M.J. Harper has proposed a new theory for the distribution of the Earth’s deserts and presents it with a DVD “The Distribution of Deserts, A New Theory, M J Harper”, available from Amazon here. It is not available on Youtube but I have watched it and it is very interesting. However a preliminary video on Youtube presents the core ideas.
The theory is based on the observation that the Earth’s atmosphere never reaches 100% saturation with water vapour because sea level has, everything else being equal, reached an equilibrium level implying evaporation from the oceans is no longer occurring; thus the origin of atmospheric water is not so much from oceanic evaporation as commonly accepted as more from vegetative transpiration. This flies in the face the hydrological orthodoxy and is Harper’s principal theory. Harper points out while a steady state between water and air in a closed container reaches a specific air saturation level in the laboratory for a particular temperature, this does not seem to occur in the real-world situation, hence the puzzle of desert formation.
Another observation made is that the Earth’s weather systems generally move from west to east, though obviously the motion of cyclones/hurricanes/typhoons belies this general assumption since these weather phenomena don’t, and often conspicuously so. But these short duration, often cataclysmic, weather phenomena are, none the less, superimposed on the more general long-term west-east movement of weather systems. (In general it may be said that the rotating atmosphere seems to drag the Earth along with it, incidentally suggesting that the driving force for the Earth’s rotation is external, with the atmosphere/liquid components rotating faster than the solid component below).
So what is the theory? Harper’s theory is that deserts form on continents when there are no trees/forests or vegetation water sources to the west along a particular latitude, so that the atmosphere of the weather systems that moves west to east are undersaturated with respect to water, and hence little to no precipitation of this water occurs on landfall to the east of the ocean, thus forming deserts. It also implies that there is also little to no evaporation from the oceans west of the landmass at a particular latitude. The corrollary is that if large vegetated landmasses exist west of a land mass, say Canada with respect to England and Europe, then deserts will not form since the westerly forrests are pumping water into the atmosphere that comes down on the landmasses etc to the east. Hence Harper’s principal theory is that atmospheric water is dominated by water emission from the plant kingdom and not from the Earth’s oceans.
The flaw in the argument is the tacit assumption that the existing landmass/ocean orientations with respect to the Earth’s rotational axis has been geologically constant. Given that the Sahara was recently a wetlands area, this suggests that during those times there may not have been an Atlantic Ocean. Or the Earth might have been rotating on a different axis than it is now. This may then explain the desert anomalies that Harper has identified, and that the anomalies are remnants of previous ‘Harper Deserts’ when his rule of thumb applied to the then landmass/ocean distribution.