The 'Global warming' phenomenon could be caused by other things

Changes in land use and urbanization may contribute more to changes in the average ground temperature than “global warming” caused by human emissions.
www.climatechangefraud.com

There are probably other factors which are contributing to the warming effect, both natural and man-made. However, calculations also show that the rise in temperature can be easily explained by the increased levels of CO2 in the atmosphere. Until another compelling explanation is put forward, this would seem to tbe the most likely cause. cause.  
 
Also, just because something else could be causing global warming doesn't mean that it is.