The often-touted decrease in greenhouse gas production applies when natural gas replaces other fuels -- particularly coal -- in electricity generation. That's important. Electricity is about 40% of total U.S. energy use. Traditionally, coal has been the dominant fuel used to generate electricity in this country and most of the world. (And no one has any serious plan to live without electricity.) Any measurable GHG reduction in the electricity sector is significant and gains achieved in that sector quickly add up.
But a good deal of the benefit of gas in electricity generation comes from the fact that it is used in modern combined-cycle gas turbine plants. A combined-cycle plant is one in which waste heat is captured and redirected to drive a mechanical system that powers a generator that creates additional electricity. These plants can be nearly twice as efficient as conventional single-cycle plants. In addition, if combined with cogeneration (the trapping of the last bits of heat for local home heating or other purposes), they can reach efficiencies of nearly 90%. That means that nearly all the heat released by burning the fuel is captured and used -- an impressive accomplishment.
In theory, you could build a combined-cycle plant with coal (or other fuels), but it's not often done. You can also increase coal efficiency by pulverizing it, and using a technique called "ultra super-critical black coal." An expert report compiled by the Australian Council of Learned Societies in 2013 compared the efficiencies of a range of fuels, including conventional gas and shale gas, under a variety of conditions, and concluded that greenhouse gas emissions from electricity generation using efficient forms of coal burning were not that much more than from gas.
What this means is that most of the benefit natural gas offers comes not from the gas itself, but from how it is burned , and this is mostly because gas plants tend to be new and use more efficient burning technologies. The lesson, not surprisingly: if you burn a fuel using twenty-first century technology, you get a better result than with late nineteenth or twentieth century technology. This is not to defend coal, but to provide an important reality check on the discussion now taking place in this country. There is a real benefit to burning gas in America, but it's less than often claimed, and much of that benefit comes from using modern techniques and new equipment. (If the coal industry weren't so busy denying the reality of climate change, they might publicize this fact.)
It's Not Just Electricity
Replacing coal with gas in electricity generation is still probably a good idea -- at least in the near term -- but gas isn't just used to generate electricity. It's also used in transportation, to heat homes and make hot water, and in gas appliances like stoves, driers, and fireplaces. Here the situation is seriously worrisome.
It's extremely difficult to estimate GHG emissions in these sectors because many of the variables are poorly measured. One important emission source is gas leakage from distribution and storage systems, which is hard to measure because it happens in so many different ways in so many different places. Such leaks are sometimes called "downstream emissions," because they occur after the gas has been drilled.
Certainly, gas does leak, and the more we transport, distribute, and use it, the more opportunities there are for such leakage. Studies have tried to estimate the total emissions associated with gas using well-to-burner or "life-cycle" analysis. Different studies of this sort tend to yield quite different results with a high margin for error, but many conclude that when natural gas replaces petroleum in transportation or heating oil in homes, the greenhouse gas benefits are slim to none. (And since almost no one in America heats their home with coal any more, there are no ancillary benefits of decreased coal.) One study by researchers at Carnegie-Mellon University concluded that while the probability of reducing GHG emissions at least somewhat by replacing coal with gas in electricity generation was 100%, the substitution of natural gas as a transportation fuel actually carries a 10%-35% risk of increasing emissions.
In the Northeast, the northern Midwest, and the Great Plains, many builders are touting the "energy efficiency" of new homes supplied with gas heat and hot water systems, but it's not clear that these homes are achieving substantial GHG reductions. In New England, where wood is plentiful, many people would do better to use high efficiency wood stoves (or burn other forms of biomass).
How Gas (CH4) Heats the Atmosphere Much More than CO2
Isn't gas still better than oil for heating homes? Perhaps, but oil doesn't leak into the atmosphere, which brings us to a crucial point: natural gas is methane (CH4), which is a greenhouse gas far more potent than CO2.
As a result, gas leaks are a cause for enormous concern, because any methane that reaches the atmosphere unburned contributes to global warming more than the same amount of CO2. How much more? This is a question that has caused considerable angst in the climate science community, because it depends on how you calculate it. Scientists have developed the concept of "Global Warming Potential" (GWP) to try to answer this question.
The argument is complicated because while CH4 warms the planet far more than CO2, it stays in the atmosphere for much less time. A typical molecule of CO2 remains in the atmosphere about 10 times longer than a molecule of CH4. In their Fifth Assessment Report, the Intergovernmental Panel on Climate Change estimated that the GWP for methane is 34 times that of CO2 over the span of 100 years. However, when the time frame is changed to 20 years, the GWP increases to 86!
Most calculations of the impact of methane leakage use the 100-year time frame, which makes sense if you are worried about the cumulative impact of greenhouse gas emissions on the world as a whole, but not -- many scientists have started to argue -- if you are worried about currently unfolding impacts on the biosphere. After all, many species may go extinct well before we reach that 100-year mark. It also does not make sense if you are worried that we are quickly approaching irreversible tipping points in the climate system, including rapid ice loss from the Greenland and Antarctic ice sheets.
It gets worse. CH4 and CO2 are not the only components of air pollution that can alter the climate. Dust particles from pollution or volcanoes have the capacity to cool the climate. As it happens, burning coal produces a lot of dust, leading some scientists to conclude that replacing coal with natural gas may actually increase global warming. If they are right, then not only is natural gas not a bridge to a clean energy future, it's a bridge to potential disaster.
Fracking
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