Green Desert

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From Climate Scepticism


As so often happens on the interweb, I found myself lost in a rabbit hole of complexity where naively I had expected to find simplicity. What began as a plan to explain to Cliscep readers the basics of the sceptic’s favourite spicy sauce, “global greening,” soon became complicated because the facts themselves are complicated. For example, there is a vegetation index that I am familiar with and know how to calculate*, which is one that has shown greening in the satellite era. However, only now reading the wider literature on global greening, I soon found out that my simple index is actually fiendishly difficult to interpret. For that reason among others, I’m not going to talk about global greening right now – that will have to wait for another day. Instead I’m going to tell you about one of the innumerable side-branches of the rabbit warren I found myself in.

I was looking for a good place to illustrate the basic concept of a gradual change in greenness over time since the dawn of the satellite era. To that end I thought somewhere in the Sahel would be a good model. On Nasa’s excellent Worldview website**, you can display many data layers, including vegetation indices. With Landsat you can go back to about 1983, and data from Terra/Aqua become available I think in 2002. So I was looking at the NDVI record for North Africa from outer space, scrolling through time, looking for a place that might make an exemplar of the theme, when I spotted what looked like a sudden blossoming of life not at the join between Sahara and Sahel but in pure desert – once utterly lifeless sand in the middle of nowhere suddenly lighting up the NDVI scale.

1986 – NDVI based on Landsat data

2010 – Aqua/MODIS

Naturally I had an idea what was going on, but nevertheless it didn’t seem to make a great deal of sense. So I turned to Google Earth in hopes that someone had helpfully uploaded geolocated photographs of the site, or else that it would be labelled with a place name that I could look up elsewhere.

Google Earth – current image

Luckily for this puzzle, a contributor called Ragab Hafiez had uploaded some photographs of the area. The first photograph I saw was what looked like barley – a typical British crop growing in the middle of the Western Desert. Another photo showed what looked like alfalfa, and others showed workers harvesting… potatoes. What gives?

“Cultivated Desert” by Ragab Hafiez on Google Earth (at about 22°
42′ N, 28° 24′ E).

Well, of course this is no natural greening caused by the CO2 fertilisation effect. This is the result of irrigation. Under the desert sand, there is water. If you put a borehole deep enough, you can reach it. Then you can simply pump out water and make it rain.

The circles on the Google Earth image are at 1 km centres, and each is 800 m in diameter (i.e. the area of each is about half a square kilometre). The irrigation systems are giant booms – obviously 400 m in length – that sweep around a central axis. The whole site (East Owainat) is easily 50 km across.

Is sucking water out of the ground to grow barley in a desert with essentially zero actual rainfall a good idea? Obviously it’s harder to grow barley in the Western Desert than, say the flatlands of Cambridgeshire. I can imagine that sandstorms threaten to bury everything, for a start. The desert is obviously sunnier, but it’s also hotter – Kharga, the nearby oasis (nearby-ish; don’t try to walk it) has an average June maximum of 40°C and a record temp of 50°C. The aquifer is going to deplete over time (according to El Alfy (2014)^, the aquifer is going down at a rate of 0.5 m per year; its thickness seems to be quite variable). Aquifer depletion generally has the potential to cause sinkholes, but here my naïve guess is that because the surface is “obviously” sandstone, there are no subsurface caverns primed to open up under the farm workers’ feet.

Why start farming in the middle of nowhere? Why not close to an existing oasis like Kharga? Well, the existing oasis already has water. And one presumes that Kharga sits at the level of the aquifer, such that local water extraction would rapidly cause obvious problems – the water at the surface would begin to disappear. No doubt the theory behind the choice of East Owainat is that it is far enough from anywhere that no amount of aquifer depletion will affect anyone else.

Why barley? Why not a plant with C4 photosynthesis, which is more productive in hot and dry conditions (maize for example)? I have no answer to that.

Nor do I know how long the desert at East Owainat will remain green. Triumph of human ingenuity, or hubristic folly?

This miniature has no direct link to climate change. Or does it? I will leave it to the reader to decide whether or not the cultivation of barley in the Western Desert might be relevant to claims about the way future agricultural productivity might be affected by the “climate crisis”.

Notes & References

You might have noticed another change between the 1986 and 2010 images. Over to the right of the second image, there is a dark area that wasn’t there in the first. These are the Toshka Lakes, fed by an overflow from Lake Nasser. These might give rise to a rapid bounty of fish and crops, but one suspects this will be temporary. The lakes are filled occasionally, and of course rapidly evaporate. Naturally over time this will lead to the concentration of minerals in the water going up.

*NDVI. “Normalised Difference Vegetation Index.” I think this was the first one invented for use with Landsat images in the 1970s. It is based on the idea that plants absorb more red light and reflect more infrared than bare ground. Now, I know what you’re thinking. Plants are green, so why do we need an index involving red and infrared? Why not just measure green? Well, if you remember those far-off days in gloomy classrooms, playing with triangular prisms and watching dust motes swarm in the gleam cutting in around the edge of the blinds, you’ll also remember that white light contains green. Plants appear green because they have preferentially absorbed blue and red light. What I am trying to get at is that a white surface (e.g. snow) has as much green light coming off it as, say, a municipal lawn.


^El Alfy, M. (2014). Numerical groundwater modelling as an effective tool for management of water resources in arid areas, Hydrological Sciences Journal, 59:6, 1259-1274. [Gettable from Google Scholar.]