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Electricity: U.S. Greenhouse Gas Emissions at 20-year Low — 
What Happened?

By Bob Shively, Enerdynamics President and Lead Instructor

Over the last 10 years, we have seen arguments ebb and flow over greenhouse gas emissions and global warming. Not too long ago, federal legislation limiting carbon emissions seemed imminent. Then the economic recession, a move to the right in the House of Representatives, and intensive lobbying and public relations efforts by certain interest groups altered public perceptions, and it appeared the U.S. would not act to reduce carbon output. 

But stunningly, the U.S. Energy Information Administration recently released data indicating that carbon emissions have fallen so dramatically over the last few years that emissions in the first few months of 2012 hit levels approximately equivalent to 1992 emissions. This surpasses the dreams of even the most optimistic policy writers who hoped to reduce carbon emissions through legislation.

So has everyone just simply decided on their own to be better environmental stewards? Not likely.  Rather, key contributing factors include:

  • reduced demand due to the economic downturn
  • renewable portfolio standards that have fostered increased renewable electric generation
  • increased energy efficiency efforts
  • replacement of coal-fired electricity generation by gas-fired generation  

To know where we stand today, it is important to understand which of the factors are most important and whether they produce long-term or short-term effects. With the following graph we can clearly see the economic downturn is not the primary factor — economic activity measured as Gross Domestic Product (GDP) has grown significantly since 1992 while carbon emissions have returned to the 1992 level. 

Using a method called “Kaya Identity” the EIA looks at causes of changes in carbon output.


Here we see a significant impact of the economic downturn in 2009, but emissions have fallen in 2011 (and as current data cited above shows they are continuing to fall in 2012) even as the economic output recovers. The largest bar for factors reducing carbon emissions in 2011 is green, indicating energy intensity. This means we are continuing to produce more economic output using less energy to do it. This is partly due to structural changes where services replace manufacturing in our economy and party due to increased energy efficiency.

Just smaller is the reduction due to carbon intensity of energy, meaning the amount of carbon emitted to utilize one unit of energy. This is dropping because we have significantly replaced coal-fired electric generation with natural gas and renewable generation. For many years, the U.S. has generated about half of its power with coal.  In 2011 coal generation was 43% of U.S. power and for the first six months of 2012 coal was only 35% of our generation mix. Why is this happening? As we can see below, this is mostly due to increased natural gas generation driven by low cost natural gas. But renewables are also starting to make a dent. 

           Source: EIA Electric Power Monthly  


And one more chart shows us the impact of changing the generation mix. Looking at the chart below, we can see that even though carbon emissions from gas units are increasing, this increase is significantly smaller than the large reduction in carbon emissions from coal generation.

So all this begs the question: Is this a permanent change? My guess is yes, despite some rhetoric you might hear during the current campaign season. Renewable portfolio standards, new EPA mercury and carbon standards, a robust supply of natural gas (Hurricane Isaac barely even moved prices), an aging fleet of coal generation plants, and upcoming significantly stricter vehicle mileage standards all suggest that in the U.S. things look rosy for continued lower carbon emissions.  Keep in mind, however, that this outlook is currently unique to the U.S. and that other parts of the world are not experiencing the same reductions.

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