Volume 92 Issue 27 | pp. 10-15
Issue Date: July 7, 2014

Cover Story

Methane’s Role In Climate Change

Whether natural gas is a savior or destroyer of climate depends on how much is leaking into the atmosphere
Department: Government & Policy
Keywords: methane, climate change, global warming, EPA, natural gas
About 40% (1 billion cu ft per day) of natural gas cogenerated during oil production operations in the Bakken formation of North Dakota is not sold or used. Most is flared as waste.
Credit: Tristan Spinski/Corbis
An aerial view of an oil fracking operation in the Bakken Formation in western North Dakota on Thursday, July 11, 2013.
About 40% (1 billion cu ft per day) of natural gas cogenerated during oil production operations in the Bakken formation of North Dakota is not sold or used. Most is flared as waste.
Credit: Tristan Spinski/Corbis

Jump to Topic:
- Methane By The Numbers

The U.S. is rushing to exploit its newfound wealth of fossil fuels. With startling regularity, a government agency or energy company announces a new natural gas or oil field, proposes another liquefied natural gas (LNG) export terminal, or lays out plans to expand a chemical plant. The bonanza is particularly important to the chemical industry as companies eagerly take advantage of these new resources for feedstocks and fuel.

But the push for fossil-fuel development and use comes at a time when greenhouse gas emissions continue their upward climb and climate scientists sharpen their calls of alarm.

For decades, human-caused climate disruption has been on the horizon, based on scientific predictions. The damage is now starting to hit home. Two months ago, a committee of leading U.S. scientists reported measurable changes in the U.S. climate because of anthropogenic greenhouse gas emissions. They stressed that all regions of the U.S. are being affected and that, without quick action, climate-related changes will accelerate and be irreversible (C&EN, May 12, page 7).

Most plans to stave off climate change have focused on limiting emissions of carbon dioxide, which make up 90% of anthropogenic releases of greenhouse gases. Yet methane—the primary compound in natural gas—is a much more potent greenhouse gas.

Methane makes up only 9% of anthropogenic greenhouse gases, but it has 86 times the global warming potential of CO2 in the first 20 years after it is emitted (see box on page 14). Scientists believe it is crucial to reduce emissions of methane now because of its potentially critical impact on the climate in the next 10 to 20 years.

Although a few years ago chemical companies complained of the high price of natural gas, today that price has tumbled. The U.S. is awash in cheap natural gas thanks to new drilling technologies, primarily hydraulic fracturing, or fracking.

EPA estimates that 29% of annual U.S. methane emissions leak from oil and natural gas production and distribution. Nearly half of those emissions come from the first of four stages from drilling to delivery; emissions are measured in metric tons of CO2 equivalent (MT CO2 eq).a An additional amount of methane is flared during oil and gas production, emitting 55 million metric tons of CO2 per year.SOURCES: EIA, EPA, DOE
Graphic shows amount of methane lost in natural gas distribution process.
EPA estimates that 29% of annual U.S. methane emissions leak from oil and natural gas production and distribution. Nearly half of those emissions come from the first of four stages from drilling to delivery; emissions are measured in metric tons of CO2 equivalent (MT CO2 eq).a An additional amount of methane is flared during oil and gas production, emitting 55 million metric tons of CO2 per year.SOURCES: EIA, EPA, DOE

The U.S. is on track to produce more natural gas than it needs by 2020. U.S. oil and gas companies intend to sell some of their bounty to the rest of the world, hence the rush to gain federal approval for building LNG terminals for exports. The race to exploit this surplus has even driven a wedge between U.S. chemical companies and their historic ally, the oil and gas industry. Chemical executives want to keep natural gas cheap for domestic use while gas and oil industry owners want to profit through exports (C&EN, March 11, 2013, page 9).

Either way, the U.S. is on the cusp of a new methane energy economy.

Despite its high global warming potential, methane has climate pluses. When burned as fuel for electricity generation, methane emits around half the CO2 of coal. This climate-related advantage has led to natural gas’s endorsement by President Barack Obama and the Department of Energy (DOE). Greater use of methane as a fuel is part of the Administration’s climate action plan to cut CO2 emissions.

Natural gas also has the potential to allow the U.S. to retain its fossil-fuel infrastructure. It offers a path to reduce CO2 emissions from the burning of coal for electricity while the nation develops carbon-free renewable energy or other electricity sources.

