1. The further back we go, the less reliable the methods of estimating such properties. We only have values for a "global temperature" from the past few years or so.

2. Weather and climate are names for the same underlying nonlinear system. This system is the archetype of a chaotic system - predictions of values and corresponding locations becomes increasingly unreliable and then impossible on larger timescales. Climate modelling was the very thing that lead to our current understanding of sensitive dependence on initial conditions.