Actually water vapor (and methane) are many times more powerful greenhouse gasses than CO2. But because of feedbacks, Earth's climate system is very sensitive. CO2 is the one that humans have been pumping inot the atmosphere, but a slight warming due to CO2 causes increased evaporation (water vapor) which makes a lot more warming. Similarly, a slight warming due to CO2 can thaw permafrost which releases huge amounts of methane and accelerates warming. CO2 is not necessarily powerful in itself, but it has giant leverage over Earth's climate system.