Climate System and Energy Balance
How incoming solar energy, outgoing thermal radiation, and the flows between atmosphere, ocean, ice, land, and biosphere set the temperature and circulation of the planet.
Definition
The climate system is the coupled set of components, atmosphere, hydrosphere, cryosphere, land surface, and biosphere, whose interactions and energy exchanges determine climate, and its energy balance is the equilibrium between absorbed solar radiation and emitted thermal radiation.
Scope
This area covers the climate system as a coupled energy machine: the global radiation budget at the top of the atmosphere and the surface, the partitioning of energy among the atmosphere, ocean, cryosphere, land surface, and biosphere, and the transports that redistribute heat from the tropics toward the poles. It treats the physical basis of the greenhouse effect, the feedbacks that amplify or damp perturbations, and the concept of climate sensitivity that links a change in forcing to an equilibrium change in temperature.
Sub-topics
Core questions
- How is the planetary energy budget partitioned between reflection, absorption, and thermal emission?
- Why does the greenhouse effect raise surface temperature above the radiative equilibrium value?
- Which feedbacks amplify or damp the response to a radiative perturbation, and by how much?
- How do the atmosphere and ocean transport heat to balance the latitudinal imbalance of radiation?
Key theories
- Planetary radiative balance
- In equilibrium the climate system emits as much thermal radiation to space as it absorbs from the Sun, and the planetary albedo together with greenhouse absorption sets the resulting surface temperature.
- Energy transport and the general circulation
- Because the tropics receive more solar energy than they emit while the poles emit more than they receive, the atmosphere and ocean circulate to transport heat poleward, shaping the general circulation.
- Climate sensitivity and feedbacks
- The equilibrium temperature response to a doubling of carbon dioxide depends on feedbacks involving water vapor, clouds, surface albedo, and the lapse rate, which collectively amplify the direct radiative effect.
Mechanisms
Shortwave solar radiation is partly reflected by clouds, aerosols, and the surface, and partly absorbed, warming the system; the warmed surface and atmosphere emit longwave radiation, much of which is absorbed and re-emitted by greenhouse gases before escaping to space. The latitudinal imbalance between absorbed solar and emitted thermal radiation drives atmospheric and oceanic heat transport, while feedbacks adjust the amount of warming needed to restore balance after a forcing.
Clinical relevance
Quantifying the energy budget and climate sensitivity is the foundation for projecting how much the planet will warm for a given change in greenhouse gases, underpinning climate policy targets and impact assessments.
Evidence & guidelines
The IPCC Sixth Assessment Report synthesizes observations and models to constrain the global energy budget and assesses equilibrium climate sensitivity to a likely range of roughly 2.5 to 4 degrees Celsius per carbon dioxide doubling.
History
Quantitative study of the planetary energy balance grew from nineteenth-century work on the greenhouse effect and early energy-budget estimates, advanced through twentieth-century radiative-convective modeling, and was transformed by satellite measurements of the top-of-atmosphere radiation budget that allowed the global energy flows to be closed observationally.
Debates
- The cloud feedback and its effect on climate sensitivity
- How clouds respond to warming, and whether that response amplifies or damps it, remains the largest single source of uncertainty in estimates of climate sensitivity.
Key figures
- Dennis Hartmann
- Kevin Trenberth
- Syukuro Manabe
- Veerabhadran Ramanathan
Related topics
Seminal works
- hartmann2016
- trenberth2009
Frequently asked questions
- What keeps Earth warmer than its radiative equilibrium temperature?
- Greenhouse gases absorb and re-emit thermal radiation, so the surface stays roughly 33 degrees Celsius warmer than it would be with a transparent atmosphere.
- What is climate sensitivity?
- It is the long-term global warming expected from doubling atmospheric carbon dioxide, assessed by the IPCC as likely about 2.5 to 4 degrees Celsius once feedbacks act.