The reason is simple: we've already put enough CO2 into the atmosphere to cause adverse changes over a long period of time. A lot of the IPCC models and their d

The reason is simple: we've already put enough CO2 into the atmosphere to cause adverse changes over a long period of time. A lot of the IPCC models and their discussion centers around more near-term climate threats over the next 100 years or so caused by continued emissions and overall severity of going ever more "over budget" with CO2 levels, but we're still 100+ ppm of CO2 over the highest average of the last half a million years or more. That CO2 is going to hang around in the atmosphere for quite some time as the natural processes like weathering that lower it are slow, and its presence will add extra heat from solar radiation while it's still there. If we suddenly stopping emitting now, we would still be above baseline but not in the apocalyptic range. If things don't change quickly though, we're going to have to suck CO2 from the atmosphere one way or another because just halting emissions won't be enough.