"Nearly 21% of the land is now national parks"
This really confuses me. A part of the "land" "IS" "parkS"?! Shouldn't it be like "21% of the lands are now parks"?
It's simply because "land" acts as a collective noun / general noun in this sentence. Compare it to the sentence:
50% of the population is in poverty
It would not make sense to say "are" because there are not multiple "lands".
However, you could reorder the sentence to avoid the awkwardness:
National parks comprise nearly 21% of the land.
I've learned that the way to decide whether to treat the subject as singular or plural is to consider the sentence without the "of " part. In this case, think "Nearly 21% is now national parks". The percentage is the subject of the sentence, not the land itself.
Land is considered singular. Eg. The land is parched. Lands on the other hand is obviously plural. Eg. The tribal lands of the different tribes are considered sacred. Another example is sea. Eg. the sea is deep. The seven seas are far and wide.