It will be assessed how it is defined by the legislation. This isn't a "gotcha" or whatever. Nearly every single measured value in human existence depends on agreed upon standards.
You misunderstood the question. It wasn't "how can anyone possibly figure out what is supposed to be measured" but rather more like "what if we measure the wrong thing".
OP's question stipulated "reasonably".
And congress has the power to define weight and measures =)
Sure, but not specifically for the sake of enabling the count of a carbon footprint. There's tinfoil hats, and then there's paper hats with big crayons.
But isn't this clause how we got NIST? Who, among other things, regulates time itself?