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.
Cat Rotator's Quarterly 
Like
My favorite technology hack thing is not just that we can read stuff we couldn’t before–I’m sure you’ve seen where they scan stuff to see what was written on it before it was scraped and reused?– but that we can now get that sample of a large geographic area almost as quickly as we can ask!
Not, dig through a print-up of books/resources a place somewhere has, and either physically go there or mail a request for a copy–but go online and check out the scans.
“Here, look at the scan of a scroll that would turn to dust if we unrolled it– so we x-rayed it, and had the computer make a copy, and simulated unrolling THAT!”
Cuneiform tablets also, that are too worn and fragile. That’s been done optically for at least 15 years with laser interferometry, and then image enhancement to read it. Mostly planar, which skips the coordinate transfer steps from a spiral or from cylindrical coordinates to 2D Cartesian. It’s just math. 🙂 [suppressing maniacal laughter from brain upsets introduced by high doses of higher-order calculus] . Sorry … it’s a lot of really neat things done by extraordinarily careful engineers, archaeologists, and museum curators., on irreplaceable artifacts. One professor in grad school had worked on the cuneiform imaging; ten minutes of fascinating example.
On original topic, I would probably both enjoy and dread being one of your students.
Worthy of note: the researcher at the heart of the Yamal tree ring sequence, had previously published about how tree rings were horrible paleotemperature proxies, as increasing levels of CO2 and increasing temperature both result in faster tree growth.
Cherry pickling the sample and using flawed statistical methods were just icing on the cake.
There’s a reason the (bleep) is so litigious.
Oooh! I just heard over at Crossover Queen’s Creative Chaos that they discovered stuff that had been painted with Egyptian Blue will glow under IR, so they can tell where the paint use to be!
Looked that up, quite interesting and cool. At first guess it may be similar to alexandrite IR properties. Egyptian Blue is a calcium silicate with copper oxide inclusion, alexandrite is beryllium aluminate with chromium inclusion. Now to find that box of notes …
I’ve been described (at the college level) as “She’s really good – but a real hardcase!” The senior professor (my indirect supervisor) was amused.