For thousands of years CO2 concentration was around 260 ppm to around 280 ppm - since the Industrial Revolution it has risen to over 400 ppm.
The graph of CO2 concentration as recorded since the 1950s is known as the Keeling Curve, named after Professor Charles David Keeling, who began the regular measurements of this value.
If we get to around 560 ppm CO2 (around twice the pre-industrial level) we might well get to more than a 3 C degree rise, as that's a plausible value of climate sensitivity as reported by research from the geological record.
Climate sensitivity is well-researched from the geological record in this paper:
a warming of 2.2–4.8 K per doubling of atmospheric CO2
Even the lower value would have serious consequences, and the probability that the value is high is much greater.