In the last 18 years, since 2005, the US has decreased our CO2 emissions due to electricity generation by 32%, 819 million metric tons of CO2 per year. Thirty percent of this decline can be attributed to renewable energy generation. But 65% is attributed to essentially replacing coal-fired plants with natural gas (NG) fired plants. The share of coal decreased from 50% to 23% while the share of NG increased from 19% to 38%. Burning coal for energy released about twice as much CO2 as burning NG. Plus, NG power plants are more efficient than coal. The net result is that NG releases about 30% of the CO2 per unit of energy created as does coal.

But – the picture is more complicated than just calculating CO2 release. The implications of a true comparison between these two sources of energy has huge implications for our attempts at reducing climate change. It’s clear that we should phase out all fossil fuels as quickly as possible. But this transition is going to take decades and cost trillions. Meanwhile, we are already skirting close to the line in terms of peak warming and the consequences of that warming. We no longer have the luxury of just developing low carbon technology with the knowledge that it will ultimately replace fossil fuels. The path we take to get to net zero matters. We need to take the path that lowers greenhouse gas (GHG) emissions as quickly as possible. So when we build more wind, solar, nuclear, hydroelectric, and geothermal plants, do we shut down coal, natural gas, or perhaps it doesn’t matter?

If we look just at CO2, it’s a no-brainer – coal is much worse and we should prioritize shutting down coal-fired plants. This may still be the ultimate answer, but there is another factor to consider. NG contains methane, and methane is also a GHG. Per molecule, methane causes 80 times the warming of CO2 over a 20 year period. So any true comparison between coal and NG must also consider methane. (A little methane is also released in coal mining.) But considering methane is extremely complicated, and involves choices when looking at the data that don’t have any clear right or wrong answers.

There are basically two variables that can dramatically affect the comparison – how do we calculate the amount of methane that leaks into the atmosphere from the entire NG industry, and how do we calculate the warming due to methane vs CO2. Let’s take the latter question first. While methane traps more heat than CO2, it survives a much shorter time in the atmosphere. Methane lasts for about 12 years while CO2 lasts for centuries. Once we release CO2 into the atmosphere it’s going to stay in the carbon cycle essentially indefinitely until we actively remove it, which we currently do not have the economically-feasible technology to do. Therefore, in order to compare the amount of heat trapped by these molecules we need to choose a timeframe. Over 20 years, as stated above, methane in 80 times worse than CO2. But over a 100 year timeframe, it is only 28 times worse than CO2. This is still bad, but not nearly as much. Which figure should we use when comparing the two? I guess that depends on the question you are asking.

The other variable concerns how much methane is leaking from pipes throughout the entire NG lifecycle. We have no great way to monitor this, and so estimate vary widely. Which estimates you plug into your equations dramatically affects the outcome. A recent EPA study found that 0.42% of  NG leaks at some point into the atmosphere. That is close to prior estimates, but showed less leakage from well and more from pipes than previously estimated. The amount of leakage relates directly to the measures put in place to minimize those leaks. However, that estimate is at the low end. Other EPA data puts the figure at 1%. A 2022 study using aerial surveys to estimate leakage from NG plants in New Mexico found leakage at 9%.

You can pretty much tell whatever story you want based on the data you choose to use. At the low end of leakage estimates, NG causes about half the warming as coal. At the high end it is as bad or even a little worse than coal. But again – depending on the timeframe you are choosing. Perhaps we should compare coal at its best (with carbon capture) and NG at its best (with tight regulations and monitoring to minimize leaks) – but then, of course, we have to actually mandate the steps necessary to optimize these fuels.

There is another interesting factor to consider. Burning coal also releases sulfur dioxide. This is a terrible pollutant which has negative health effects. But it is also highly reflective – it actually mitigates the warming effect of burning coal a little by reflecting some sunlight back out into space. Do we count the effects of sulfur dioxide also? Shouldn’t we be trying to minimize the release of this harmful particulate matter? Even still, it does impact net warming.

There are other factors to consider as well. Coal is the deadliest form of energy, costing 24.62 deaths per TWh. NG causes 2.82 per TWh. Meanwhile wind is 0.04, nuclear is 0.03, and solar is 0.02.

From one perspective, the answer is simple – we need to reduce our burning of fossil fuels as quickly as possible. This means building up other sources of power and quickly as possible, while electrifying other sectors, such as transportation and industry. But we also need to consider what we will be doing over the next 30 years while we are working toward this goal. It matters. All things considered, I still think we should prioritize shutting down the burning of coal over NG. Meanwhile we need to enforce strict standards in terms of minimizing NG leakage.

The global context here is also sobering. While the US, Canada, Australia, and Europe have made strides in reducing CO2 release, and South America and Africa remain extremely low overall, Asia (especially China and India) have increased their carbon footprints by even more. The net result is that in 2022 the world burned more coal than any previous year, and released more CO2 into the atmosphere than any previous year. China continue to build more new coal-fired plants, choking its own cities. This does not mean the rest of the world should do nothing – we are still better off the more everyone reduces their carbon emissions. We should do everything we can, while putting extreme pressure on China to do the same. Otherwise, debating which is worse, coal or NG, is nothing more than a sideshow.