Clean Energy

Clean Energy is really a misnomer.  There is no such thing as "clean energy".  All energy sources emit pollutants during their life-cycles.

All we can say is that there are cleaner energy sources.  In other words, some sources are cleaner than others.

Since CO2 is the main green-house gas we are now concerned about, following is a comparison of carbon emissions when different energy sources are used for generating electricity.

"A literature review of numerous energy sources CO2 emissions by the IPCC in 2011 found that that the CO2 emission value, that fell within the 50th percentile of all total life cycle emissions studies conducted, was as follows." (From Wikipedia):

Now, depending on what lobby calculates the above numbers they tend to vary somewhat, but the point is that NO energy source is clean.  Sure, there is no doubt that coal is the dirtiest one but on the other hand it is significant that, at least in this particular study, solar (which is considered by many the ultimate green energy) is dirtier than nuclear energy.

Why is this?  

Answer: although solar panels produce no carbon emissions during operation, the emissions produced during their manufacture have to be "amortized" in the energy produced by the panels during their useful life.  And these emissions are not negligible.  The silicon has to be mined, purified, fused, doped, cut, soldered, transported, installed, etc.  Plus the aluminum and glass components of the panels also go through an energy intensive manufacturing process.

And by the way, oil is located somewhere between coal and natural gas.

So, even though ALL energy sources emit carbon dioxide when you factor in their complete life cycle,  it is obvious that fossil fuels are the dirtiest.  All the rest we could label as "low carbon" energy sources.  

What humanity needs to do to prevent a climate catastrophe is to move from "high carbon" to "low carbon" energy sources.

Today the USA is significantly reducing it's carbon emissions by switching part of their coal electric utilities to natural gas.  This is probably the simplest and fastest way to reduce carbon emissions but is obviously only an intermediate step in the solution.

Long term we have to move aggressively to "low carbon" sources.