Greening deserts

[Hans de Vries]

Would altering the albedo of deserts by (for example) planting trees or building a large number of solar panels cause them to cool down and have more rain?

[pavelcherepan]

The main reason why deserts have little to no rain has very little to do with temperature, mainly it's because whatever moist air is coming their way ends up as rain before reaching the desert.

 

Some deserts like Atakama or Gobi are pretty awfully cold and still have very little rain because of mountains blocking moist air from ocean.

 

So the answer to your question is most likely no.

Edited January 13, 2016 by pavelcherepan

[tantalus]

Its long been known that the Sahara was a green temperate grassland until 5 to 6 thousand years go. More recent evidence indicates that it abruptly changed to desert in 1 to 2 centuries. The cause of the wet, green sahara was linked to changes in earth's tilt and rotation that affected the sun's radiation in the region, triggering monsoon rains that swept onto the continent.

 

The fact that the change from desert to grassland occurred so quickly indicated additional feedbacks, because the change with the sun couldnt cause it so quickly (apparently), the below paper speculated on an a further feedback by the growth of vegetation in the formerly desert region that sped up the process and they modelled it.

Simulations with a synchronously coupled atmosphere–ocean–vegetation model show that changes in vegetation cover during the mid-Holocene, some 6000 years ago, modify and amplify the climate system response to an enhanced seasonal cycle of solar insolation in the Northern Hemisphere both directly (primarily through the changes in surface albedo) and indirectly (through changes in oceanic temperature, sea-ice cover, and oceanic circulation). The model results indicate strong synergistic effects of changes in vegetation cover, ocean temperature, and sea ice at boreal latitudes, but in the subtropics, the atmosphere–vegetation feedback is most important.

 

http://science.sciencemag.org/content/280/5371/1916.full

 

so it discusses your albedo effect.

 

I guess this indicates on a large enough scale the importance of vegetation under this alternative climate scenario, but note the change in rainfall patterns came first.

 

I cant see how you could plant trees in a desert like the Sahara currently on a large scale that could have any significant impact without a change in rainfall patterns that would keep them alive.

 

Anyway, it is possibly rainfall that constrains tree growth in the sahel, and even in the savanna further south in favour of grasses that can outcompete tree seedlings in lower rainfall ecosystems, so you would have a better chance with grasses initially, although now I am just becoming a little speculative to say the least.

Edited January 13, 2016 by tantalus

[pavelcherepan]

so it discusses your albedo effect.

 

I'm pretty sure you've misread it. The passage talks about albedo and insolation of Northern Hemisphere in which the biggest driver is glaciation in high latitudes. Increase or decrease in the amount of glaciation changes the insolation regime of the whole hemisphere by a lot.

Edited January 13, 2016 by pavelcherepan

[tantalus]

 

I'm pretty sure you've misread it. The passage talks about albedo and insolation of Northern Hemisphere in which the biggest driver is glaciation in high latitudes. Increase or decrease in the amount of glaciation changes the insolation regime of the whole hemisphere by a lot.

Ya I miss understood that. Thanks