I suggest that the reader visit the wikipedia article on the Global Warming Controversy before reading further. There are many references there which may lead you to more complete knowledge of the subject.
There are lots of reasons to be skeptical about whether global warming is actually taking place, and whether mankind is causing it.
This is true in spite of a large consensus among climate scientists that it is real, and it’s due to emissions of CO2. Arguments supporting the increasing temperature trend are many. There are also claims that the ocean level is rising, and the polar ice caps are melting. Some supporting facts:
- Since ~1890, the average world mean temperature has risen about 1.3 degree Fahrenheit, from about .4 below the accepted mean (57 degrees) to recently (2008), .6 degree above.
- The arctic ice cap is either shrinking or growing – you can find articles supporting both claims. Likewise, conflicting claims are made about the Antarctic ice cap.
- The sea level is rising – or not. Here again, the claims conflict. It was recently revealed that scientists have been fudging the estimates upward.
- The years 1998 and 2010 are cited as the warmest on record – yet, ocean temperatures fell – we had “La Niña ” last year. (El Niño is characterized by unusually warm temperatures and La Niña by unusually cool temperatures in the equatorial Pacific.) These temperatures have been mostly below average since around 2000.
- In the long range temperature plot by Harris-Mann, covering the period from ~2500 BC to present, we see the “Little Ice Age” from ~1350 AD to ~1850. Since that time, a general warming trend has taken place, beginning before mankind could have affected the climate very much. This natural cycle may be continuing.
There seems little doubt that a long-term warming trend is in place, although it has been interrupted from time to time. If you go back to the coldest point of the “Little Ice Age” – around 1600 – it has steadily increased by around 3.3 degrees F.
Whether mankind is causing the warming is controversial. There are numerous uncertainties and controversies:
- The value of climate sensitivity of carbon dioxide doubling isn’t known. Estimates range from less than 1 degree F to several degrees. 
- There is controversy over whether increasing carbon dioxide levels will cause runaway positive feedback, or not. 
- There is controversy over the effects of increased water vapor in the atmosphere (caused by rising temperatures). 
- There are arguments that no action by the United States can have could have any significant effect on the global warming trend. 
- There are recent predictions that the world is about to enter a new “Little Ice Age” within a decade due to new lows in the sunspot cycle. 
- The effects of man-made aerosols and particulates (dust) emitted into the atmosphere isn’t fully understood. 
 Climate sensitivity is defined as the average increase of the temperature of the Earth that you get (or expect) by doubling the amount of CO2 in the atmosphere – from 0.028% in the preindustrial era to the future value of 0.056% (expected around 2100).
 Feedback is the amplification of an effect – in either direction. Runaway feedback is considered a possibility – a degree of warming adds moisture to the air, and moisture is a much more effective greenhouse gas, so the temperature rises due to that, which adds more moisture . . . etc.
 Earth’s atmosphere is constantly circulating moisture into and out of the air. It leaves as rain or snow. Snow cools the surface, and reflects sunlight back into space, resulting in less warming.
 The rest of the world may do nothing whatever to counter emissions of carbon dioxide and other greenhouse gases.
 Sunspots have so diminished recently that they may disappear completely for a time. During the “Maunder minimum” – a long period with very few sunspots — the Earth went through the “Little Ice Age” – which lasted for 500 years (around 1350 to 1850) and saw the average world temperature fall (at maximum) by about 2.7 degrees F. This cycle was due to nature, with little to no assistance by mankind.
 Dust blocks sunlight. Volcanoes create a great deal of it, which is why volcano activity can quickly cool the earth’s average temperature. Mount Pinatubo, in 1991, was a notable and recent example.
The Harris-Mann website A beautiful chart of the long term temperature fluctuations of the Earth may be found here.
American Institute of Physics This site explains the history and science of climate change thoroughly.