Lapse rate feedback

In the tropics the tropopause is much higher in altitude than in temperate regions. The reason is for this is that warm ocean surfaces cool by evaporation moving latent heat upwards where it heats the upper atmosphere through condensation. The lapse rate moves towards the moist lapse rate. The tropopause is where the lapse rate peters out because radiation from GHGs to space dominates over convection. The tropopause is much higher in the tropics because there is so much water vapour in the atmosphere that radiation can only dominate where H2O wavelengths are no longer opaque. Despite this the temperature at these heights is not less than in temperate zones and the radiation loss is significant. This shows how the troposphere lapse rate and height is defined by greenhouse gas concentrations. If you add more water vapour to the atmosphere the troposphere increases in height. This is a negative feedback because more H2O does not lead to enhanced surface warming. On the contrary surface temperatures are stabilised by evaporation

What then happens if you add more CO2 ? Essentially the same negative feedback must occur. The effective height for CO2 radiative cooling to space increases to an initially colder level so leading to a slight increase in forcing. This slight forcing at the surface must also lead to more evaporation from the ocean reducing the lapse rate and thereby increasing the temperature of the effective height for CO2 radiation to space. The result is a negative feedback counteracting the original CO2 forcing.

This same effect happens every day in the tropics as solar radiation increases around midday then so too does evaporation. This is a strong negative feedback keeping ocean temperatures below ~30 deg.C . Exactly the same effect must occur for CO2.

I have been away from the UK for 5 weeks in Australia and Vietnam. Even Darwin ocean temperatures are below 30c while the air approaches 100% relative humidity and 38 C during midday. It then rains most days cooling the surface. Extreme temperatures only occur in deserts where there are no evaporation sources. Otherwise the oceans stabilise temperatures on Earth.

    Scary 5.5 meter crocodile on the Adelaide river just outside Darwin. Keeping cool for the last 50 million years!


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4 Responses to Lapse rate feedback

  1. Euan Mearns says:

    Its interesting Clive. I’ve always taken the view that convection would be the dominant negative feed back. Haughton does say that convection is the dominant mechanism for removing heat from the surface – heat moved up towards the tropopause from where it radiates away through a much thinner column of GHG. A little bit warmer, leads to a little bit more convection leads to increased rate of heat loss. Simplistic I know, but I am a simple geologist 😉 How does this notion of increased convection mesh with what you are saying here? Completely different, or a different flavour of the same thing?

  2. Thanks for this post and I’ll reblog it if you don’t object…

    Here are 2 related recent papers of interest:

    1) “A minimum atmospheric temperature, or tropopause, occurs at a pressure of around 0.1?bar in the atmospheres of Earth1, Titan2, Jupiter3, Saturn4, Uranus and Neptune4, despite great differences in atmospheric composition, gravity, internal heat and sunlight.” “A common dependence of infrared opacity on pressure, arising from the shared physics of molecular absorption, sets the 0.1 bar tropopause.”

    2) New paper finds a negative feedback mechanism of water vapor that may cause global cooling

    A new paper published in Nature Climate Change finds warming of sea surface temperatures in the Indian and Pacific Ocean ‘warm pool’ is causing less water vapor to enter the top of the troposphere and could cause global cooling from this negative feedback.

    I’m interested in your comments on both if you like.


  3. Clive Best says:

    Point 1 is interesting. I think it says that the mean free path for IR photons is on average similar for different greenhouse gases -usually CO2. Of course the details are far more complex as there are hundreds of individual quantum excitation lines but overall this is a god approximation. At 0.1 bar photons escape to space and the troposphere runs out of steam. Probably there are factors of 2-10 differences in pressure depending on GHG levels.

    Point 2 : NVAP data also shows no sign of increased water vapour at the top of the troposphere. H2O and CO2 for that matter in the stratosphere leads to cooling because temperatures start to increase leading to higher radiation losses. You can see that in a infrared spectrum for CO2 and Ozone. The central line shows an upward spike where radiation originates from high up in the stratosphere.

    Water vapour has 2 net negative feedbacks and 1 positive feedback.

    Increased clouds
    Lapse rate reduction


    Increased GHG warming. If now this is not demonstrated in the data then it’s import ace has surely been exaggerated in GCM models.

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