Has the US cut its emissions?

You may have seen reports that US greenhouse gas (GHG) emissions fell 3.8% last year; they were all over the media last week. The fall since 2005 is said to be 12%. The main factor driving the fall is a switch from coal to natural gas due, in turn, to the availability of cheap gas from fracking (shale gas) though the recession has also contributed.

That’s good news – but nothing like as good as it looks.

The emissions numbers count gas burnt but ignore gas lost during extraction and transfer to users (Wall Street Journal (18/4/13, http://tinyurl.com/WSJ-US-GHG). Since natural gas is mainly methane and methane is much worse for the climate than CO₂ (http://en.wikipedia.org/wiki/Greenhouse_gas) it takes only a little methane leakage to offset that saving in CO₂ emissions.

To see how much we have to set a timescale and I choose 20 years because that is the critical period for addressing climate change. If we have not solved it well within that period it will be a lost cause.

Because methane is such a potent greenhouse gas there will only be a saving of greenhouse gas emissions relative to coal if leakage is less than 0.7%. In 2011 Robert Howarth and co-workers at Cornell University estimated the leakage in the US at 3.6% to 7.9%, exclusive of accidents. They conclude that switching from coal to shale gas will increase the greenhouse effect of this fuel use by a factor of 1.2 to 2 over 20 years! The effect over 100 years is roughly zero.

Suppose that half of the apparent fall in greenhouse gas emissions was due to a switch from coal to shale gas (which is plausible).  Then the 12% reduction in CO₂ would imply that the reduction in total greenhouse gas emissions has been somewhere between 5% and nothing. 

The switch from coal to shale gas is not a solution to the problem of climate change and has no part to play unless the leakage can be very greatly reduced and the other environmental effects eliminated. 

Nor is that the end of the bad news – unlike the policy-driven reductions we seek the US reduction is price-driven and is therefore not locked in. Specifically:

• Part of the GHG reduction is due to the recession and will likely be reversed as the recession ends.
• Natural gas prices have been rising so US users have begun to switch back to coal.

Finally, low gas prices in the US have led the US to export the coal it would otherwise have burnt – mostly to Europe!

Fossil fuels are a global market and local prices falls will only increase fuel usage. We can only reduce GHG emissions if we raise the price of fossil fuel.

Nasty but necessary.