"Of course. But that 0.038% is responsible for 9-26% of total greenhouse effect. Even at the lower figure, obviously a small increase in CO2 will have a large effect on greenhousing."

If you look at the ref you cited. The variance in the number is depending upon if you exclude the other greenhouse gases:
"The higher ends of the ranges quoted are for the gas alone; the lower end, for the gas counting overlaps"
http://en.wikipedia.org/wiki/Greenhouse_gas
So, 9-26% isn't meant to be a range of error. 26% would be if there were no other greenhouse gases, but since there are other greenhouse gases it's generally going to be closer to the 9% number unless there is a lower then average amount of other greenhouse gases present. This leads to another aspect. What area on Earth receives most to the Sun's energy?
The answer is: the tropics [and the most significant area of the tropics in regards to warming planet Earth, is the oceans of the tropics]. In the tropics you generally have very high humidity and the warmer air can hold more humidity than colder air. So even without a greenhouse affect, the tropics would generally recieve more energy from the Sun per sq meter than anywhere else, but it's also got near the maximum of greenhouse affect from water vapor. So you could say that the Tropics are near the highest efficency possible in terms of gaining heat from greenhouse affect- if you could lessen the amount clouds- you could improve this significantly- high humidity and no clouds and you are really cooking.

But let's say that small increases in CO2 does have a large affect. CO2 is said to have increased by 87 ppm, and during this time it could have caused .6 C increase in global temperature, though nobody assumes that all the increase is due to this CO2 increase. But let's just say for fun that per 87 ppm you get .6 C. So when we reach around 460 ppm- nites in the north [in northern the hemisphere] are slightly warmer [1 C].
But it is possible that humans could produce say, four times the CO2 they currently producing per year and global temperatures could lower and you could have a lower global content of CO2- in other words, global temperature *might* have more to do with CO2 levels than human activity. The ice core show about a 100 ppm of CO2 rise for every 1 C increase in global temperature- before humans were burning coal and oil- but of course not before forest fires or volcanos. So you could assume that something is causing an ever increasing amount CO2 over thousands of years which pushes up temperatures. Or you could assume that as temperatures increases, CO2 levels rise- regardless of what plants are growing or Volcanos/forest fires are raging.
Or as Forest Gump says, a little of both.