Can any good be wrung from such misery and suffering?
Well, perhaps that depends where your priorities lay -- people or carbon. How Genghis Khan cooled the planet discusses a Carnegie study by Julia Pongratz and Ken Caldeira that examined large scale historical disasters, the Black Death and the like, and estimated their effect on carbon production. From the post:
"It's a common misconception that the human impact on climate began with the large-scale burning of coal and oil in the industrial era," says Pongratz, lead author of the study in a press release. "Actually, humans started to influence the environment thousands of years ago by changing the vegetation cover of the Earth‘s landscapes when we cleared forests for agriculture."
The answer to how this happened can be told in one word: reforestation. When the Mongol hordes invaded Asia, the Middle East, and Europe they left behind a massive body count, depopulating many regions. With less people, large swathes of cultivated fields eventually returned to forests, absorbing carbon dioxide from the atmosphere.
Seems all a bit cold-blooded to me and I suspect their estimates are conveniently aimed guesses, but to be fair to Julia Pongratz and Ken Caldeira I haven't read their original paper, only this brief report on it, so I have no idea how sensationalized the article has painted their findings. Still, I wonder if they tried to match the Mongol Hordes' rampages with the actually historical temperature? I imagine they are trying to tie the Medieval Warming period to Genghis and his heirs, but it isn't clear from the part quoted.
At any rate, I'll leave you with John Wayne's portrayal of the newly anointed Mongol Eco Hero and Interpetive Dance enthusiast.