CO2 is a greenhouse gas, true, but the atmosphere is not a greenhouse.
A greenhouse is enclosed, a controlled environment. You put IR blocking panels up in a stagnant structurally enclosed area where air movement is a non-issue, and walla, a heat increase from trapped energy.
The atmosphere is a complex system with multiple countering forces, dominated by water. There's nothing stagnant about it. Our actual greenhouse effect is much greater than the temperature variation, but there are substantial cooling factors, namely conduction, increased albedo, and condensation.
CO2 is a trace element, a minor factor. Even by IPCC standards(and these are hilariously flawed) it only accounts for about two degrees of our temperature variation over what we'd expect without an atmosphere. The rest is the magic feedback, where somehow, the water cycle becomes some magical addition because the CO2 increase is anthropogenic.
In the real world, more surface heating results in more heat loss through conduction. More moisture uptake results in greater heat loss, higher albedo through cloud cover. The greater the temperature variance, the faster the warmer air rises. The warmer the water, the faster it evaporates and transports that water to the upper troposphere, to release the heat into the stratosphere as it condenses and falls back to earth, absorbing heat from the atmosphere on the way down.
Saying CO2 is a greenhouse gas, so obviously the temperature is increasing, is like saying you pissed on a plant, so obviously it will grow more. If it all evaporates without being absorbed by the plant, or other factors hold sway, you haven't accomplished anything. Perhaps you just increased the acidity of the soil to a point where the plant gets sick and dies.
We have a great many factors to take into account, and everything but CO2 is being written off as minor. The variation between the stratosphere and troposphere, the amount of scattering at the surface, there are secondary factors in abundance to explain this phenomenon, but CO2, which doesn't, has been locked into as the cause. It could be nothing more than the aerosols two stratovolcanic eruptions put into the atmosphere, combined with our own CFC production. If they're simply underestimating the impact of the ozone layer, we've got a radically different impact from the claim. If water is 98% of the greenhouse effect(the other end of the spectrum) then CO2 levels would need to be high enough to kill us before we changed the climate to the degree the IPCC claims just a few hundred more would manage.
From what I've learned over the years, CO2 has a potential impact, but it's minor if there at all. Our sharp rise in temperature is the result of rewriting the past. The current trend of stagnant temperatures as it peaked and went into decline, and the preceeding two decades of increases, are an entirely natural cycle. We're in a repeat of the late 40's right now. The 90's were comparable to the 30's, less extreme, but possibly a year or two prolonged. For some strange reason no one seems to be able to find arctic ice sheet extent from the early to mid 40's, which would be the period of interest regarding the recent record low levels in the arctic. It's entirely possible that the record lows we saw were also less extreme than the past cycle was, further indicating a gradual cooling trend over the last century. That gradual trend being exactly what was shown to be occurring in the surface station record until the late 90's, despite large quantities of heat biased stations in urban locations. Today it shows an abnormal rise instead. This is explained as being corrections for biases in measurement, but I'm quite certain this is a lie. Having looked at the station records myself for supposed bias corrections I feel quite safe in the opinion that the people making them sold their professional ethics for government funding.