Replacing coal gives you other benefits as well, such as reducing the sulfate pollution that causes acid rain, particulate emissions that cause lung disease, and mercury that causes brain damage. And if less coal is mined, then occupational death and disease can be reduced in coal miners and the destruction caused by damaging forms of mining, including the removal, in some parts of the country, of entire mountains can be reduced or halted.
Those are significant benefits. In part for these reasons, the Obama administration has made natural gas development a centerpiece of its energy policy, and environmental groups, including the Environmental Defense Fund, have supported the increased use of gas. President Obama has gone as far as to endorse fracking—the controversial method of extracting natural gas from low permeability shales—on the grounds that the gas extracted can provide "a bridge" to a low carbon future and help fight climate change.
So if someone asks: "Is gas better than oil or coal?" the short answer seems to be yes. And when it comes to complicated issues that have science at their core, often the short answer is the (basically) correct one.
As a historian of science who studies global warming, I've often stressed that anthropogenic climate change is a matter of basic physics: CO2 is a greenhouse gas, which means it traps heat in the Earth's atmosphere. So if you put additional CO2 into that atmosphere, above and beyond what's naturally there, you have to expect the planet to warm. Basic physics.
And guess what? We've added a substantial amount of CO2 to the atmosphere, and the planet has become hotter. We can fuss about the details of natural variability, cloud feedbacks, ocean heat and CO2 uptake, El Niño cycles and the like, but the answer that you get from college-level physics—more CO2 means a hotter planet—has turned out to be correct. The details may affect the timing and mode of climate warming, but they won't stop it.
In the case of gas, however, the short answer may not be the correct one.
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 US 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.
A great deal of recent public and media attention has been focused not on gas itself, but on the mechanism increasingly used to extract it. Hydraulic fracturing—better known as fracking—is a technique that uses high-pressure fluids to "fracture" and extract gas from low permeability rocks where it would otherwise be trapped. The technique itself has been around for a long time, but in the last decade, combined with innovations in drilling technology and the high cost of petroleum, it has become a profitable way to produce energy.
The somewhat surprising result of several recent studies (including one by an expert panel from the Council of Canadian Academies on which I served) is that, from a climate-change perspective, fracking probably isn't much worse than conventional gas extraction. Life-cycle analyses of GHG emissions from the Marcellus and Bakken shales, for example, suggest that emissions are probably slightly but not significantly higher than from conventional gas drilling. A good proportion of these emissions come from well leakage.
It turns out to be surprisingly hard to seal a well tightly. This is widely acknowledged even by industry representatives and shale gas advocates. They call it the problem of "well integrity." Wells may leak when they are being drilled, during production, and even when abandoned after production has ended. The reason is primarily because the cement used to seal the well may shrink, crack, or simply fail to fill in all the gaps.
Interestingly, there's little evidence that fracked wells leak more than conventional wells. From a greenhouse gas perspective, the problem with fracking lies in the huge number of wells being drilled. According to the US Energy Information Administration, there were 342,000 gas wells in the United States in 2000; by 2010, there were over 510,000, and nearly all of this increase was driven by shale-gas development—that is, by fracking. This represents a huge increase in the potential pathways for methane leakage directly into the atmosphere. (It also represents a huge increase in potential sources of groundwater contamination, but that's a subject for another post.)
There have been enormous disagreements among scientists and industry representatives over methane leakage rates, but experts calculate that leakage must be kept below 3% for gas to represent an improvement over coal in electricity generation, and below 1% for gas to improve over diesel and gasoline in transportation. The Environmental Protection Agency (EPA) currently estimates average leakage rates at 1.4%, but quite a few experts dispute that figure. One study published in 2013, based on atmospheric measurements over gas fields in Utah, found leakage rates as high as 6%-11%. The Environmental Defense Fund is currently sponsoring a large, collaborative project involving diverse industry, government, and academic scientists. One part of the study, measuring emissions over Colorado's most active oil and gas drilling region, found methane emissions almost three times higher than the EPA's 2012 numbers, corresponding to a well-leakage rate of 2.6%-5.6%.
Some of the differences in leakage estimates reflect differing measurement techniques, some may involve measurement error, and some probably reflect real differences in gas fields and industrial practices. But the range of estimates indicates that the scientific jury is still out. If, in the end, leakage rates prove to be higher than the EPA currently calculates, the promised benefits of gas begin to vaporize. If leakage in storage and distribution is higher than currently estimated—as one ongoing study by my own colleagues at Harvard suggests—then the alleged benefits may evaporate entirely.
And we're not done yet. There's one more important pathway to consider when it comes to the release of greenhouse gases into the atmosphere: flaring. In this practice, gas is burned off at the wellhead, sending carbon dioxide into the atmosphere. It's most commonly done in oil fields. There, natural gas is not a desirable product but a hazardous byproduct that companies flare to avoid gas explosions. (If you fly over the Persian Gulf at night and notice numerous points of light below, those are wellhead fires).
In our report for the Council of Canadian Academies, our panel relied on industry data that suggested flaring rates in gas fields were extremely low, typically less than 2% and "in all probability" less than 0.1%. This would make sense if gas producers were efficient, since they want to sell gas, not flare it. But recently the Wall Street Journal reported that state officials in North Dakota would be pressing for new regulations because flaring rates there are running around 30%. In the month of April alone, $50 million dollars of natural gas was burned off, completely wasted. The article was discussing shale oil wells, not shale gas ones, but it suggests that, when it comes to controlling flaring, there's evidence the store is not being adequately minded. (At present, there are no federal regulations at all on flaring.) As long as gas is cheap, the economic incentives to avoid waste are obviously insufficient.