Because orbital dynamics are also driving it.
The total temp swings are about 10 degrees peak-peak, so +/-5deg about a mean. From memory this is about half from solar forcing, half from GG forcing. However, the root cause is solar or rather orbital dynamics making an effective solar forcing. The GG contribution is a feedback function.
If by some magic, you could make the GG portion causative (by say, digging up and burning fossil fuels at a time of your choosing) then you can make the GG out of phase with respect to the orbital dynamic/solar contribution. That is what is happening now. You will note on that graph that we should be starting the downhill slide to colder temps, and lower CO2 levels. Note how brief the prior peaks were, they are quite sharp. Instead, we have turned on another source of warming, and this is sufficiently great to place us on an upward trend rather than downward, with increasing CO2 levels rather than falling ones as we would expect from the historical pattern shown in the graph.
Us insignificant little humans can actually drive a GG forcing of sufficient amplitude to negate 100% of the orbital solar forcing by swinging the CO2 levels only about +/-150ppm or so, something which looks well within our capabilities given that we have more or less done so without even intending to. You might also note, that it is much easier to do so keeping us out of an ice age (long time period for the slide) then it is to cool us down when the orbital dynamics wants to warm us up (that is the 5K year problem). However, 5K years is way longer than we need to pull 300ppm of CO2 from the atmosphere. Anyway, that problem is 100K years away. We have the easy part first. |
