According to the Telegraph, these were the parting words to members of the G8 from George W. Bush:

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." - The Telegraph

The joke not only fell flat, but wasn't even accurate as China recently surpassed the U.S. as the world's biggest polluter. No surprise that he didn't know that, apparently in the Bush White House, climate change and pollution are just one big joke.