Publication Abstracts

Lacis 2018

Lacis, A.A., 2018: Explaining climate. In Our Warming Planet: Topics in Climate Dynamics. C. Rosenzweig, D. Rind, A. Lacis, and D. Manley, Eds., Lectures in Climate Change: Volume 1. World Scientific #EP 27, pp. 3, doi:10.1142/9789813148796_0001.

It all begins with the Sun. The global-mean solar illumination of the Earth has been determined to be about 340 W/m2 of which about 100 W/m2 is reflected back to space. Hence, the Earth absorbs a global annual-mean 240 W/m2 of solar energy. This amount of energy is just sufficient to support a global-mean temperature of 255 K. However, the global-mean surface temperature of the Earth is known to be about 288 K, which implies that the Planck emission from the ground surface must be about 390 W/m2. It is this missing energy circumstance that led Joseph Fourier to conclude back in 1824 that there must be an atmospheric greenhouse effect causing thermal heat energy to be radiated downward from the atmosphere in order to supply the additional heat energy needed at the ground surface. This is because in the absence of the greenhouse effect, 240 W/m2 of absorbed solar energy is not sufficient to sustain a surface temperature of 288 K.

The flux difference of 150 W/m2 between the 390 W/m2 emitted by the ground surface and the 240 W/m2 of longwave (LW) flux going out to space at the top of the atmosphere is direct measure of the strength of the terrestrial greenhouse effect. The greenhouse action builds up the surface temperature and the surface-emitted flux to 390 W/m2, and is also responsible for the ensuing reduction by 150 W/m2 of the surface-emitted flux as it makes its way to space all of that is accomplished by radiative energy processes (via sequential emission, absorption, and re-emission interactions). Detailed calculations of radiative flux attribution show explicitly that water vapor accounts for about 50 of the 150 W/m2 greenhouse effect, and that LW cloud opacity accounts for 25. Both of these radiative contributions stem from the fast feedback processes of the climate system. The remaining 25 of the terrestrial greenhouse effect comes from the radiative forcings contributed by the non-condensing greenhouse gases (GHGs, which coincidentally also act to sustain the terrestrial greenhouse effect at its present strength). Of these non-condensing GHG contributions, CO2 is by far the strongest contributor accounting for about 20 of the 150 W/m2 greenhouse effect, with the remaining 5 due to minor GHGs such as CH4, N2O, O3, and CFCs (Lacis et al., 2010).

The key point is that these non-condensing GHGs behave as the principal radiative-forcing agents of the climate system because of their thermodynamic, chemical, and radiative properties. CO2 and the minor GHGs are chemically slow-reacting with atmospheric lifetimes that range from decades to many centuries. Once introduced into the atmosphere they effectively remain there indefinitely because they do not condense or precipitate at the prevailing atmospheric temperatures while continuing to exert their radiative forcing. Since CO2 is by far the strongest and most effective of these non-condensing radiative-forcing gases, it follows that CO2 can be identified as the principal LW control knob that governs the global climate of Earth. The fact is that the other forms of radiative climate forcing (e.g., changes in solar irradiance, surface albedo, and aerosol forcing) are small by comparison.

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BibTeX Citation

  author={Lacis, A. A.},
  editor={Rosenzweig, C. and Rind, D. and Lacis, A. and Manley, D.},
  title={Explaining climate},
  booktitle={Our Warming Planet: Topics in Climate Dynamics},
  publisher={World Scientific
#EP 27},
  series={Lectures in Climate Change: Volume 1},

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RIS Citation

ID  - la00500y
AU  - Lacis, A. A.
ED  - Rosenzweig, C.
ED  - Rind, D.
ED  - Lacis, A.
ED  - Manley, D.
PY  - 2018
TI  - Explaining climate
BT  - Our Warming Planet: Topics in Climate Dynamics
T3  - Lectures in Climate Change: Volume 1
SP  - 3
DO  - 10.1142/9789813148796_0001
PB  - World Scientific
#EP 27
ER  -

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