The gas boom is a boon, according to Energy Secretary Ernest J. Moniz. “First of all, we know that it is already partially responsible for the decrease in CO2 emissions that we have experienced over the last years,” he says. Moniz calls natural gas a “kind of bridge to a very low carbon future.” U.S. natural gas resources, he says, could afford more time for the nation to develop new alternative energy technologies, lower their costs, and achieve significant market penetration.

Not everyone agrees. Because of methane’s global warming potential, leakage of methane to the atmosphere makes this “bridge” a fallacy, several scientists say. They note that the current natural gas glut—and subsequent low prices—means there is little monetary incentive to stanch methane leaks.

They point to places such as the Bakken oil field, where output, thanks to fracking, has made North Dakota the U.S.’s second-largest oil-producing state. Natural gas and oil are usually found together, notes the Energy Information Administration (EIA), adding that more than half the 40,000 wells drilled last year in the U.S. held reserves of both. At Bakken, however, there is no infrastructure to move the gas to market. Consequently, methane is vented to the atmosphere or flared on-site as if it were waste. It is a mere by-product as energy companies rush to exploit the field’s more valuable oil resources.

About 29% of U.S. methane emissions are from leakage in the oil and gas production and distribution chain, according to the Environmental Protection Agency’s Greenhouse Gas Inventory. That makes the oil and gas industry the nation’s second-largest source of methane releases, ranking barely a percentage point below emissions from livestock.

Oil and gas drilling and field production constitute nearly half this loss, according to Paul Gunning, director of EPA’s Climate Change Division in the Office of Air & Radiation, who spoke at a recent webinar. The rest is divided among gas processing, transmission, storage, and distribution.

The actual methane leakage numbers—stated as a percentage of production—are between 1 and 2%, according to EPA’s annual inventory, based on the agency’s interpretation of industry-reported data. This number varies yearly and has shown a decline over time, according to EPA’s assessment of industry reports.

Global methane concentrations remained steady at about 0.7 ppm for hundreds of years, then began to rise quickly during the modern industrial age.NOTE: Data for 1750–1982 are from CSIRO Marine & Atmospheric Research. Data for 1983–2014 are from NOAA.SOURCE: Ed Dlugokencky/NOAA
Graph shows global methane concentration initially steady, then rising during the modern industrial age.
Global methane concentrations remained steady at about 0.7 ppm for hundreds of years, then began to rise quickly during the modern industrial age.NOTE: Data for 1750–1982 are from CSIRO Marine & Atmospheric Research. Data for 1983–2014 are from NOAA.SOURCE: Ed Dlugokencky/NOAA

However, many atmospheric scientists question these numbers, saying they are too low, and point to a host of new studies conducted in recent years.

First to voice a strong challenge to EPA’s data was Robert W. Howarth of Cornell University. Three years ago, Howarth warned that overall methane emissions from natural gas activities could be 7.9% of production, far above EPA’s calculation. He blamed EPA’s low estimate on missed releases during fracking, when methane and other hydrocarbons flow to the surface, carried up by fracturing fluids when wells are initially drilled and fractured (Clim. Change Lett. 2011, DOI: 10.1007/s10584-011-0061-5).

A study published in 2012 appears to support Howarth. Scientists from the National Oceanic & Atmospheric Administration (NOAA) and the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado, Boulder, accidentally discovered high methane levels being released from a Colorado oil and gas field. They were conducting tower-based air measurement experiments near a freeway (J. Geophys. Res. 2012, DOI: 10.1029/2011jd016360).

The group continued their methane experiments and shifted to a large oil field in Utah, where they used aircraft to take air samples over natural gas and oil fields in Uintah County. There, the research team, led by Anna Karion, found methane leakage rates of 6 to 12% of production. The study was published last year (Geophys. Res. Lett. 2013, DOI: 10.1002/grl.50811).

This group of researchers also conducted a follow-up study to the earlier one in Colorado and found methane levels that were at least three times higher than EPA’s estimate. Their data were published last month in the Journal of Geophysical Research (2014, DOI: 10.1002/2013jd021272). These studies are highly structured and adjust for other sources of methane near the oil and gas fields, the researchers note.

A growing body of literature supports the view that methane emissions are higher than levels reported by industry and EPA. A study released earlier this year by a team led by Adam R. Brandt of Stanford University examined some 20 years of technical literature and 200 papers and found that federal officials have consistently underestimated actual methane emissions, which the report says are some 50% higher than EPA’s estimates (Science 2014, DOI: 10.1126/science.1247045).

Other peer-reviewed studies have raised similar challenges to EPA’s estimates, all pointing to levels above the agency’s estimates.

