A published article from years ago pointed out that the human influences on the climate likely began around 10,000 years ago, due to deforestation and changes in land use. Ruddiman was the author if you want to look it up.
We have only accelerated the pace now, quite substantially, through the use of fossil fuels, and have moved the CO2 concentrations in the atmosphere from 280 ppm to >400 ppm. And we absolutely are 'on pace' to double CO2 levels within just a couple hundred years - that is a pace of change that is not seen anywhere in the geologic or ice core records. Two or three of the biggest extinction events that have ever occurred on Earth are now attributed to certain types of volcanic activity, releasing levels of CO2 on par with what we are doing today; only THOSE releases of CO2 likely occurred over a few thousands of years, not just 150 years.
So, yes, we are 'pretty sure'; we are 'sure enough' that any reasonable level of 'risk management' would identify that we need to be carbon neutral by the end of this century, or we will risk some pretty ugly consequences.