Desert Nights, Magic and Global Warming

We all know the theory. The average temperature of the Earth would  only be about 6 degrees C. if it had no atmosphere. This temperature can be derived from Stefan Boltzman’s law assuming that the Earth is a perfect Black Body in thermal equilibrium with the Sun’s radiant energy. Luckily greenhouse gases in the Earth’s atmosphere actually keep the Earth’s temperature at a more comfortable ~ 15 degrees C. Human activity has led to an increase of about 25% in CO2 concentration in the Earth’s atmosphere  and this is now predicted to increase average temperaturesby a couple of degrees over the next century. Glaciers will melt, sea levels will rise, people will migrate, our way of life is to blame – we must repent etc.  So if all this is now accepted as factual evidence, then how come the desert is so cold at night ?

Temperatures in the Sahara and other deserts can reach 45 degrees during the day but then magically fall to below freezing at night. Now the CO2 concentration in the air is exactly the same over the Sahara as it is anywhere else e.g. in Hawaii where long term  measurements are made. How does this magic work? – surely human additions to CO2 that we are so alarmed about must also cause greenhouse warming in deserts.  The greenhouse effect acts like a blanket helping  block infra red radiation losses to space and thereby keeps the Earth warm. So what is going wrong and why is it that the greenhouse effect simply  doesn’t seem to work at all deserts ?  The answer to this question is not only interesting  scientifically but because it exposes a flaw in the selective use of data to overstate the climate change issue for political reasons.

The desert temperature falls dramatically at night simply  because the air is dry. There is little water vapour in the air and it is water vapour which dominates all other greenhouse gasses thereby contributing  about  75% of the greenhouse effect on planet Earth, while CO2 causes only  10% of the warming. The  main reason for this discrepancy is that there is a tiny amount of CO2 in the atmosphere compared with the amount of water vapour.  This is not about clouds which are another story – it is just about the number of H2O molecules in clear air  compared to the number of CO2 molecules. The amount of water vapour in the air varies dramatically from the tropics to arctic and desert regions but on average it is about 1-4 % at the surface. CO2 is just 0.038% so at the surface there are about 100 water molecules for every CO2 molecules and averaged over the entire atmosphere there are at least 10 times more water molecules. Without water vapour  the greenhouse effect would be at least one order of magnitude smaller, and the direct evidence for this are those very cold nights with sub zero tempreatures in the desert where the air is dry.

Water vapour is often taken out of the equation  because it is a supposed feedback greenhouse gas. The air can hold more water vapour the warmer it gets so the argument goes that CO2 is the real driver for warming so that as the atmosphere warms it holds more water vapour thereby warming even more – a positive feedback. As air rises so it cools and given sufficient concentration water vapour condenses out as clouds at a certain height once it reaches the dew point. Clouds then can  have two separate effects : 1) They reflect incoming sunlight back to space thereby cooling the land and 2) They reflect infra-red back to the earth thereby warming. During the day the former usually wins which is why it feels cold when the cloud blocks the sun and at night the second effect dominates keeping the land warmer than it otherwise would be. So it is not clear whether clouds warm or cool the Earth or whether they produce a negative or positive feedback. Imagine a world completely covered in white clouds. Would the temperature at the surface be higher or lower than today. My gut feeling is that it would be cooler as more of the sun’s energy would be reflected back to space.

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One Response to Desert Nights, Magic and Global Warming

  1. Hi, I’m surprised that as you are a scientist and traveller you have fallen for a widely repeated ‘truism’ concerning AGW and desert temperatures. Sure one of those things learnt at school is the wide temperature differential in deserts because the lack of cloud cover but this truism is neither the full or detailed picture. I refer you to mean high and low temps in Death Valley http://en.wikipedia.org/wiki/Death_Valley
    As you can see deserts experience seasons with winter nights being cold and summer nights 30c, which is hardly freezing. Having spent time in the middle of the Sahara I can vouch for cool even cold nights in winter months and uncomfortably hot nights in summer, and I can assure you the relative humidity during the day was very low, yet at night condensation would occur.

    It is a fascinating subject that deserve more research.

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