An interesting question. Because my planet is so far advanced of yours, we experienced our major climate changes about a millennia ago.
Like you are currently experiencing, on Neecknaw we discovered that our oil and coal-based economy increased the carbon dioxide load in our atmosphere past the point of the planet being able to deal with it. (Unlike Earth, the main culprit was not the car but our massive fez and tutu industries.)
As is happening here on Earth, the primitive nation-states of Neecknaw were unable to agree on ways to reduce the carbon emissions and so, ameliorate the changes. We did use a few mitigation strategies, such as seeding the atmosphere with sulphur dioxide to block solar radiation, but this had the effect of killing many of our freshwater lakes and waterways with massive storms of acid rain. The acid rain also played hell with everyone’s tutus and fezzes, which meant that we had to produce more tutus and fezzes, resulting in an unforeseen positive feedback loop.
So we all agreed that seeding the atmosphere with SO2 was a bad idea. (You must understand how attached the average Neecknabian is to their fez/tutu.)
But we didn’t ever tackle the CO2 problem, and the warming continued.
Eventually, there was a massive spike in temperatures, which happened very quickly. Most of our coastal cities drowned, the equatorial regions became uninhabitable, and even then, most of our resources went into fez and tutu construction. Roughly ninety percent of the Neecknabian population died, civilization collapsed, and the scarcity of food resulted in a somewhat loose interpretation of cannibalism. (These are sometimes referred to as the “Tasty Ape Age”, though I would never be so crass.)
Luckily, this die-back had the effect of weeding out the weakest of our species, and when Magnok the Foot-Eater conquered the continent of Floog, he was able to quickly rebuild society. Eventually, a brilliant gorilla invented fusion and our economy was based on that power source instead of carbon.
But you know, even though everyone understood the fez and the tutu were the cause of this disaster, Neecknabians are still devoted to the sartorial splendor they impart; however, a series of brutal conquerors have reserved these items of clothing for only the most elite troops.