The argument is not we don't need to do something because the other guy is worse...it's about not needing to institute specific policies that cannot be shown to be effective in addressing the problem from a global perspective while at the same time, imposing a disproportionate local burden on the societies which take those ineffective actions. 

Rather than the professor's chocolate bar moral argument, which is flawed simply because stealing always has a victim, irrespective of whether the amount stolen is large or small....a better way to judge the global warming action argument is the 2 trains speeding towards each other on the same track analogy.  If the trains are already so close that both need to apply the full brakes to avert complete disaster , and yet one engineer refuses to do so, there is no moral reason to stop serving lunch in the dining car of the other train....

climate change is a technical global issue, it requires an enforceable co-ordinated technical response