A Bridge to Nowhere?
Natural gas was supposed to be a bridge fuel, helping us shift away from coal while we waited for clean energy costs to fall to more affordable levels. Articles like this one from Inside Climate News do a really good job of digging into the question of whether or not that’s actually a good idea. Methane leaks are a real problem and undermine a lot of the climate benefits of increased natural gas use. One thing that consistently drives me crazy about articles like this is that they don’t seem to answer the question: how bad do the leaks have to be before the climate benefits are gone?
I think one reason that articles like these don’t answer this question is that it’s much harder than it seems. It would take a real scientist to actually figure this out, and I’m sure at least one already has, but I haven’t seen it. So I tried my hand at calculating the simplest version of the question: how high would the methane leakage rate have to be before switching from coal to gas was no longer beneficial from a climate perspective.
If I’m right, the answer is not encouraging. I’m probably wrong, of course, but I do think I’m in the right ballpark. The answer I came up with is either 1.8% or 5.5%. I know that’s a big margin of error, but it’s actually not my fault. Here’s the math:
Switching from Coal to Gas:
Gas-fired power is less carbon-intensive than coal for two main reasons. First, burning natural gas releases less CO2 per unit of energy than coal does. Second, gas-fired generators tend to create more electricity per unit of energy than coal-fired ones.
Looking at the EIA Annual Energy Outlook for 2020 (Appendix A, where all the good stuff is) I got carbon emissions, fuel consumption, and electricity output for both coal and gas fired power, here’s what I found:
This means that switching 1 billion kWh from coal to gas would reduce carbon emissions by .58 MMT, making gas fired power about 40% as carbon intensive as coal. So far so good.
Now, to replace that 1 billion kWh of coal fired power would require 7.3 BCF of natural gas. Methane accounts for about 95% of the content of natural gas (ethane accounts for most of the rest), and unburned methane is a much more powerful greenhouse gas than CO2. According to the IPCC, over a 100-year time horizon, methane is about 28 times more powerful than CO2, and 84 times more powerful over a 20-year horizon. This is why methane emissions are so important to the climate problem.
Using the 100-year time horizon, the question is how much gas would have to leak to make switching to gas a break-even proposition from a CO2 perspective. At that time frame, releasing 1 BCF of methane has the same climate impact as about 1.5 MMT of CO2. To release the equivalent of the 0.58 MMT of CO2 we saved by switching (accounting for the methane content of gas), we would have to release about 0.4 BCF of unburned natural gas, or about 5.5% of the gas we needed for the switch.
Looking at the 20-year time frame, methane is much more powerful, so the answer is much worse, and a leakage rate of 1.8% would be enough to make the switch carbon neutral.
So the next question is how much gas actually leaks from the system? Fortunately, someone has already taken a good look at that for us. In a 2018 study published in Science Magazine, Alvarez et. al. estimated that in 2015, about 2.2% of gas was lost to leaks in the supply chain (not including leaks in local distribution or oil refining and transportation).
And there’s the bad news. If my calculations are right and if gas heading for generators leaks at the same rates as the overall supply chain, switching to gas is already doing more harm than good over a 20-year time horizon. Since climate is a long-term problem, it’s probably more appropriate to look at the 100-year horizon, in which case we’re only losing about a third of the climate benefit. But then again, gas was supposed to be a near-term bridge to a lower carbon energy system.
Looking at these numbers, it’s not clear where this bridge leads.