Radiative Equilibrium & Convection

Surface heat loss by moist convection dominates over radiative heat loss from the surface. If somehow you could switch off convection then surface temperatures on Earth would need to rise by about 60 deg.C to bring the atmosphere into a pure radiative equilibrium with space.

Figure 1: Comparison of the net upward  radiative flux in the main CO2 absorption band. In black - with convection. In red - pure radiative energy balance.

Figure 1: Comparison of the net upward radiative flux in the main CO2 absorption band. In black – radiative transfer with convection. In red – pure radiative energy balance without convection.

I became fascinated by an article written by Richard Lindzen about 10 years ago [1] in which he wrote, when discussing the greenhouse effect on Earth.

When all these things (greenhouse gases,clouds, water vapour) are accounted for in sophisticated radiative transfer models, the equilibrium surface temperature works out to be 350K or about 80C. But the equilibrium temperature decreases rapidly with height reaching a minimum of 210K (-60C) about 8 miles above the surface…. But the rapid decrease in temperature with altitude cannot be sustained because it is unstable to convective overturning.

In the absence of convection  the surface could only cool  through radiation alone. A pure radiative equilibrium on Earth would then result in a much larger  greenhouse effect with a surface temperature of 350K !

How moist convection reduces the greenhouse effect through the lapse rate.

How moist convection reduces the greenhouse effect through the lapse rate.. The dashed curve represents the lapse rate for pure radiative equilibrium.

Convection, evaporation and the environmental lapse rate reduces the greenhouse effect on Earth because they act somewhat like a “release valve” moving heat up more efficiently through the atmosphere to then radiate out to space. The more humid the air the less steep the lapse rate and the less effect any increases in CO2 will have on the surface temperature. Just how important convection is to the Earth’s climate can be seen in Figure 1. I ran my radiative transfer model for the CO2 15 micron band first for the observed surface temperature and lapse rate shown in black, and then a second time for the pure radiative energy balance and steeper lapse rate shown in red.

Convection is mainly determined by gravity and the  heat capacity of gases in the atmosphere, modified by latent heat of water.

But supposing that the atmosphere was stratified in some way to stop convection – something like a series of transparent shells blocking air movement vertically but still transmitting radiation both up and down. In such a hypothetical case the surface could only cool through radiative transfer. The surface would then heat up enough to sustain a temperature gradient for 100% radiative cooling reaching a surface temperature of 350K.

Convection and evaporation really do moderate the greenhouse effect.

1. Greenhouse Effect: A Scientific Analysis” (Grazing Lands)

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5 Responses to Radiative Equilibrium & Convection

  1. admin says:

    Convection also moves heat from the equators to the poles and makes global temperature more uniform. I am not sure what you mean by moderate the greenhouse effect. It is not like CO2 is concentrated in one region over the Earth and it takes convection to spread that out to “moderate” its effect.

    • Clive Best says:

      Yes convection drives the weather systems on Earth moving heat to the poles.

      I really just meant the heat loss vertically to space averaged over all the surface. If the Earth only had radiative transfer available to lose heat then the surface would need to be 60C warmer than it is today. Convection and Latent heat are more efficient at moving heat from the surface to the upper atmosphere than radiative transfer by about a factor 2.

  2. Cry Wolf says:

    Clive, Houghton says p 18:

    “Within the atmosphere itself (at least within the lowest three-quarters or so of the atmosphere up to a height of about 10 kms which is called the troposphere) convection is, in fact, the dominant process for transferring heat.”

    I am a geologist, and this makes sense to me as a thermal regulation system. If the lower troposphere warms a bit this enhances convection (thunder storms) transferring heat from surface to tropopause where heat may be lost radiatively passing through a much thinner column of GHG, enhancing the heat loss rate. A powerful, regulatory, negative feed back mechanism?

    Can you tell me if GCM’s incorporate variable heat loss via convection as a negative feed back loop.


    • Clive Best says:

      I don’t claim to be an authority on GCMs. Convection and evaporation are the dominant processes for surface heat loss in the troposphere. Radiation from the surface is about half this value. Increased evaporation/condensation (latent heat) reduces the lapse rate acting as a negative feedback. This is partially included in GCMs, but the full picture must be complex including changes in cloud cover (albedo) – another net negative feedback.

  3. nuwurld says:

    Hi Clive, I think the issue is that the lapse within the troposphere hardly differs from that predicted by totally ignoring internal radiative exchange. -g/Cp only acknowledges vibrational bands in a reduction of the lapse from a mean radiative height by increased specific heat capacity. This must be more significant than suggesting that convection has somehow hauled the lapse away from a radiative balance that “never existed”?

    Regards. Geo

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