So, this has been bothering me for a while now. One of the arguments for fracking–an environmentally destructive method of extracting natural gas–is that it is a relatively clean fuel with a lower carbon footprint than coal or petroleum products. How could any fossil fuel have a lower carbon footprint? Isn’t the chemistry of burning carbon compounds basically the same everywhere?
Well, it turns out that no, the chemistry isn’t always the same. Natural gas is relatively clean, with a lower carbon footprint. There are some complications, of course.
Natural gas is so called in contrast to manufactured gas, which was typically made from coal. Manufactured gas was very popular through the 1800’s and early 1900’s. The gas lights of gas-lit London (and elsewhere) burned manufactured gas, as did the household ovens that were so tragically easy to use in suicide–manufactured gas is very toxic. The manufacturing process for gas was an environmental nightmare.
Natural gas, in contrast, exists in gaseous form naturally, hence the name. It burns relatively clean. A natural gas oven cannot kill a person (unless the gas explodes), so the switchover actually caused a dramatic drop in suicide rates–apparently, making self-injury just a little harder gives a lot of people enough time to change their minds..
Natural gas is mostly methane, which really does release less carbon dioxide per unite of heat released than any of the other fossil fuels.
This is because the energy released from burning any substance comes from all of the chemical reactions that occur during the fire. All fossil fuels are hydrocarbons–chemical compounds made mostly out of carbon and hydrogen. When these burn, the carbon combines with oxygen, releasing energy and creating carbon dioxide. But the hydrogen also combines with oxygen, a second chemical reaction–this one releases heat, too, but creates only water.
Because methane has the least number of carbon atoms per hydrogen atoms of any fossil fuel, burning it creates the least amount of carbon dioxide per unit of heat–but by the same principle, burning methane produces more water.
Water vapor is also a greenhouse gas.
What About Water Vapor?
Water vapor is our most important greenhouse gas. Most of it is natural; humans didn’t create the greenhouse effect, we’re just adding to it.
But human activity is adding water vapor to the sky. Besides the chemical production of water through burning fossil fuel, irrigation and other industry exposes more water to evaporation (and transpiration by plants). And, the warmer the planet gets, the more water the atmosphere can hold and so the more water is sucked up into the sky. This is part of how heat waves make droughts worse.
So, what is all this extra water vapor doing to the climate?
It’s hard to tell for sure, because there is a lot scientists still don’t know about the hydrological cycle–including how much water vapor, exactly, is in the sky. Humidity is very variable, so, depending on where and when you measure, the atmospheric concentration of water vapor could be anywhere from zero to 4%.
Climate scientists do know the feedback loop between hotter weather and increased evaporation is very serious. The more water evaporates, the hotter the planet gets, and the hotter the planet gets, the more water evaporates. This is just one of the several feedback loops that could easily make global warming become a frighteningly self-exacerbating problem.
But the extra water vapor we add directly (through irrigation, and so forth) it more confusing. Climate scientists typically ignore this extra humidity, in part because it isn’t clear that it has a global impact. A huge amount of water goes up into the sky–the entire flow of the Colorado River and most of the Aral Sea, for example, both are sucked up by human activity and almost all of that water either evaporates or is transpired. But at least some of that water probably falls back down again pretty quickly, so the total amount of water vapor in the air might not increase all that much. Still, at least some of that water vapor probably stays up there for a while, plus both groundwater mining (pumping well water out faster than it can recharge) and fossil fuel use add water to the cycle that wasn’t in it at all before. It seems plausible that there is at least as much extra water vapor in the sky as extra carbon dioxide. That must be having some effect.
Anthropogentic (human-caused) water vapor could be one of the things science is wrong to ignore. But on the other hand, there is a lot more water vapor than carbon dioxide up there. The concentration of CO2 has gone up by about 100 parts per million (PPM) since the Industrial Revolution, meaning that just over a third of what’s in the atmosphere is our doing. In contrast, if the concentration of water vapor has also gone up by about 100 PPM as a direct result of human activity (that is, not counting the feedback loop), then only about one ten-thousanth of the water vapor up there is our doing. The extra might well be lost in the shuffle.
The above figure assumes that the global concentration of water vapor is 1% which, as noted earlier, might well be wrong–the true figure could be anywhere from 4% to zero, but given how much of the planet is either ocean or humid landscape, 1% seems plausible. The point is that, whatever the real numbers are, we’re looking at a difference of several orders of magnitude between the concentrations of the two gases. Of course, while an extra 1oo ppm of water vapor might mean nothing over, say, a rainforest, over a desert where natural humidity approaches zero, the difference might be quite real. So, whether anthropogenic water vapor matters might therefore be a very complex question, depending on where the increase occurs and what happens to the global climate if certain areas warm disproportionately.
The reason I bring all this up is that while coal is a very dirty, destructive fuel on almost any conceivable level, burning it produces no water vapor at all. When methane (natural gas) burns, for every one molecule of carbon dioxide produced, we get two molecules of water.
Bringing It All Together
On balance, I’d say that burning methane is better for the sky than coal is, and may be better than gasoline and other petroleum products. Much of our natural gas now comes from hydraulic fracturing, or fracking, which is an environmental horror show (flammable well-water, increasing earthquakes), but so is coal mining (mountain-top removal) and petroleum (oil spills, groundwater contamination). The EPA’s new rules for CO2 emissions will probably encourage the natural gas industry at the expense of the coal industry, and that’s ok.
But the issue with water vapor is only one place where the environmental impact of natural gas might be more complex than it sounds. Clearly, the stuff is no panacea.
Ultimately, we’re going to have to get off fossil fuel entirely, and that is where our efforts need to go–towards renewable energy sources and energy conservation. Anything else is probably a distraction, although any step that lowers our carbon emissions is an improvement and needs support.