Objection: So 2005 was a record year. Records are set all the time. One really warm year is not global warming.
Answer: This is actually not an unreasonable point — single years taken by themselves can not establish or refute a trend. So 2005 being the hottest globally averaged temperature on record is not convincing. Then how about:
every year since 1992 has been warmer than 1992;
the ten hottest years on record occurred in the last 15;
every year since 1976 has been warmer than 1976;
the 20 hottest years on record occurred in the last 25;
every year since 1956 has been warmer than 1956; and
every year since 1917 has been warmer than 1917.
The five-year mean global temperature in 1910 was .8 degrees Celsius lower than the five year mean in 2002. This, and all of the above, comes from the temperature analysis by NASA GISS.
Global Temperature Land-Ocean Index
There is an interesting quote from that page:
Record warmth in 2005 is notable, because global temperature has not received any boost from a tropical El Niño this year. The prior record year, 1998, on the contrary, was lifted 0.2°C above the trend line by the strongest El Niño of the past century.
So, yes it is true that one record year does not make a long term trend, but that is clearly not the whole story.