Yes, the Cambrian explosion indeed was an even wilder change in environment and ecosystem than what we're going through now. A hypothetical human time traveler, however, wouldn't survive a hypothetical time travel back to that.
I assume you are referencing about CO2 levels. Yes, around ~500 million years ago, CO2 levels reached 10,000ppm, which would be hazardous (but not fatal) to a human traveler. However if you pick a point between 300 million years ago and today, when CO2 levels were at 1000-2000ppm, a human traveler would have no problem surviving in that environment. Heck, it is even possible to hit those CO2 levels in a modern day warehouse or office that is poorly ventilated.
Increased CO2 levels are very survivable and do not mean the end of the human race.
Let's get everything on the table. Are you actually okay with dozens of millions to hundreds of millions of people dying -- and many others suffering immense drops in quality of life -- as a result of human-caused climate change? Despite the fact that we have the capacity to heavily mitigate such risks?