We don’t know precisely what concentration of CO2 in the atmosphere will keep temperature rising less than 2°C, which is the increase experts hope will keep us safe.
For thousands of years before the industrial revolution the concentration had been near 280 ppm (parts per million).
Problem is that we’re already well above 350. We’re in fact above 390 and that doesn’t even count other greenhouse gasses like CFCs and methane.
What’s more, to get back down to 350 doesn’t just mean reducing emissions, it means that the planet has to reabsorb all that CO2 above 350. That’ll take a while.
Plus at this point, with developing nations like China and India just gearing up their economies we haven’t even begun reducing emissions, we’re still increasing them, and at an accelerating rate.