I think that it's misleading (at the least) to say that 'CO2 traps relatively little heat'.
CO2 currently makes up a very small percentage of our atmosphere (~ .036%) but it's properties allow incoming solar radiation to pass through while it blocks a substantial amount of earth's radiant heat from escaping. It's true that methane has a higher capability to traph heat (20x) than carbon dioxide, but it stays in the atmosphere for a shorter time (only 10 years). Both are important, but generally carbon dioxide is considered to be more so because it is present in larger quantities and it hangs around for longer. So, relative to methane, CO2 does trap little heat, but its overall contribution in our atmosphere is very high.
The strong correlation between atmospheric carbon dioxide levels (from ice cores) and temperatures (from multiple proxies) is striking. When carbon dioxide has been high, temperatures have been high. So, the wisdom of pumping billions of tons of carbon dioxide into the atmosphere seems very suspect, even if modelers cannot nail down exactly how clouds (ie. water vapor) will perform with increasing temperatures. Thus, the argument for some preventive action.
But which causes which is uncertain. Most likely there is warming from some other cause (e.g. wobble, sun, etc) Then the oceans release CO2, then terrestrial sequestration increases and CO2 slowly decreases. The positive feedback from CO2 to water vapor to warming to warming is one theoretical part of the warming. There is a lot more theory that limits that positive feedback which must also be considered.