Liet me ask you a question about a big desert.
March 8, 2011 4:12 PM   Subscribe

How would global climate and soil conditions change if the Sahara Desert wasn't a desert anymore?

You've got a lot of relatively flat/open land for wind to blow across, and that wind picks up sand and dust and deposits it all over the world (though mostly on east coast of North America? maybe?). So what happens if the Sahara becomes grasslands? What about a forest? This is putting aside any other conditions necessary for such a change to take place. Just, tomorrow, the Sahara is no longer a desert. How does that affect the rest of the planet?
posted by curious nu to Science & Nature (8 answers total)
You can't seperate it. The conditions that make it a desert (a global belt of desert clustered around the 30th parallel that results from air circulation up at the equator and that air falls, dry and warm, around the 30th parrallel).

my guess would be as to effects though would be mostly related to the dust that blows out across the atlantic. This will affect how much nutrients are available for the ocean ecosystems and how bright the sunlight is on a major hurricane forming area.

However don't believe we have any real idea about cause and effect with climate science. They are really just beginning to get this into some kind rational field with useful tools and nobody really can saw with any accuracy-what happens if we do this. It is a chaotic system where minute changes in input can have drastic and unpredictable changes down the road and large changes in input can momentarily change things by the system then settles back down to previous patterns. This is the real danger in increasing CO2 levels. We just don't know what we are doing to our life support system. The changes may be much less thant he models predict and then maybe we will drastically alter the climate with just 1 more ppm. Scary stuff.
posted by bartonlong at 4:24 PM on March 8, 2011

Putting aside other changes that would cause the sahara to stop being a desert, then all the grass in the sahara dies in a month and it turns back into a desert.

Some other change would have to happen first.
posted by tylerkaraszewski at 4:29 PM on March 8, 2011

OH! OH! I know!
*waves hand*

The climate of the Sahara has undergone enormous variation between wet and dry over the last few hundred thousand years.[11] During the last glacial period, the Sahara was even bigger than it is today, extending south beyond its current boundaries.[12] The end of the glacial period brought more rain to the Sahara, from about 8000 BC to 6000 BC, perhaps because of low pressure areas over the collapsing ice sheets to the north.[13]

Once the ice sheets were gone, northern Sahara dried out. In the southern Sahara though, the drying trend was soon counteracted by the monsoon, which brought rain further north than it does today. The monsoon season is caused by heating of air over the land during summer. The hot air rises and pulls in cool, wet air from the ocean, which causes rain. Thus, though it seems counterintuitive, the Sahara was wetter when it received more insolation in the summer. This was caused by a stronger tilt in Earth's axis of orbit than today, and perihelion occurred at the end of July around 7000 BC.[14]

Copied that right out of this Wikipedia article, I did indeed.

If that makes no sense, we could try the way-back machine to the Pliocene.
posted by BlueHorse at 4:59 PM on March 8, 2011

The Bodélé, a region of the Sahara not far from Lake Chad, is the source of more than half the material that fertilises the Amazon rainforest.

So I think it would be reasonably safe to say that if the Bodélé was grassed over, there may be a reduction in the nutrients deliverd to the Amazon, which would have consequences for the type and growth rate of plants there.
posted by girlgenius at 5:15 PM on March 8, 2011 [1 favorite]

And to be more specific and address your question about climatic changes, a large scale change to the plants in the Amazon would likely affect the rainfall patterns across large areas of South America.
posted by girlgenius at 5:17 PM on March 8, 2011 [1 favorite]

There's two answers. If you're talking about a hypothetical situation where the Sahara magically became vegetated and that vegetation could somehow be indefinitely sustained then, yes, there would be a lot less dust blowing westward. That would mean a lot less nutrients in the ocean and over the Amazon as others have mentioned.

Saharan dust plays a role in Atlantic tropical cyclone formation and frequency. Less dust is, not surprisingly, correlated with more solar radiation reaching the ocean surface (i.e. more ocean heating) and a greater frequency of tropical cyclones. This Foltz and McPhaden article spells out some of the complexities of the Saharan dust-tropical cyclone relationship.

More realistically, if the Sahara were to become vegetated and left to its own devices it would quickly revert back to a desert as tylerkaraszewski said. Initially the air over the newly vegetated surface would be warm and humid. If there were enough moisture you might get some small-scale circulation, an oasis equivalent to a sea breeze, that might produce rainfall.

However, there are much larger forces at work! That latitude is where the sinking portion of the Hadley Cell is located. Air gets heated and rises near the equator (if you look at global satellite images you'll often see an irregular line of clouds just north of the equator - that's the InterTropical Convergence Zone where rain is falling out of the rising air) then moves poleward, cooling enough to start sinking around 30 degrees latitude. The air that is sinking is dry and sinking air warms as it gets compressed, moving it further from saturation. Over the long run there's no way a vegetated Sahara could produce enough moisture to overcome the larger-scale circulation. Rain would end, the vegetation would die, and the area would revert back to desert.
posted by plastic_animals at 6:28 PM on March 8, 2011

To add on what girlgenius said, the Amazon is dependent on that deposition for several nutrients (especially phosphorous). Tropical soils are so highly leached from constant rainfall that they're quite nutrient poor.

But you know, it's really hard to answer this question because as pointed out above, changes in the Sahara would depend on things being different elsewhere. Biomes exist because of global weather patterns. You can't theorize about one just changing, because for that to happen everything else would have changed/be changing.
posted by oneirodynia at 6:28 PM on March 8, 2011

Response by poster: Right, I get that there's a fantastically complex system in place that both caused the Sahara to exist and maintains that existence. In retrospect, my question is more: "What are the most obvious effects the Sahara desert has on the Earth's ecosystem?" with the understanding that everything feeds into everything else and you could follow threads around forever if you wanted to.

So the dust plays quite a few roles in different things around the Atlantic, at least. What does all that sand laying out there do for the planet's albedo(correct term here?)?

If anyone has journal articles or textbooks or something that deals with all this in the nitty-gritty, I would appreciate those 100%. I don't want layman science books unless they're exceedingly well done as I find them to be too short on facts and numbers and too long on flowery prose.
posted by curious nu at 9:31 PM on March 8, 2011

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