Filming real ice shelves, for example. That's why Al Gore used CGI footage from the movie The Day After Tomorrow in his documentary, An Inconvenient Truth.
As it happens, this doesn't affect the validity of his argument either way, but it's unsettling that it doesn't seem to have been disclosed before last Friday.
UPDATE: Gore wants to make a sequel:
Mr Gore added: “I have to say the situation has not improved since I made the movie in 2006. Sure, awareness has grown and more people are concerned since scientists said we had just ten years to take action to halt rising sea levels.I'm afraid that last assertion doesn't seem to be true. Well, I'm glad it isn't true, but afraid he'll include it in his sequel.
“But the situation has got worse. The entire North Polar ice cap is melting and could be gone in some areas in as little as five years."