However, not all studies found problems in EPA’s data. An examination by scientists at the University of Texas, Austin, backed EPA’s numbers (Proc. Natl. Acad. Sci. USA 2013, DOI: 10.1073/pnas.1304880110).

The Texas researchers found methane leakage levels from drilling and production to be near or even slightly lower than EPA’s estimates. The study, funded and organized by the Environmental Defense Fund (EDF), an environmental group, and several oil and gas companies, measured methane emissions from operations on the ground at the oil and gas fields, rather than air measurements taken during flights over the sites.

This study is the first in which oil drillers, which control the sites, granted access to nonindustry researchers. Consequently, this study provides on-the-ground measurements. But like EPA’s inventory, it carries the stigma of industry control.

Industry and researchers negotiated to select the test sites, noted David T. Allen, a UT Austin chemical engineering professor, who led the study. Speaking at the webinar, Allen acknowledged that the tests may be measuring best conditions—sites with so-called green completions that use new technologies to capture methane leakage during drilling and fracking. Still this study advances the discussion by measuring on-the-ground emissions, he said.

The difference has been characterized as “top-down versus bottom-up”—field measurements at a specific site versus measurements from above in an aircraft or tower.

Allen, Gunning, and other researchers attribute the high losses that NOAA and CIRES found to so-called superemitters—badly run, old, and leaking well sites or abandoned wells from partially depleted fields.

They stress the difficulty in measuring emission from huge fields holding tens of thousands of operating wells and an unknown number of closed sites. EIA estimates that there are some 500,000 wells operating at some level of production in the U.S.

The disparity might be resolved by upcoming research that will merge top-down and bottom-up approaches to emissions measurements. Those studies are just getting under way, notes Drew Nelson, EDF senior manager for natural gas projects. EDF and the gas and oil industry are funding 16 studies involving 90 academic and industry collaborators, and results are expected this year, Nelson says.

Several of these studies will be done by the NOAA-CIRES team of scientists, says Jeff Peischl, CIRES associate scientist. Their plan, he says, is to conduct top-down measurements of an oil and gas field by aircraft and back up this data with mobile lab measurements on the ground. NOAA-CIRES will use two aircraft and four mobile units and will look for other hydrocarbons as well as methane. The researchers will start these investigations within a month, he adds.

Bottom-up measurements are difficult, Peischl says, but combining them with top-down measurements may help solve the puzzle. “You find a leaking valve in an oil field and you measure it, but you have to ask—will this be the same for thousands of other valves in a field, or is it just one valve that someone forgot to tighten?”

Cornell’s Howarth remains convinced of his early argument. In a recent paper, he says that EPA and Allen are “measuring the best possible performance by the industry” by relying on industry data and giving industry control of sampling points. He also stands by his earlier conclusion that natural gas and coal have similar greenhouse gas impacts when methane leakage is included with CO2 from burning methane. He warns that rather than a bridge fuel, natural gas is “a bridge to nowhere” (Energy Sci. Eng. 2014, DOI: 10.1002/ese3.35).

“So much is happening right now and ... the literature is growing so rapidly that we need to understand more about top-down studies before we change estimates,” he said during the webinar. “We need further evaluation. It is too early to make any conclusions in my opinion.”

But Gunning sticks by EPA’s numbers.

“The number 1.2% doesn’t seem like much. But if the U.S. lost this many LNG tankers, the industry and public wouldn’t tolerate it,” he says.

Technologies exist to capture about 40% of lost methane from the oil and gas system, according to analysis by EDF and the Natural Resources Defense Council. The technologies are in use, but it is unclear how many sites are using them.

So far, EPA has been unwilling to directly regulate methane emissions. Under court order, the agency issued regulations in 2012 intended to reduce emissions of volatile organic compounds and hazardous air pollution from gas wells. Although methane wasn’t specified in the rules, the gas will be captured in these efforts as a cobenefit. The regulations affect new wells beginning in 2015 on a phased schedule.

But existing wells are not covered. Because of this, an EDF-funded study by the engineering consulting firm ICF International estimates methane emissions from the oil and gas industry will grow by 4.5% by 2018, with that year’s releases coming primarily from existing wells.

“Existing wells are a big hole in the regulatory structure,” Nelson says.

Meanwhile, natural gas industries and their political allies are continuing to press ahead with proposals to sell U.S. LNG to the rest of the world. In June, the federal government approved the Cameron LNG project in Louisiana, the nation’s second natural gas terminal to get government go-ahead.

