Physiorg's elegant explanation of GW rings true as yours truly is way beyond 30, so us old ones remember how different the weather was back in the day, something my son, who is slightly north of 30, will never experience as we move further into an increasingly hot 21st century.
The variability we observe in surface temperature comes primarily from understood patterns of weather. Many have heard of El Niño, when the eastern Pacific Ocean is warmer than average. The eastern Pacific is so large that when it is warmer than average, the entire planet is likely to be warmer than average. As we look at averages, 30 years, 10 years, or even one year, these patterns, some years warmer, some cooler, become less prominent. The trend of warming is large enough to mask the variability. The fact that there have been 30 years with no month below the 20th century average is a definitive statement that climate has changed.