Here is a small list of human activities to consider...

• Houses in colder climates burning fuel of some sort (wood, gas, etc..) all winter to heat up there homes
• Massive amounts of coal burned to generate electricity
• Massive amounts of oil burned in the form of gasoline and diesel...just imagine dumping a gallon of gasoline on the ground and lighting a match...thats what we do every time we drive like 20 miles depending on what kind of car we have)

Now all that heat that is generated...diffuses in the environment...and spreads out. The gas composition of the air is changing as well. The earth does go through natural cycles of hot and cold, but human activity is most certainly affecting it, because on Earth, everything is connected. For the most part it is a closed system and anything that happens has some effect, even if its small. Its not a complete closed system though because of the affects from the the sun and other cosmic energy.

Regardless of whether the earth is warming up or cooling down right now, humans have set drastic changes on Earth in motion... I just think when some people deny global warming they don't consider the huge impact humans have made on Earth.