But Spain's desert is small compared to Sahara's, that is as big as Europe. Its dunes are as big as buildings, with pictures you can't really grasp the enormity of this dessert. When Sahara grows each year it literally engulfs entire villages.
In Egypt and center of Africa there are cities that were prosperous in the past that were engulfed by dessert and disappeared.
Honestly I would not shed a tear for big parts of the big dessert going away. When solar energy and desalination becomes cheaper I expect Sahara to be one of the most fertile places on Earth.
You can probably preserve the biodiversity freezing eggs and sperm of the creatures that live there. There are not so much. I believe that with pesticides and herbicides we have destroyed way more biodiversity of our world(not the desert but our forest) and nobody has complained.