"With only a small variation in fertility, particularly in the more populous countries, the total could be higher: 10.6 billion people could be living on Earth by 2050 and more than 15 billion in 2100, " says the Population Division of the U.N.

CNN: U.N.: World can 'thrive' as population reaches 7 billion

Yet the overall variation is tiny, just 0.1% of the sun's total output, which seems insufficient to be the cause of a warming Earth.

ECONOMIST: The sun has gone quiet

But since some variation is permitted at most positions within a binding site, it means that the total number of different possible versions of a site is often large.

ECONOMIST: How (and why) to find a needle in a haystack | The