Doing this is the opposite of being a doomer. I love the concept of terraforming the desert and desert walls. This is positive, inexpensive and we should do much more of this.
Isn't it possible for a complex ecosystem to develop without knowing exactly how it happens? My compost heap hosts a fairly complex ecosystem - more complex than I understand, and I created it merely by heaping my yard waste and food scraps in the corner of my lot.
If someone gave you $15B, how would you use it to put a dent in climate change?
You can't put a dent on climate change until you find a way to reduce greenhouse gas emissions. That's what matters. Trees have to last a 100 years to be effective carbon stores.