Are humans to blame?

Global warming can be broken down into two main components: natural and anthropogenic. Natural warming occurs with processes that would have taken place without any human interaction. The Milankovitch cycles have produced warming and cooling periods since the beginning of time and are shown further in Figure 6.  These cycles are forced by the tilt and rotations (about the axis) that the earth makes during a certain period in time.

Figure 6: Milankovitch Cycles

 Along with the Milankovitch cycles, vegetation and bodies of water will emit/absorb high levels of greenhouse gases over time.  The reason that substantial warming does not occur with natural processes, is due to the ability to absorb (take out of the atmosphere) greenhouse gases.  This is not to say that natural processes do not cause warming at all!

To understand anthropogenic warming, analysis of the word itself can be used to simplify meaning.  Break the word down into anthropo- (meaning men) and -genic (meaning cause).  Now put together it is easier to see that anthropogenic warming is that which is caused by humans.  The scientists that study anthropogenic warming say that humans have increased the global temperature 1.5* F since 1850 and this is how they are proving it.  Greenhouse gases such as methane, carbon dioxide, and nitrous oxide have significantly increased since this time (Industrial revolution and so on).  As more and more gas is present in the atmosphere, its ability to "trap" radiation from earth is increased and effectively heats the earth. 

Figure 7:  Temperature and Carbon Dioxide Data

Though the analysis seems simple, Figure 7 shows how correlated the data is (shown for carbon dioxide).  Burning of fossil fuels accounts for 60% of greenhouse gas emissions which is causing a sense of urgency for the use of alternative energy sources.  Even with decreases in these greenhouse gas emissions, will the global temperature ever be restored?

Understanding the potential damage of greenhouse gases will more than likely lead to the beginning of an era to restore (or at least reduce) levels of such gases.  So pretend that all greenhouse gases were "shut off" or no longer allowed to be released into the atmosphere.  The global temperature would slowly decrease right?  According to the NOAA, no it would not!  Currently, the oceans are soaking up a lot of the planet's heat (as well as the carbon dioxide) and will eventually start being released back into the atmosphere.  This process may take hundreds (maybe thousands) of years and in the eyes of scientists, is considered irreversible.  Figure 8 shows how gas levels (carbon dioxide) may decrease if emissions completely stopped. 

Figure 8:  Results of stopped carbon dioxide emissions

 Changes in temperature without greenhouse gas emissions will most likely be caused by changes in the earth's rainfall, which will in turn affect the supply of water, agriculture, and the ecosystems.  Also shown in the figure is sea level rise, which will increase two meters by  the year 3000 (and this is without any more carbon dioxide emissions!).  All put together, greenhouse gases pose a threat to our planet not only today, but hundreds of years in the future.  Even with small (or even large) changes today, future generations are still going to feel the effects of what has already occured. 


References: 
http://ossfoundation.us/projects/environment/global-warming
http://www.pewclimate.org/global-warming-basics
http://monthlyreview.org
http://www.greenmontclair.org/greenmontclair/The_Science/Entries
http://www.npr.org/templates/story/story.php?storyId=99888903