It's all been one, long private joke, I think.
Via the Telegraph:
“The American leader, who has been condemned throughout his presidency for failing to tackle climate change, ended a private meeting with the words: ‘Goodbye from the world’s biggest polluter.’ He then punched the air while grinning widely, as the rest of those present including Gordon Brown and Nicolas Sarkozy looked on in shock.”
“One official who witnessed the extraordinary scene said afterwards: ‘Everyone was very surprised that he was making a joke about America’s record on pollution.’”
