Not only is global warming real, but mankind's activities contributed greatly to our recent warming.
A. Since the turn of the last century, the average global temperature has risen by about 0.8°C (about 1.4°F). Most of this increase has come within the last thirty years. http://data.giss.nasa.gov/gistemp/graphs/Fig.A2.lrg.gif
B. CO2, methane, water vapor are all greenhouse gases. Over the last ~150 years, the concentration of CO2 has increased by 35%. More greenhouse gases in the atmosphere will means that Earth's surface will become warmer.
C. Most, if not all of this increase in CO2 is the fault of mankind. We know this because of changing ratios of carbon in the atmosphere.[1]
D. Due to increasing concentrations of greenhouse gases in the atmosphere, the average global temperature is expected to rise by another 1.1 to 6.4°C by 2100.[2] The most likely increase will be between 1.7 and 4.4°C.
E. If the world warms by another 1.7°C, then there is a laundry list of bad effects that will most likely come true. That laundry list includes: more evaporation, and heavier precipitation and severe storms. Heavy precipitation often leads to flooding. And because there is heavy precipitation in one area, the next area over will get less precipitation. This is because the storm clouds have already dropped much of the accumulated water vapor. Inland, droughts are expected to worsen. A 1.7°C increase hastens the melting of glaciers and snow pact. Much of the world receives their fresh water from glacier melt. With the glaciers gone, many people will have no drinking water for much of the year... etc. etc....
F. The climate can also change for a number of "natural" reasons as well. Such natural changes include:
1. Solar variations: This is thought to be responsible for the Medieval Warm Period and the little ice age. http://en.wikipedia.org/wiki/File:2000_Year_Temperature_Comparison.png
2. Orbital variations: Orbital variations are though to be responsible for the coming and going of ice ages. http://www.technologyreview.com/articlefiles/climatechart.pdf
3. Plate tectonics: The movement of continents and the building and eroding of mountains have greatly changed the climate of earth over the course of millions and billions of years.
4. The chemical composition of the atmosphere affects climate as well. Most molecules in the atmosphere are immune to outgoing radiation (sunlight that bounces off of earth). The greenhouse gases trap this outgoing radiation close to the surface of earth.
Some particles such as dust and ash reflect sunlight before it even has the chance to warm Earth. Dust and ash from volcanoes and meteor impacts can cool off earth for years and even decades.
The evolution of single cell organisms, and then animals and plants, gradually changed the composition of the atmosphere over the course of billions of years.
G. The bottom line is that all natural cycles have been ruled out as to the cause of our recent global warming. Changes in solar output, solar cycles, orbital variations, cosmic ray variations are not responsible for our recent global warming. The deniers want us to believe that some yet unexplained magical cycle is responsible, but I'm not buying their argument.
