Thankfully, most of Asia, besides the Indian subcontinent, has done a great deal to arrest its population growth. Even the Indian subcontinent has made some progress.

Your last line is perfect Andrew:

"But where is the court that could enforce such judgments?"

Indeed. Because you can bet your bottom dollar that if such a court existed, the very first country to opt out would be the US of A. And it wouldn't be the first time!

But in fact a court in name might not needed if you could have a sort of defacto court.

One idea could be that the "externality charge" of your pollution gumming up my factories could be neatly collected by a bilateral trade sanction. 15% extra on Chinese-made imports, to pay for the clean-up, until the dust stops flying...

The defacto court capable of handling this in fact already exists -- all the countries mentioned are members of the WTO.

Getting the WTO to recognize such environmental externality sanctions in its rule system, now that would be the trick!

All they need is some invader bush.

Acacia Senegal, one of the species encompassed by the term "invader bush," which as you've pointed out grows uncontrollably in Namibia ravaging the savanna and causing major erosion problems, is also one of the species of plants used successfully for land restoration purposes in the more arid Sahel region of Africa, which borders the Sahara desert (which is also expanding.)

Acacia Senegal, more commonly known as gum arabic, is also widely imported and used in China for medicinal purposes, especially to treat the inflammation of throat and stomach.

Perhaps by importing some live Acacia bushes from Namibia and strategically planting them on the expanding dunes of the Gobi Desert, the Chinese could, as they love to do, kill 3 birds with one stone.

And if it doesn't work they can always sell their invader bush to the Koreans to cure their inflamed throats after those irritating dust storms.