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.
When we don’t have direct data, we use proxy data. The best known (famous or infamous) is tree-ring data, with ice-cores and pollen coming close behind. All those can give us comparative patterns, especially when done properly over a large geographic area. The mega-droughts of the 1200s in the American Southwest were first identified through tree ring data. Certain ponds and marshes in Europe and Britain preserve records of what was growing in the area, or blowing into the area, going back to the last ice age. Those don’t tell us exact temperature or precise details, but they do give us broad trends and patterns – drying, getting wetter, warmer, cooler – based on what was growing in the area. If you can combine those with archaeological support, then you can get a very good sense of the big picture.
There are also written or drawn data proxies. We don’t have exact data about what happened in the Northern Hemisphere in AD 536-39 CE. We do have written descriptions of the weather and skies getting strange, and folk memories of terrible weather and population collapses. [H/T Grim of Grim’s Hall blog]. When combined with other sources of proxy data (archaeology, pollen counts, tree rings, written accounts from farther south) we get a solid overview.
Other sources for “what was it like, ish” include looking at oxygen isotopes, the composition of gas bubbles in ice cores, shellfish rings, fossilized grass roots (Great Plains/ High Plains), the presence and absence of humans and other animals, rings in stalactites and stalagmites, the composition of ancient soils (when you get a thick layer of nothing but dust, it’s a good indicator of a bad drought), and other things. These all have large caveats, and are very imprecise. We can’t tell when that horrible dry period was, but we can get a sense of sequence, and if we can combine that with other things (carbon or Kr/Ar dating, artifact sequencing, pollen below and above the sterile dust layer) a very broad-brush idea of climate emerges.
All of these are subject to human error, to mis-interpretation, to manipulation for funding or political ends (the Yamal tree ring sequence may live in infamy for that reason). We have to remember that pine pollen found in an ancient pond layer in the High Plains might not signify a cool, wet phase in that area. Pine pollen travels on the wind, and it might have blown in from the Rockies. However, pine pollen plus heavier pollen from plants that need cool, moist conditions, plus certain diatoms that need cool, fresh water, plus lots of fresh-water snails . . . The combination is what confirms.
When we sort out climate for places and times that didn’t have modern thermometers and rain gauges, we rely on a lot of other things, along with common sense. And some of what we find upsets some popular applecarts, or helps explain things from the written record. Climatology is a giant puzzle, with pieces that come from all scientific disciplines. As you can tell, I find it a lot of fun.