Carbon dioxide and other greenhouse gases in the atmosphere, including those from the burning of fossil fuels and other human activities, act like a blanket to warm up the Earth. The Sun's rays of visible light easily come in; but the infrared radiation that the Earth emits has a harder time getting out as the concentration of greenhouse gases increases. As a consequence, the atmosphere, on average, heats up, though actual changes vary by location. In Massachusetts, the average annual temperature has risen about 1 degree F in the past hundred years.
Another human-related phenomenon that is affecting the temperature in the Blue Hills and Boston is the urban heat-island effect.
Graph courtesy of Blue Hill Observatory and Science Center, East Milton, MA. http://www.bluehill.org/