out = 65 absorbed. So the average temperature stays steady.
• Exam practice: draw three boxes (space, atmosphere, surface). Add arrows for 35, 14, 51, 17, 6, 9, 19 and 48. Check that 65 in equals 65 out.

Albedo, latitudinal balance and today's imbalance

• Albedo: the share of incoming sunlight a surface reflects without being heated by it. A bright surface has a high albedo. A dark surface has a low albedo.
• NASA puts the Earth's albedo at about 30%, measured by CERES satellite sensors (Clouds and the Earth's Radiant Energy System) [2][3]. NCERT gives 35%.

• Albedo largely sets the planet's average surface temperature [3]. An Earth fully covered in ice would have an albedo of about 0.84. One fully covered in dark green forest would have about 0.14 [3].

• Latitudinal heat balance (Fig. 8.3): the global budget balances, but not at every latitude.

• About 40°N to 40°S: surplus. The sun is high in the sky all year.
• Poleward of 40°: deficit. The sun's rays are slanting, and winters bring long darkness.

• Winds (Hadley, Ferrel and polar cells, and cyclones) and ocean currents carry the extra heat poleward.

• Earth's energy imbalance (EEI): today more energy comes in than goes out, by roughly 0.5–1 W/sq m.

• It about doubled over 2005–2019 (joint NASA–NOAA study, 2021) [4].
• Causes [4]:
  • More greenhouse gases and water vapour trap outgoing long waves.
  • Fewer clouds and less sea ice lower the albedo, so more sunlight is absorbed.
  • A warm phase of the Pacific Decadal Oscillation (PDO: a long-lasting pattern of warm or cool sea-surface temperatures in the North Pacific) from about 2014 reduced clouds over the ocean.
• About 90% of the extra heat goes into the oceans [4][6].

Where it occurs

• The whole planet: the 100-unit budget is a global average for the entire Earth–atmosphere system.
• Surplus belt, about 40°N to 40°S: all of India lies in it. So does the Indian Ocean.
• Deficit regions poleward of 40°: the Arctic and Antarctica, which depend on heat brought in by winds and currents.
• Gulf Stream: 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. It keeps ports in Norway and the UK ice-free in winter.
• High-albedo surfaces: fresh snow on the Greater Himalaya, Antarctica and Greenland. Sea ice in the Arctic Ocean and Southern Ocean. Desert sand in the Thar (Rajasthan) and Sahara.
• Low-albedo surfaces: forests of the Western Ghats, the Amazon and the Congo basin. The open Indian and Pacific Oceans.
• Arctic: warmed nearly four times faster than the globe over 1979–2021 [5]. The ice-albedo feedback is a key cause.
• Himalaya (India): shrinking snow cover lowers the albedo and speeds up melting. This affects the Indus, Ganga and Brahmaputra basins.
• Measurement: CERES satellites track radiation at the top of the atmosphere. Argo ocean floats track how fast the oceans warm [4].

Don't confuse with

• Insolation vs terrestrial radiation: insolation is incoming solar energy as short waves. Terrestrial radiation is heat given off by the Earth as long waves. The budget balances the two.
• Heat budget vs Earth's energy imbalance (EEI): the heat budget is the long-term state where energy in = energy out. EEI is the present-day gap, where more comes in than goes out, so the planet is gaining heat.
• Albedo vs absorption: albedo (35 units in NCERT) is reflected without heating anything. The 65 absorbed units are what actually warm the Earth and then leave as long waves.
• Global balance vs latitudinal balance: the globe as a whole balances. Individual latitudes do not. There is a surplus between about 40°N and 40°S and a deficit poleward of that.

Prelims Hooks

• NCERT 100-unit budget: 35 reflected, 14 absorbed by the atmosphere, 51 absorbed by the surface. The atmosphere is heated mainly from below.
• The largest flow from the surface into the atmosphere is latent heat of condensation (19 units). It is bigger than convection and turbulence (9) and direct absorption (6).
• Of the 35 reflected units, cloud tops reflect 27. Snow and ice reflect only 2.
• Outgoing radiation: 17 (surface) + 48 (atmosphere) = 65 = units absorbed.
• Earth's albedo: about 30% (NASA/CERES) vs 35% (NCERT). Albedo from highest to lowest: fresh snow > sea ice > desert sand > grassland > forest > open ocean.
• EEI doubled over 2005–2019 (CERES + Argo). About 90% of the extra heat is stored in the ocean, not the air [4]. Ice-albedo feedback is a positive feedback.

Mains Points

• Heat transfer drives world circulation (GS-I)
• There is a surplus between about 40°N and 40°S and a deficit poleward of it.
• This drives 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 high latitudes would freeze permanently. This is the root cause of world wind and ocean-current patterns.

• Albedo as a climate lever (GS-I/GS-III)

• Shrinking snow, sea ice and cloud cover lower the albedo. More sunlight is absorbed, and the warming feeds on itself (the ice-albedo feedback).
• Result: Arctic amplification, nearly 4 times the global rate [5]. In India, less Himalayan snow cover speeds up melting and changes river flow in the Indus, Ganga and Brahmaputra basins.

• Deforestation and urban heat islands also change the local albedo and the local heat balance.

• EEI as a climate indicator (GS-III)

• EEI is about 0.5–1 W/sq m and doubled over 2005–2019 [4]. It shows global warming directly. WMO's State of the Global Climate 2025 listed it as a key climate indicator for the first time and reported it at its highest in the 65-year record [6][7].
• About 90% of the heat goes into the oceans. This causes sea-level rise through thermal expansion (water expanding as it warms), marine heatwaves, coral bleaching and stronger cyclones. All of these matter for India's 11,000-plus km coastline.
• Way forward: cut greenhouse gas emissions, and expand ocean observation (Argo floats) and satellite radiation monitoring (CERES).

Related concepts

• Albedo
• Latitudinal heat balance
• Earth's energy imbalance

Read more

• The heat budget of the Earth and albedo