We have millions of years of climate history of which only a tiny fraction has substantial data. The rest is theoretical data that can only be verified by a web

We have millions of years of climate history of which only a tiny fraction has substantial data. The rest is theoretical data that can only be verified by a web of theoretical means. Variance accumulates across models. The less actual data you have the wider the variance is on your estimators. When your data is mostly projected the confidence interval on your outcome makes it meaningless, e.g. average temperatures will change by plus or minus 20 degrees.