The American Petroleum Institute, an oil trade association, called the decision “great news for Louisiana workers and the U.S. economy” and noted that the U.S. has become the “world’s top natural gas producer.” It urged the government to speed up the export approval process.

But the Chesapeake Climate Action Network, an environmental group, is more circumspect after parsing a recent DOE report on LNG exports to Europe. It finds that if the oil and gas system methane leak rate exceeds 1.9%, U.S. natural gas loses its greenhouse gas advantage over European coal. To retain a climate benefit, natural gas leakage must be below 1.4% if shipped to Asia, the group says. The climate action network, which opposes LNG terminal expansion, is urging federal regulators to consider global climate impact when permitting new LNG terminals.

Gunning says EPA has several proposals in the works to reduce methane emissions. The agency is planning for new, tighter greenhouse gas reporting requirements, which may clear up the disagreements about leakage rates and boost confidence in EPA’s figures. The agency will also prepare a series of technical white papers this autumn examining ways to limit methane emissions. If regulations are necessary, EPA intends to propose them by 2016.

On the other hand, the Obama Administration underscores the importance of methane “as a source of clean energy that generates revenues, spurs investment and jobs, improves safety, and leads to cleaner air.”

Arguing about emissions levels is not the point, says EDF’s Nelson. An overall system leak of 1.2% of production is worth between $1.36 billion and $5.15 billion in lost revenue, he says. That amount of methane equals the annual greenhouse gas emissions of 112 million cars and 140 coal-fired power plants. It’s also equivalent to the capacity of 119 LNG tankers.

Methane’s role as either a climate saver or destroyer may be determined by these white papers. If EPA finds leakage to be small, the gas may serve as a clean energy bridge, as Energy Secretary Moniz says. Otherwise natural gas will simply speed the rate of climate change that Earth already faces.

Key numbers provide insight on the impact of two critical greenhouse gases.
Table shows key numbers regarding methane and CO2 concentrations and their impacts.
Key numbers provide insight on the impact of two critical greenhouse gases.
Methane By The Numbers
Global greenhouse gas emissions, as estimated by IPCC, indicate that methane will exceed CO2 in the near term. The temperature increase stemming from those estimated methane and CO2 emissions will be nearly the same during the first 10 years, but CO2 will dominate later on.SOURCE: IPCC
Graph shows effects of methane and carbon dioxide on temperature and warming over time.
Global greenhouse gas emissions, as estimated by IPCC, indicate that methane will exceed CO2 in the near term. The temperature increase stemming from those estimated methane and CO2 emissions will be nearly the same during the first 10 years, but CO2 will dominate later on.SOURCE: IPCC

Most concerns about human-caused climate change have focused on carbon dioxide emissions. But now that the U.S. is exploiting a newfound wealth of oil and natural gas, additional concerns have arisen about methane emissions.

Scientists have spent decades deciphering the complex chemistry and physics of Earth’s atmosphere to get a handle on how greenhouse gas emissions might impact future climate. One way scientists evaluate greenhouse gases is through their global warming potential (GWP). This is a relative scale to compare the ability of individual compounds to trap heat and impact climate over time. GWP is defined as the amount of energy added to Earth’s climate system by 1 kg of a gas relative to 1 kg of CO2. As the reference greenhouse gas, CO2 is given a GWP value of 1.

To determine GWPs, researchers collect gas-phase infrared spectra of compounds to calculate their radiative efficiency. This is a measure of how much and at which wavelengths a compound absorbs energy from Earth’s sun-warmed surface and atmosphere, preventing the energy from escaping into outer space. Scientists couple the radiative efficiency with information on the atmospheric lifetime of the compound to calculate its GWP.

For example, the 20-year GWP of methane is 86. This means that if the same amount of methane and CO2 were released into the atmosphere at the same time, methane with its four relatively floppy C–H bonds would trap 86 times as much heat as CO2 with its two relatively rigid C=O bonds over the following 20 years. GWP values are variable for most compounds because they dissipate in the atmosphere. Given that methane lasts in the atmosphere for only 12.4 years on average, its 100-year GWP drops sharply to 34.

Part of the difficulty in estimating global warming and forecasting climate change is that time dependence. For example, assigning an atmospheric lifetime value for CO2 is tricky because it cycles through terrestrial plants and the oceans and does not follow an exponential decay pattern like other greenhouse gases. The lifetime concept therefore is not as meaningful for CO2, yet scientists know that its impact on climate remains significant even after 1,000 years.

