A hard-science professor e-mails:

I understand your scepticism regarding the temperature record. Temperature fluctuates by a few degrees every day and you may have a hard time understanding how the average temperature can be reported with the accuracy of one-tenth of a degree.
It nonetheless can be done. When temperature is monitored continuously throughout the day, week after week, the hourly and daily fluctuations average out.
There are also spatial fluctuations: weather varies from place to place. However, averaging over the globe helps get rid of this source of noise, too.
If you are interested in further details, more information is available at the National Climatic Data Center .