6. Stock and flow
The US has emitted about a quarter of all the CO₂ in the atmosphere and emits an eighth of today's. China is the mirror image.
CO₂ accumulates, so a country's share of the stock in the atmosphere and its share of this year's flow answer different questions.
A line that falls from left to right is a country whose share of today's emissions is smaller than its historical share. The stock is the sum of every tonne on record: OWID counts from each country's first year of data, which is 1750 at the earliest.
The UK, the first industrial economy and 0.8% of emissions today, still carries 4.3% of the cumulative total against 3.6% for India, which emits 10 times as much every year and has 21 times as many people.
So what. Two defensible metrics, opposite rankings: the US leads on the stock, China on the flow, and each is correct for the question it answers. Every ranked KPI has this property. The decision is which definition goes into the target and gets reused: defined once in the warehouse, not re-derived in each dashboard query by whoever wrote it.
Who acts: whoever owns metric definitions. Cost of getting it wrong: two teams presenting different leaders from the same warehouse in the same meeting, with neither of them wrong.