I think Global Climate change has a completely different effect on the first timeline from September 11th 2001:

 I was thinking about this today.

Imagine this on September 11th 2001 5 billion people died and all civilizations ended and all technology ended permanently until around 6000 AD. Why?

Because people reacted badly around the world the first time this occurred and freaked out and all the doomsday weapons of every country (and not all of them are nuclear because some are biological or chemical or others or nanobots or you name it. It seemed like everyone had a way to end all the other life on earth if they were attacked and destroyed as a nation too.

So, basically as a result all civilization ended within a month or two at most when the last normal people died even though genetically many survived but were not right after that and more like wolves than anything else most place except above 3000 feet in elevation where radiation dust storms were less likely to take that radiation so everyone didn't have to breathe in the particles of radiation that much. So, even the Wolf people lived rarely beyond 20 to 30 years old because of radiation caused lung cancers I think you would call them (like John Wayne died from) (from filming downwind a cowboy movie where nukes had gone off on a testing site. He breathed in a few radioactive particles and this eventually killed him along with other members of the filming crew.

So, imagine this happening to most wolf people (I call them that because they are not practical or rational like most present day humans because of genetic damage from radiation poisoning. 

So, only people who continued to live above 3000 feet in elevation had any chance of staying anywhere close to the genetic norms we presently have on earth on the 2nd timeline that also began on September 11th 2001.

However, because no civilization or technology or cars or trucks or factories existed or governments or farms etc. below 3000 feet in elevation this also changed our relationship to the weather and global Climate changes. 

So, it didn't get as severe as it is now and will continue to get from too many people, too much technology, too many cars and trucks, too many forest fires, melting snow and ice caps etc.

So, it is still unknown how this century turns out for mankind but my best guess is that populations will begin to decrease if for no reason that Global Temperature changes alone will make it impossible to grow food many places where it has been grown for thousands of years already. So, the main culprit in depopulating the humans on earth will likely be starvation because of not being able to afford the food that is grown and not being able to grow your own food some places on earth anymore above ground because of winds, flooding, droughts etc. as well as the desertification of many new lands as well.