By well-placed assumptions!
The bottom line is, we can't accurately predict temps on a 10-day forecast (I know, I'm pouring cement and watching the forecasts like a hawk).
But we are sure as heck supposed to believe "models" involving trace gasses and their effect on earth's temps to the nearest degree C over the period of decades.
Nope, brainwash young, cap-and-tax, that's all the politicians care about, and scientists are NOT unanimous that the trending is man-made.