Because the rate is higher than normal, because we've increased CO2 concentration by 40%. That means two major things. 1) Human caused climate change will cause a mass extinction because species can't evolve fast enough and can't change their ranges quickly enough, especially with the massive amount of deforestation. 2) The climate will go from being ideal for humans to not ideal, in a matter of hundreds instead of thousands of years.
If you don't get the rate thing, consider this. Every month your wife bleeds at a certain rate. If one month she bleeds at 100x that rate, do you say "You bleed every month" and refuse to take her to the hospital? No, because then you'd be a dumbass. Yet, when it comes to things other than bleeding, a lot of people seem to not grasp the concept of rate.