How do we know what the weather was like “back then?” We don’t.
We can get some good ideas about climate, however, so long as we acknowledge that our data and ideas may be flawed and subject to change as we learn more. The sources we use to track long term weather patterns (aka climate) fall into two very broad categories, and there is some overlap. Those categories are direct data, and proxy data.
Direct data are what we can and have measured using modern instrumentation (for the then-present definition of modern). Numeric temperature, barometric pressure, snowfall and rainfall totals, sunspot counts, those are direct data. Continue reading