2000 years??????
Well you see, they scoop out sand at the bottom of the sea, then take measurements of the microscopic dead critters, then figure out how much they should have measured 2000 years ago, divide the difference by the Roman Emperor’s hat size, 2,000 years ago, multiply it by 2000, divide it by the size of the recent dead critters, and subtract the difference, check which way the political winds are blowing, make a computer model to fit all that, and presto chango, there you go!