“Little Ice Age?”
~1250- ~1850. Khan died in 1227.
It takes a few hundred years for the forests to grow proper and have their desired effect.
Also, the Mongolian Empire’s lasted in Russia until almost 1500, and Moscow did not fully recover until the early 1600’s. I would not put it past some greens to argue this. They have certainly managed to politicize the weather, and once you are able to do that, you can do or say almost anything.