December 26 – What’s for desert?

Today’s factismal: The Sahara desert covers an area larger than the entire contiguous United States.

One of the more interesting questions in science today is “what makes a desert?” We know what a desert is (an area where there is very little precipitation and very little vegetation). We know where the deserts are (on all seven continents) and when they are (in all ages of the Earth from the Precambrian to today). But we don’t know how once-fertile land is transformed into a desert nor (more importantly) how we can transform a desert into fertile land.

The Nebraska Sandhills a once (and future?) desert (Image courtesy National Park Service)

The Nebraska Sandhills a once (and future?) desert
(Image courtesy National Park Service)

Many people assume that all it takes is water. “Add water to the desert,” they think, “and it will bloom once more!” Sadly, this is not true. That’s because the soil that creates fertile land requires more than just sand and water; it also has a large amount of organic material, not to mention worms, bacteria, and other living critters, that help the plants grow. As proof of this, we have the Nebraska Sandhills region which was a desert in 1250 BCE and is just now developing a complex prairie grassland even though it has had plentiful rainfall for nearly a thousand years.

Adding water to the Sahara desert is a temporary solution at best (Image courtesy NASA)

Adding water to the Sahara desert is a temporary solution at best
(Image courtesy NASA)

Another good example of this is the Sahara. This desert was much larger during the last ice age but when it ended about 15,000 years ago, the extra water and monsoon rains helped shrink it and created verdant grazing lands along the southern edge; the change took a mere 2,000 years. Unfortunately for the folks who lived there, the monsoon rains that kept the area green shifted further south about 6,000 years ago and allowed the southern edge of the desert to expand to the point that the desert now covers an area the size of the contiguous United States. In addition, the introduction of new types of grazing to the area may have helped the desert to extend further south than it would have otherwise.

But we aren’t sure how much of the change was due to rainfall patterns and how much was due to changes in land use. What we do know is that we are seeing a lot to land use changes around the world, right now. And we are pretty sure that those changes are going to affect how much of the Earth is covered by desert and how much is fertile land. If you’d like to explore the question for yourself (and maybe add a little data of your own), then head on over to the GeoWiki Project. These researchers are looking for folks who are willing to add what they know about local land use to a global map that will help answer questions such as “How many people can live in the Sarahan region?” If you’d like to take part, march on over to:
http://www.geo-wiki.org/

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