An energy efficient light bulb would use far less electricity than an ordinary one. By far in this country, most electricity is generated by burning fossil fuels. This puts carbon dioxide into the atmosphere in huge amounts. So, it follows that if you use CFL (compact fluoerscent lightbulbs) you use far less electricity to produce the light, thus reducing the amount of carbon dioxide out-put. Electricity is also produced hydroelectrically, through solar cells and geo-thermally and also through wind power, but these methods, while not producing carbon dioxide, as yet are far in the minority.