What’s more, the amount of CO2 in the atmosphere is much greater than that of methane, about 402 ppm versus about 1.8 ppm. Although methane has a higher GWP than CO2, the sheer quantity of CO2 and the time it remains in the atmosphere make it the most important greenhouse gas.

But that isn’t the end of the story. Methane’s high GWP means it still plays a supersized role as a greenhouse gas during its short atmospheric lifetime. Global methane emissions from all sources are projected to slightly exceed CO2 emissions during the next 10 years. Unless methane emissions are reduced immediately, according to the Intergovernmental Panel on Climate Change, Earth’s average surface temperature will continue to warm significantly by 2030, whether or not CO2 emissions are reduced. This warming could possibly lead to climate feedbacks that exacerbate global warming, such as releasing methane now trapped in frozen Arctic tundra and elsewhere.

Scientists therefore believe reducing methane emissions in the short term is important to slow the current rate of global warming and enable reductions in CO2 emissions to be effective later on. This time sensitivity is now creating a sense of urgency among scientists and policymakers about the need to control methane emissions.—Steve Ritter


Chemical & Engineering News
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Mervyn K.Vogt (Mon Jul 07 10:58:34 EDT 2014)
Though the article is well written and interesting it seems to ignore the methane elephant in the room. It does not address the question of the melting tundra which threatens to release massive amounts of methane that has been trapped there for millenia. Short sightedness of this sort could lead power giants being the richest corpses in the history of the planet. Greed is not good greed is ghastly. Greed is destructive.
Mervyn K. Vogt

Barbara Warren, RN, MS (Mon Jul 07 13:04:06 EDT 2014)

Excellent reporting and graphics. I hope you will consider a follow-up article on the rest of the methane leaks from processing, transmission and distribution.

Thank you.
Max (Mon Jul 07 17:25:37 EDT 2014)
I don't believe that the greenhouse gas potential is the only metric by which natural gas should be compared to carbon dioxide emissions. Along with comparisons on the release of radioactive material, fracking wastewater, and sulfur/other compounds that are burned concomittantly with any energy source, carbon dioxide is also a major driver of ocean acidification. Lower pH ocean water may end up being a greater problem for environmental well-being than greenhouse gas related climate change.
L. Chris Hager (Mon Jul 07 19:29:41 EDT 2014)
Thanks for bringing information on this important risk to your readership. But are we seeing here THE major threat to the atmosphere, or are two other sources competing in a rush to destroy the atmosphere? These two are non-point-source methane releases from the melting permafrost, and methane from hydrates in the Arctic oceans, where a small effervescence of the gas is a common characteristic of more than a million square miles of ocean. Worse, occasionally the Arctic ocean "burps" a large plume of methane, formerly capped at the sea bottom, directly into the atmosphere, releasing more methane than any single source in the industrial world.
Both the land-based and ocean-based releases in the now-warmer arctic threaten to overwhelm both methane leakage Mr Ritter describes in the fuel cycle as well as the combustion byproduct carbon dioxide we have been watching for several decades. Can he or another researcher compare the risks of these different methane sources, the man-made gasses vs the 'natural' releases under the now warmer Arctic conditions?
Joanne Corey (Mon Jul 07 19:40:30 EDT 2014)
There is also science showing that the EPA has underestimated the amount of methane leakage in other ways. There are several studies showing extensive leakage from distribution lines in cities, such as Boston. http://biology.duke.edu/jackson/ep2013.pdf A recent doctoral dissertation showed leakage from old oil and gas wells in PA, even those that had been plugged prior to abandonment. http://dataspace.princeton.edu/jspui/handle/88435/dsp019s1616326 Methane plumes have been documented in areas away from the wellbore. http://www.damascuscitizensforsustainability.org/wp-content/uploads/2012/08/Leroy2-072512-report-FINAL.pdf It is vital that all fossil fuel related leakage is quantified and that the data is used to enact responsible policy, even if that means ending shale drilling.
Frank G. Chernega (Wed Jul 09 10:09:31 EDT 2014)
Methane leakage has been researched by Cornell professors Dr. Lawrence Cathles, Andrew Hunter, and Larry Brown of the Earth and Atmospheric Sciences Dept. and their conclusion is that it is easily mitigated. In fact the aforementioned professors have debunked the findings of Cornell fracktivists Anthony Ingraffea and Bob Howarth. Here is the Cathles/Brown/Hunter paper which elaborates on methane emissions - http://www.geo.cornell.edu/eas/PeoplePlaces/Faculty/cathles/Natural%20Gas/2012%20Cathles%20et%20al%20Commentary%20on%20Howarth.pdf
Chad Brick (Mon Jul 07 19:41:36 EDT 2014)
"U.S. is awash in cheap natural gas thanks to new drilling technologies, primarily hydraulic fracturing, or fracking."

