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The Greenhouse Effect and Energy Imbalance

Earth's climate is set by a balance between energy from the Sun and energy radiated back to space. Human activity has now disrupted this equilibrium.

The Greenhouse Effect and Energy Imbalance

The Earth stays habitable because of a delicate balancing act. It sits in the cold vacuum of space, constantly receiving an enormous flow of solar energy that heats it up. To keep its temperature stable, it must release an equal amount of energy back into space. Currently, greenhouse gases emitted by humans have upset this balance, leading to an accumulation of thermal energy within the Earth system.

Without an atmosphere, the Earth would be a frozen desert with an average temperature of -18 °C. Instead, our planet averages a life-sustaining 15 °C. This 33 °C difference is provided by the greenhouse effect.

Greenhouse gases act like a one-way filter: they are transparent to incoming shortwave solar radiation but absorb the longwave thermal radiation emitted by the Earth as it passes through the atmosphere. Then they re-radiate that energy in all directions and send a substantial portion back down toward the ground – called back radiation – which warms the surface to the current level.

The most powerful greenhouse gas is water vapour (H₂O), responsible for over half of the effect. It acts as an amplifying feedback: as the air warms, it holds more vapour, which traps more heat in turn.

While much less abundant than water vapour, carbon dioxide (CO₂) plays a crucial role. Because it stays in the atmosphere for centuries, it sets the baseline temperature that allows other factors, like water vapour, to react.

How does the Earth exchange energy?

The Earth’s climate is governed by the exchange of radiation between space, the atmosphere, and the planet’s surface. The energy fluxes are measured in Watts per square meter (W/m2).1

  • Incoming solar radiation: The Sun delivers approximately 340 W/m2 of shortwave light. About 100 W/m2 of this is immediately reflected away by clouds, atmospheric particles, and bright ice. The remaining 240 W/m2 is absorbed by the surface (160 W/m2) and the air (80 W/m2).

  • Thermal radiation from the surface: Like all physical objects, the Earth emits thermal radiation proportional to its temperature – about 398 W/m2.

  • Additional heat transfer: Energy also leaves the surface through direct heat transfer to the air (sensible heat, 21 W/m2) and through water evaporation (82 W/m2 – energy stored in water vapour and released higher in the atmosphere when the vapour condenses).

  • Back radiation: The atmosphere with greenhouse gases intercepts much of this thermal radiation and sends it back to the ground (342 W/m2).

  • Outgoing thermal radiation: 239 W/m2 escapes from the Earth to space. To maintain a stable temperature, the planet should lose an equal amount of energy as it receives from the Sun.

Why is there an energy imbalance now?

By emitting greenhouse gases, humans have strengthened the greenhouse effect. The graph shows that the radiation going out of the Earth system is about 1 W/m2 lower than the radiation coming in, resulting in about 1 W/m2 more energy (and growing in the last decades)2 warming the Earth’s surface and creating an energy imbalance.

A single Watt per square meter may sound negligible, but the scale of our planet turns this into a massive amount of energy – equivalent to the energy of eight Hiroshima-style atomic bombs exploding every second. Over 90% of this surplus heat is being absorbed by the oceans, the rest by land, by ice sheets, and (about 1%) by the atmosphere.3 Until humanity stops emitting greenhouse gases, the planet will continue to store more heat.

Footnotes

  1. The graphics including the numbers are based on Figure 7.2 of IPCC AR6 WGI report. Intergovernmental Panel on Climate Change. (2021). Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to AR6. DOI: 10.1017/9781009157896. ↩

  2. Mauritsen, T. et al. (2025). Earth’s energy imbalance more than doubled in recent decades. AGU Advances, 6, 3. DOI: 10.1029/2024AV001636. ↩

  3. See Table 7.1 of IPCC AR6 WGI report. Intergovernmental Panel on Climate Change. (2021). Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to AR6. DOI: 10.1017/9781009157896. ↩