Anyone who's spent a day on the beach has noticed the water level changing minute by minute as waves swash in and out. But spend enough of the day there and you'll see that the water level inevitably trends higher or lower depending on which way the tide is flowing:
Tide time-lapse from Protar on Vimeo.
No one would be foolish enough to say the water level was rising or falling based on one or two individual waves.
Now look at this plot of temperature data from the Blue Hills Observatory, located on Great Blue Hill south of Boston. This station has been collecting weather data for over a century. The individual data points (blue diamonds) are the averaged temperature at BHO for every year between 1895 and 2009:
The black line is a moving 10 year average and the red line is the linear trend for the whole set of data. Just like the waves and the incoming tide, the average temperature goes up and down from year to year, but over a century the trend is inexorably upward. No one would be foolish enough to say the climate at BHO is warming or cooling on a year to year variation, right? It only makes sense to look at the long-term.
Tide's comin' in folks.