Why there is so much concern on global warming due to carbondioxide when the gas is soluble in water .?

Carbondioxide is soluble in water. Hence its percentage in atmophere can be depleted due to rain and moreover,it is absorbed by plants during photosynthesis. Other gases like methane produced due to decomposition of organic matter and producing greenhouse effect is more responsible.for global warming. I donot understand why there is much hype about carbondioxide.