Really? That's pretty horrible understanding of stats you have there. By your logic, it makes no sense to say that the fertility rate is 1.81 and has changed by

Really? That's pretty horrible understanding of stats you have there. By your logic, it makes no sense to say that the fertility rate is 1.81 and has changed by 0.8 to now be 1.73 because a woman can either give birth to 1 or 2 kids. We do measure Earth's average temperature for what it's worth and while different metrics might give different results, the absolute value is not really useful. What is useful is the change in that metric. And if our global temperatures change by 0.1C by any one metric, it is a very significant change given that the metric is well designed. And yes, it is possible to design a good metric.