The average price of natural gas over the last twelve months has been about $3.50 per thousand cubic feet at the wellhead. This exceeds its inflation-adjusted price for 72 of the 92 years during which the EIA has been track such data. By what criteria is natural gas currently cheap, other than arbitrarily comparing it to its mid-aught spike prices, or noting that oil prices have gone up even faster?
Chad Brick (Tue Jul 08 03:48:08 EDT 2014)
It is a myth that natural gas is currently cheap. Here are the historical prices (unfortunately in nominal dollars) going back over ninety years.


If you apply an inflation adjustment to those numbers, you will find that the only times natural gas has been more expensive than the last twelve months are the early eighties and most of the aughts. Over 75% of the time in modern history, it has been cheaper in real terms than today.

Natural gas prices have been rising, albiet erratically, for around fifty years. It only appears "cheap" because other forms have energy have risen in price even faster, and most people have forgotten the low prices that prevailed as recently as 15 years ago.

Craig Dillon (Tue Jul 08 13:23:29 EDT 2014)
1. We need to focus on the standing level of methane in the atmosphere. That means using the 100 averaging technique for measuring the impact of a single methane release is silly. We need to use 100 as the multiplier to equate methane's warming impact compared to CO2.

2. Someone needs to measure and calculate the impact of methane released to the atmosphere from fracked fields. Fracking breaks up the rock strata holding methane. Not all of that gas will be collected. What percentage is not collected? What percentage of the methane not collected finds it's way to the surface and is released to the atmosphere?

3. Huge releases of methane from the Arctic ocean floor have been reported by Prof. Simelitov. Methane clathrate releases will be a game changer, if true. We won't be talking about a slightly warmer planet, with changing agricultural zones. We will be talking about a drastically changed planet, ten to 10 degrees Fahrenheit warmer, with the potential of shutting down the ocean conveyor that would create a global anoxic ocean.

The ocean is big. Simelitov seems to be the only one actually looking for the methane releases. Why is there not a multi-national effort to do an exhaustive observation and analysis of methane clathrate releases in the Arctic?
ray delcolle (Tue Jul 08 20:12:56 EDT 2014)
"Does methane trap heat? Yes. But carbon dioxide has caused three times more warming than methane." http://clmtr.lt/c/Jt50Bbk0cMJ
Barry Messina (Wed Jul 09 05:59:22 EDT 2014)
Here's a growing body of scientific evidence for you: the EPA notes that (http://epa.gov/climatechange/ghgemissions/gases/ch4.html)

"Methane (CH4) emissions in the United States decreased by almost 11% between 1990 and 2012. During this time period, emissions increased from sources associated with agricultural activities, while emissions decreased from sources associated with the exploration and production of natural gas and petroleum products."

According to the EPA 2014 Inventory of U.S. Greenhouse Gas Emissions (http://epa.gov/climatechange/Downloads/ghgemissions/US-GHG-Inventory-2014-Chapter-Executive-Summary.pdf) Table ES-2, Recent Trends in U.S. Greenhouse Gas Emissions and Sinks, the leading source of methane is enteric fermentation . . . that is, methane from cows. This is not the manure treatment process, which is another line in the table. Enteric fermentation replaced natural gas as the leading source of methane in 2009.

Even as the number of wells producing natural gas increased, the amount of methane released into the atmosphere went down.

Is that a clear enough statement for you? The EPA reports that since 2009 cows are producing more methane than any other source in the US.
greener6 (Wed Jul 09 10:55:58 EDT 2014)
CH4 going to 1.8 ppb and then using very limited data to predict the massive climatic events is science from C&E News? So, exactly how many degrees of global warming have we gotten from CH4 emissions since 1850? Since the rate of global warming increase has been about zero for almost 18 years and CH4 and CO2 have increased, shouldn't we be about adjusting the global warming potentials? The "pause" was not predicted by any of the 114 models that have an exceptionally poor track record on prediction, even when a number of erroneous models are averaged. (Is that the theory of two wrongs making a right and the more wrongs included the righter it gets?)

Limited aerial sampling and expanding those data to some universal conclusion is an exceptionally poor war of doing this.

Years ago, I thought C&EN did better science. Quite a disappointment. I can see why I stopped reading it.
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