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By: David Biello

A drop in U.S. carbon dioxide pollution in recent years stems fromburning natural gas instead of coal. Or does it? Given that the U.S. bid tocombat climate change through actions like the Clean Power Plan relies on moreburning of gas than coal in power plants, that answer is both politically andscientifically important.

Compared with coal, burning natural gas results in roughly half theamount of CO2 per megawatt-hour of electricity. Yet even half the CO2, whenspread over hundreds of power plants,is too much to achieve such goals as aCO2-emission reduction of 80 percent by 2050 or 100 percent by the end of thiscentury, in order to avoid more than 2 degree Celsius of global warming, moreacidic oceans, inexorable sea level rise and extreme weather, among otherunpleasant impacts predicted by scientists. Under the terms of the Clean PowerPlan, the most advanced natural gas burning power plants can still emit 771pounds of CO2 per megawatt-hour of electricity produced. So is natural gas abridge to a cleaner energy future or a slightly longer route to climatecatastrophe?

To answer that question the past may provide a rough guide. At roughlythe same time after the turn of the 21st century the U.S. underwent arecession, an energy transition to more natural gas and a move away fromproducing highly polluting products such as steel. So which of these factorsdeserves the most credit for the accompanying drop in the nation’s globalwarming pollution? It's an important puzzle to solve because recessions are notwidely viewed as a policy option (advocates of “degrowth” notwithstanding)whereas the export of fracking—the process by which natural gas is extractedfrom shale rock—to other countries to help deliver a cleaner fossil fuel habitis.

To disentangle all these competing explanations or at least find outtheir relative importance, scientists have used a mathematical technique knownas structural decomposition analysis. Here's how it works: There is a bignumber to look at, say, total consumption in an economy. This big number can bebroken down into contributing factors, such as population size and consumptionper person. Hold population steady through time as total consumption changesand you derive the change in total consumption caused by a change inconsumption per person. Then hold consumption per person steady while changingpopulation size and you derive how much of total consumption comes from each ofthese factors.

A group of scientists and economists used such a mathematical analysisto look at U.S. CO2 emissions between 1997 and 2013, a period that saw totalpollution drop by nearly 800 million metric tons, or roughly the annualpollution of Germany. The group looked at six different factors: population andconsumption per person, but in addition shifts in consumption patterns; shiftsin industry; the energy intensity of the economy; and changes in the fuel mix.

The study found that prior to 2007 (and, hence, the start of therecession), U.S. CO2 pollution continued to grow, largely because the economycontinued to expand; people bought more and more. After 2007 U.S. CO2 pollutiondropped but roughly 80 percent of that decline was because people and companiesbought and built less stuff, supplemented by the shift away from heavyindustry. This finding matches a slew of previous analyses by the U.S. EnergyInformation Administration, Pacific Northwest National Laboratory and CO2Scorecard, among others, that have also concluded that the 2008 great recessionwas largely responsible for the observed emission reductions. In fact, one warmwinter in 2012 alone played an outsize role in recent CO2 reductions.

Coal's share of electricity generation in the U.S. has been droppingsince 2009 and more than 180 gigawatts of power plants that burn natural gashave been built since 1990. The electricity from a one-gigawatt coal-firedpower plant can be replaced by burning one billion cubic meters of natural gasinstead, resulting in an annual savings of roughly three million metric tons ofCO2 in addition to reductions in other air pollution, like the sulfur dioxidethat causes acid rain or the nitrogen oxides that create smog. There is now 1.5times more potential electricity generation from burning natural gas than fromburning coal in the U.S., and coal-fired power plants representing roughly 7 percentof this country's electricity generation are retiring this year, mostly in theeastern half of the country. An analysis by the National Renewable EnergyLaboratory suggests that natural gas and renewables like wind turbines andsolar panels have picked up the slack produced by missing coal—the beginningsit seems of a long-term energy transition. More simply put, natural gas may bekeeping a lid on growth in CO2 emissions from generating electricity in theU.S. at present.

But natural gas hasn't just killed coal. From Florida to Wisconsin,gas-fired power plants are replacing nuclear ones. That fuel switch actuallyincreases CO2 pollution, however. And, in the absence of mandates likerenewable portfolio standards—mandates for a certain percentage of electricityto derive from renewable resources—natural gas could also prevent the buildingof wind and solar farms or geothermal power plants.

Furthermore, all those power plants that burn natural gas will stillspew CO2, albeit less than the equivalent coal-fired power plant. In a worldaimed at zero emissions, that reduction is not good enough ultimately. In fact,the more than 1,000 gigawatts of natural gas–fired power plants built aroundthe world would spew roughly 300 billion metric tons of CO2 if operated overthe next 50 years—or more than half of the world's remaining carbon budget.Exceeding that budget may lock in the worst of climate change, whether fast sealevel rise or extreme weather. Cheap natural gas may even slow the shift awayfrom heavy industry in the U.S.: New fertilizer plants and chemical plants havealready been built as a result of cheap and abundant natural gas and new steelplants may not be far behind. Finally, natural gas can leak, adding methane tothe atmosphere, which also exacerbates global warming.

In the context of an energy transition that may take decades the U.S.does not have 20 years for natural gas to kill coal and replace oil in powerand transportation, respectively, followed by another 50 years needed to replacenow-entrenched natural gas with renewables and/or nuclear power plants poweringelectric cars and trucks. The U.S. Environmental Protection Agency expectsnatural gas to be producing one-third of U.S. electricity in 2030 andtechnologies that might make natural gas near-zero carbon, like those thatcapture and store CO2, have yet to be tried or even tested on the gaseousfossil fuel.

For all these reasons, natural gas makes for a weak bridge to azero-pollution future and truly clean power—one that cannot span more than afew decades. Still, a bridge made of gas is better than none at all.

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