There are three EtOM videos, and they explain how they use geology and plate tectonics and other fields, too. We know the past fairly well - at one point after Pangaea split up but before what now is India rammed into southeast Asia and started to push up the Himalayas - there was ~1,000ppm because of all the volcanic activity, and the relatively slower weathering process. After the Himalayas started forming (and India slowed way down), the balance tipped toward faster weathering.
There was no ice anywhere, and huge continental-sized land masses were still moving around. There alligators living on what now is Alaska, and Antarctica was a tropical place, too. Ocean level was 100's of feet higher than it is now.
It took about 1,000,000 years to "wash" 100ppm out of the atmosphere (I'm going from memory here, and I could be wrong on the specifics). That is a long, slow change. We know about when the Antarctica ice started to freeze (about 450ppm) and when the Arctic ice started to freeze year round (about 350ppm).
We also know fairly well when the earth's axis changed, and when the sun's output changed, and when the ice ages occurred, etc. We know where carbon 14 and carbon 13 and carbon 12 come from. We know the chemistry that achieved the 170-270ppm stable plateau that we had for ~650,000 years - all lifeforms were changing the atmosphere and all subsequent lifeforms had to adapt to the resulting climate. Before cyanobacteria and plants there was no oxygen in the atmosphere. Volcanoes "saved" the earth from being a perpetual snowball. We know about things like gravitational changes because of lots of ice. All that is history and we have a pretty good idea about the big-picture stuff.
We know the physics of carbon dioxide in the atmosphere. We know about systemic causation - all weather occurs within the climatic conditions of the moment. We know that temperatures are trending higher, and we know that the relative number of record high temperature vs record low temperatures are changing. We know
the ocean is getting more acidic because it is absorbing carbon dioxide as carbonic acid - the ocean has absorbed about HALF of the additional carbon that we humans have emitted. The acidification is worse in areas adjacent to granite-based land, and not as bad adjacent to limestone-based land. The thing is that the granite areas in the USA are also the best fishing areas.
We see amplifying feedback loops occurring: lower albedo, more water vapor - about 5% more, massive methane "burps" all over the tundra, more massive fires, forced deforestation, a massive (~40%) loss of plankton since the 1950's. We see more massive floods, and longer droughts, weakening jet streams, lower salinity and warmer water, rising ocean levels, we see the Arctic ice going into a "death spiral" - all around global weirdness.
Will we suffer more from bigger storms, or bigger droughts, or bigger floods, or the rising ocean? Will we run out of water before we have a crop failure? Will we keep burning fossil fuels until we pass 450ppm or 550ppm? Will we be forced to switch to all renewable energy by the high costs of oil, or the high costs of climate change? What are the health costs of burning fossil fuels vs the cost of transitioning to renewable energy? Will the insurance companies drive where people (re)build their lives after a storm/flood/fire/drought, or the government? When will the human costs from climate change be paid for by the profits of the fossil fuel industry?