Boycotting the Climate Summit

Global Post reports that UN Secretary General Ban Ki-moon hosts a summit on global warming:

While some world leaders (including President Obama) will spend time at the UN summit discussing how to prevent the human species from hastening its own demise, the countries most responsible for climate change aren't bothering to show up. Of the five worst carbon emitting nations, China (1), India (3), Russia (4), and Japan (5) are all skipping. (Most nations are sending heads-of-state and/or foreign ministers, but China and India are sending low-level representatives.)

Which suggests Japan and Russia aren't attending at all. We are the (2) missing in the above list. If the other four worst polluters won't bother to show, why should the U.S. President?