Even if we do everything wrong and burn all the fossil fuels that exist on this planet (releasing enough CO2 to make us noticeably dumber as well as melting all the ice caps and glaciers), that’s unlikely to knock us back further than the dawn of agriculture, when we had a population of 1-10 million, compared to the population bottleneck possibly caused by the Toba catastrophe when there were 1-10 thousand of us.
Even if you take us back down to 10,000 humans, way worse than you should expect even in a nuclear war, after one or two hundred thousand years the CO2 will be gone. We’d get back up to at least as far as the early Industrial Age in a relative blink of an eye, though further than that would require a different path, one without fossil fuels, because those wouldn’t have regenerated over that period.
https://www.xprize.org/prizes/elonmusk
https://scied.ucar.edu/learning-zone/climate-change-impacts/...
None of these tipping points are considered very likely to occur over the next several decades.
A recent report confirms that ice sheets in Greenland and Antarctica, whose mass-loss rates have been rapidly increasing, are matching the Intergovernmental Panel on Climate Change's worst-case sea-level rise scenarios.
"For near-term time horizons, we think it's actually the best choice because it matches cumulative emissions. What happened over the last 15 years has been about exactly right compared to what was projected by RCP 8.5." That holds especially true for medium-term planning through 2050.