43. A Short Course in Greenhouse Gases
A Short Course in Greenhouse Gases
The various “greenhouse gases” can be a source of confusion: carbon dioxide, methane, nitrous oxide, refrigerants (f-gases), sulfur hexafluoride, water vapor….How should we budget our concern about each of these? What is their relative importance in the strategy for addressing atmospheric warming?
Back when some of us were young we learned all about the greenhouse effect as a GOOD thing: carbon dioxide (and a few other gases) in the atmosphere insulated the earth so all life could thrive. Now students have to learn about too-much-of-a-good-thing. Since the beginning of the industrial revolution the amount of these gases has steadily increased as a result of human activity. Primary among the greenhouse gases is carbon dioxide (CO2), but those above are also in the mix.
For each of these gases we need to consider four factors that determine their long-term impact on global warming: the concentration; the global warming potential; the “lifetime” of the gas in the atmosphere; and the rate that we are emitting them.
CO2 currently has a concentration of about 420 parts per million (ppm)–up from 280 ppm at the start of the industrial revolution. Methane has a concentration of about 1.8 ppm which is about double the pre-industrial level. Nitrous oxide (N2O) is at about 320 parts per billion, up 17% from a pre-industrial 270 ppb. The f-gases (not present before industry) are in much smaller concentrations usually measured in parts per trillion.
If you think of these gases as a blanket, the concentration is a measure of the thickness of the blanket, and we refer to their Global Warming Potential (GWP) for their “insulating” value. We give CO2 a GWP of 1. Other GHGs typically have much higher values. Methane, for example, has a 100-year effective GWP of about 30. That means 1 ton of methane has the same impact as 30 tons of CO2. But since its concentration is only 1.8 ppm, its net impact is equivalent to 54ppm of CO2. This is referred to as CO2e, or “CO2 equivalent”.
At this point, a table with some sample comparisons might be helpful.
The f-gases typically have much higher GWP than CO2 - like 10,000 times higher - but the concentrations are so low that their combined warming impact is only about 2% that of CO2. Though this number may be small we should not ignore these gases, as they are relatively easy gases to control - There are alternative refrigerants that can be substituted, and it is theoretically possible to remove old refrigeration equipment from the waste stream (bounties on junked refrigerators?). When governments banned certain chlorofluorocarbons in the 1980, the world watched in relief as the ozone hole in the atmosphere shrank back to a less dangerous size.
Of the other GHG gases, sulfur hexafluoride wins some kind of prize with a GWP of 23,500. We were recently asked with some alarm about the dangers of SF6, which is used in high voltage switches. There are lots of those around, right? Perhaps, but at this point SF6 concentration in the atmosphere is 11 parts per trillion, and its warming impact is only 0.04% of the total.
But while the contribution of these other gases to global warming is relatively small, they have the potential to increase over time so they warrant attention. The good thing is that f-gases are relatively easy to keep out of the atmosphere, either by keeping them contained or by using alternatives.
CO2, N2O, and methane are a different story. They are the most difficult but have by far the most impact. Stopping the use of fossil fuels for energy would reduce GHG emissions by more than 80%. It would still take decades for atmospheric GHGs to return to safe levels, and at this point we will most likely also need to actively remove CO2 from the atmosphere with as-yet undeveloped technologies.
One of the circulating “Here’s the REAL global warming culprit” stories is that the biggest problem is water vapor. Yes, water vapor is actually the most abundant greenhouse gas, but this has very little to do with human activity. The water content of the atmosphere is actually driven by warming—not the other way around. Human-induced increases in CO2 concentrations lead to warming, which can then cause atmospheric water vapor to increase and accelerate the warming.
The urgency of reducing global warming is mounting every day. Our focus must be aimed accurately on the most significant factors in global warming and ways to target them. A basic understanding of the relative importance of the above gases is an important part of this focusing task!
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