Read here and here. Multiple studies are indicating that any global warming that is occurring happens mostly during night hours. The global average temperature is actually an average of the maximum daily temperature and the minimum daily temperature, and what studies are revealing is that daily maximum temperatures are not rising but the minimum temperatures are. The reason? Human caused land-use change (urban heat island is one example) is radiating more heat at night, thus impacting the daily minimum temperatures being measured near the surface. The large impact on minimum temperatures can be seen in the chart on the left when comparing a recently urbanized area with a more rural area. In contrast, note how the daily maximum temperature differences in the chart on the right have stayed remarkably close for the two different areas. This is not a result of human CO2 emissions; it's a result of humans changing the earth's terrain and landscape. (click images to enlarge)