I've posted about this before, it's the rate of change in the concentration of carbon dioxide in the atmosphere not the level, that matters. If the CO2 increases slowly, the planet is able to make an adjustment, but when it accelerates it can't.
Skeptical Science makes the same point but with graphics.
For 1 million years life on earth has adapted itself to going into and out of ice ages over approximately 100,000-year cycles. We come out of ice ages in about 10,000 years, with CO2 rising 100 ppm in that time. That is a rate of increase of about 0.01 ppm/year. If we use this as a typical rate to which nature has adapted, and has done so already for at least 10 cycles, then we can determine how much faster than this we are now moving. The idea is that if we limit CO2 rise to this rate, we expect nature will adapt; the further away we move from this base rate the more difficulty nature will have adapting.
Even before the age of big oil and coal, we were already pushing nature more than 10 times the base rate. The rapid industrialization sparked by WWII marked a rapid increase near 1950 that has continued, unabated, yielding a rate now more than 100 times the base rate.
This is comparable to the effect of running into a parked car, and if left unchecked, we may soon be running into a brick wall.
Climate change deniers without the math or science background have trouble with this concept. They need to understand this. It's like the difference between finding something out over time versus in seconds. Compare what your reaction might be if you've known for sometime that your significant other was leaving you versus finding out a few minutes ago when you thought everything was fine.