Three quick points. First, I don't think anyone outside of wikipedia or RealClimate believes anthropogenic CO2 increases are anywhere near 40% since the beginning of the industrial revolution. Remember that the beginning of the industrial revolution also coincides with the end of the Little Ice Age (unless you're one of the true zealots who has revised out the Little Ice Age to make Mann's hockey stick graph work) and in particular an end to the especially bitter part of the Little Ice Age in the late eighteenth and early nineteenth century. Remember that CO2 naturally lags temperature (plants respond more slowly to temperature increases than do animals) and thus CO2 would be expected to increase as the LIA ends.
It must be satisfying to be a true believer in climate denial. Just a little inconvenient point: Over the past 2.1 million years, over cycles of ice ages and re-warming, C02 levels haven't approached the current levels. In fact, the average C02 level during the warm periods between ice ages of the past has on average been almost 40% lower than during the current period.
If warming following the little ice age is the reason for the 40% increase in C02 levels, then why didn't similar huge increases occur after past ice ages? The fact is, you can't justify your claim. You just choose to believe it.
Second, the greenhouse effect is subject to saturation and scattering. To truly relate the greenhouse effect linearly to CO2 concentration, one would have to show that all the available radiation in the spectra affected are not absorbed out. To show that a near-linear effect is occurring, one needs to show that a linear amount of radiation is being trapped and converted to heat in the atmosphere. Correct me if I'm wrong, but I think the atmosphere has had little of the measured temperature increase; rather, the measured temperature increase has been largely in the oceans and on the land (and especially so around big cities, which are known heat sinks.)
It's so interesting to see how you blithely pass this off as though you know what you're talking about. First of all, it's not the greenhouse effect that's subject to saturation, it's the C02 level itself: There's some theoretical C02 level at which the atmosphere won't hold any more. But that point obviously hasn't been reached: C02 levels are increasing at an ever-increasing rate. If we were approaching the C02 saturation point, we'd expect to see C02 levels leveling off.
Perhaps what you intended to say was that as C02 levels increase, the marginal greenhouse effect of each additional C02 molecule is theorized to be less than that of the previous molecule. The current atmospheric models assume that the the greenhouse effect is proportional to the log of the C02 concentration. Thus, the models assume that a doubling of C02 concentration leads to a 30% increase in the greenhouse effect of the C02.
As to scattering, I'm not sure why you're relating that to C02. On other planets, C02 in ice form is believed to cause atmospheric scattering of thermal radiation. But dry ice isn't a factor here on earth.
Maybe your overall point is that a 40% increase in C02 isn't the same as a 40% increase in the greenhouse effect of the additional C02. That various factors reduce the effect of increasing C02 concentrations. That's a fair point. But note that atmospheric models attempt to take these factors into account, yet they still show significant warming due to human activity. Try as you might, you're not going to get that 40% increase down to no effect at all.
Third, the fluctuations in temperature and CO2 are larger than the anthropogenic contributions, even assuming worst case. Assuming that anthropogenic CO2 production and associated temperature increases will cause catastrophe when natural oscillations did not assumes that there is something about manmade sources that make it not assimilate. To date, no such mechanism has been described, much less proven in repeatable studies.
The bolded statement is like saying that the total mass of living organisms (including humanity) on earth is greater than the total mass of humanity. Or that the total number of people with H1N1 influenza in Maryland is less than the total number of people with H1N1 influenza in the United Sates. Well duh! Why would you even make such a statement?
Temperature fluctuation is the SUM of all contributors. If mankind's actions add 1 degree of warming, then that means that that 1 degree will ride on top of the other effects: A 25 (Celsius) degree day absent humanity becomes a 26-degree day with humanity. A minus 40-degree day becomes a minum 39-degree day.
Remember, climatologists aren't predicting that we'll experience 20 degree increases in average temperature. The prediction is that if nothing is done to curb C02 levels, global temperatures in 2050 will on average increase by 1.0 degrees Celsius. 1 degree.
I think what continually boggles the climate-denial crowd is that seemingly small changes can be catastrophic. The deniers argue from emotion rather than science. They think small temperature changes - and small percentages - don't mean anything.