You raise a valid point. China, in particular, produces twice as much CO2 as the US. And they have a long history of ignoring environmental concerns.
Obviously, tackling this issue requires international cooperation. No easy task. But, to suggest we are powerless to do anything is not true. Everyday we make consumer decisions that affect businesses and, in this case, polluters. China doesn't want to curb CO2 production? If I don't like that, I can choose not to buy stuff from there. Given that China is hugely dependent on the US to purchase their goods, this would have an effect.
Nothing is immediate. This is a long term problem that needs a long term solution and patience. Sticking our heads in the sand, denying reality and/or claiming we're powerless are all cop-outs. A combination of carrots and sticks hopefully gets us going in the right direction.