There's negative feedback cycles involved too. More carbon in the air means more plant growth, more heat means more plant growth, and that would lock up more carbon and prevent runaway greenhouse effect. Especially because we're close to the lowest CO2 levels in geological history, which happened during the last ice age, and could have lead to plant starvation. Arguably all plants now are in a semi-starving state due to low levels. Long story short, I don't think we'll see massive temperature increases or sea level rises, but even if we did, we'd see huge fertility boosts in Canada and Russia due to more arable land being warm *and* due to extra carbon in the air.