First, policies that accomplish "almost nothing" on a worldwide basis can still be very beneficial. The idea that a cut of 5% in world CO2 emissions is so insignificant that we might as well pretend it's zero is a classic logical error. You can't use your subjective judgment about what constitutes "insignificance" to arbitrarily set benefits at zero before even comparing them to costs! As we emit more carbon, the consequences for the climate become more difficult to predict, and the last 5% of emissions are the most dangerous. On a worldwide basis, they will do a great deal of harm.
You might claim that carbon emissions will simply shift overseas, but this goes much too far. Not all carbon-intensive production will move, and an enormous fraction of our emissions come from electric power generation, transportation, and other functions that can't be readily outsourced. An aggressive emissions policy by the United States will lead to real cuts in worldwide carbon output, and if you acknowledge the tremendous worldwide costs of climate change, you have to admit that there's potential for global benefit even from unreciprocated American effort. If you dispute this on technical cost/benefit grounds, fine, but you need to make this more explicit than a handwaving dismissal of "small" progress.
Admittedly, these are global benefits we're discussing. If you assume that there is zero chance of multilateral bargaining on climate change, and that the United States will be completely alone in whatever it does, the American economy itself will be a net loser. But even putting aside the absurdity of the assumptions, does this really mean that we should do nothing?
Let's try a thought experiment. Say that households in the United States emit a large amount of deadly poison, which kills tens of thousands annually around the world. Other countries emit poison too, but the United States is responsible for a vastly disproportionate amount. Even if it doesn't act as part of an international coalition, and it will suffer a net economic loss from changing policy alone, isn't the US morally obligated to address this? As long it's the single most culpable nation in the deaths of countless people, I think it is. When poison coming from the US is killing thousands elsewhere, it's patently unethical to interpret policy through the narrow prism of American benefit. I cannot imagine any consistent moral philosophy suggesting otherwise.
Of course, "poison" here is a philosophical stand-in for climate change. I plead guilty to overdramatizing my example, but I don't think that it makes any difference to the core issue: whether it's right for the United States to disproportionately harm the rest of the world as long as it's in the nation's interests to do so. In fact, thinking about a case as simple as "poison" allows us to clarify our moral intuition, because it sets aside complications—like the temporal displacement of cause and effect, and the fact that costs lie on a probability distribution rather than at a single point—that have no bearing on the basic moral question yet tend to confuse us.
And remember that the US can still benefit! There is no basis for assuming that concerted global action on climate change is impossible. Yes, China and India will probably be reluctant to take costly action when the US still emits far more carbon per capita. (Frankly, this is reasonable: why should your underdeveloped nation make cuts when a far richer country emits so much more?) But this won't last forever. If we make cuts now, our per capita emissions will be much closer to theirs in just a few decades. At that point, the need for dramatic changes will be even clearer, and since there will not be such obvious "winners" and "losers,", a fair and Pareto-improving policy will be accessible. If we do nothing, however, we risk a perpetual cycle of blame and inaction, as large developing countries refuse to make sacrifices when the US is still polluting so much more.
It's easy to dismiss carbon policy because it "won't make a difference." But I have never seen anyone making this argument appropriately deal with the economic, moral, and political issues I've discussed here.