"it is now believed that 1934 was probably the hottest year on record"
Factcheck required; I think this quote may well be misleading, though I am open to correction. I googled it and it appears that this statistic relates only to the weather in the 'lower 48' United States - not global average temperature and in particular not polar regions.
The correction from 2005 to 1934 directly corresponds to the dataset Gore used in An Inconvenient Truth. However, there are plenty of other major flaws with Gore's film. He has already freely admitted to the necessity of using exaggeration to provoke a motivated response.
For a detailed critique of the abundant flaws in this movie and the above quote, this link is very interesting.Gore shows a chart purporting to be actual temperatures since the Civil War. (Note that the film doesn’t expose the dependent axis. If he did it would emphasize how small the changes in temperature were.) Gore states: “These are actual measurements of atmospheric temperature since our civil war. In any given year it might look like it’s going down, but the overall trend is extremely clear. In recent years it is uninterrupted and it is intensifying. In fact, if you look at the 10 hottest years ever measured in this atmospheric record, they have all occurred in the last 14 years. The hottest of all was 2005.“
He can be forgiven for having been misled by a close advisor, Dr. James Hansen who heads the Goddard Institute for Space Studies (GISS). Hansen has recently been forced to admit that errors were made in correcting raw data and that the hottest year was 1934. Here is the new ranking: 1934, 1998, 1921, 2006, 1931, 1999, 1953, 1990, 1938, 1939.