What are the main causes of Global Warming?

The main human activities that contribute to global warming are the burning of fossil fuels (coal, oil, and natural gas) and the clearing of land. Most of the burning occurs in automobiles, in factories, and in electric power plants

no. people are not the main cause. the earth has actually been known to go through phases of extreme heat and extreme cold. but through the past couple of thousand years, the earth has been reported to have been cooling down. people only really predict for their lifetime and a few more future lifetimes.they don't always look at the big picture. so the causes behind global warming are,....natural causes. nothings wrong with the slight heating up in temperature. Although humans do contribute to global warming, its not much. The news just makes a big deal about it.

There are many causes.  Here is a really good history:

http://www.aip.org/history/climate/co2.htm

There are many things that go into the climate we have today.  Human activity just adds a little to the other things that are maintaining the climate.  But that "little" is critical.  That little is pushing us toward a climate we may not like.  

The Earth's temperature is about 287 degrees Kelvin.(about 14 degrees C or 57 degrees F).  Subtract just a few degrees K and you have an ice age.  Add just a few degrees K and you have the ice caps melting and most of Florida under water.

The best web site for answers on global warming is http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-faqs.pdf
The people who put this site together are actually climate scientists.
Some facts that can't be denied about global warming are:
  1)  Carbon dioxide is a greenhouse gas - that is it traps heat in the earths atmosphere, thereby warming the planet.
  2) The amount of carbon dioxide in the atmosphere has increased quite considerably from 270 parts per million prior to the industrial revolution to about 385 parts per million now.
3) Humans are responsible for this increase. Even though the human sourced carbon is only about 3% of the carbon cycled through the atmosphere each year, the system was in balance, but now each year shows an increase in carbon dioxide. In fact the increase each year is only about half  the human contribution, so the system is trying, but failing to compensate.
4) Climate fluctuations in the past correlate with carbon dioxide levels.

The evidence seems to me overwhelming. The consequences of a small change in sea level are quite serious if you happen to live only a few feet above sea level - like most of London and New York and some entire Pacific Ocean countries.