I'd say it's just as likely that the retreating glaciers changed the climate here to that of Saudi Arabia.
I'd say it was probably a little (or a lot) of both. It wouldn't take much in terms of weather pattern or rainfall changes coupled with complete deforestation to destroy the ecosystem.
I've seen it happen in huge areas of the Philippines, which is clearly a tropical setting with rabid vegetation growth. But, with no land management regulating the harvesting of natural resources, things like complete or deforestation occurred throughout the second half of last century. Now nothing, not even weeds grow in some places. Erosion of the topsoil accelerates leading to complete collapse of the ecosystem. It's quite something to see in person.
Now, if that happened in this area coupled with some mild weather pattern shifts, I could see how it could easily be catastrophic. I'm not a enviro-whacko, but I do believe we should take reasonable care of what God gave us.