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Energy Matters/Short lesson on greenhouse gases

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The different “greenhouse gases” can be a source of confusion: carbon dioxide, methane, nitrous oxide, refrigerants (fluorinated gases), sulfur hexafluoride, water vapor… How to budget our concern for each of between them ? What is their relative importance in the strategy to combat atmospheric warming?

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 that all life could prosper. Now students have to learn too many good things. Since the beginning of the industrial revolution, the quantity of these gases has been constantly increasing due to human activity. Chief among the greenhouse gases is carbon dioxide (CO2), but those above are also in the mix.

For each of these gases, four factors must be considered which determine their long-term impact on global warming: concentration; global warming potential; the “lifetime” of the gas in the atmosphere; and the rate at which we emit them.

CO2 currently has a concentration of around 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, about double the pre-industrial level. Nitrous oxide (N2O) is around 320 parts per billion, up 17% from pre-industrial 270 ppb. Fluorinated gases (which were not present before industry) are in much lower 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 generally have much higher values. Methane, for example, has an effective 100-year GWP of around 30. This means that one tonne of methane has the same impact as 30 tonnes of CO2. But since its concentration is only 1.8 ppm, its net impact is equivalent to 54 ppm of CO2. This is called CO2e, or “CO2 equivalent”.

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At this point, a table with some sample comparisons might be helpful.

Gas GWP at M. concentrate CO2 equiv. % total
CO2 1 420ppm 420 73.5%
N20 265 0.330ppm 87 15%
Methane 30 1.8ppm 54 9.5%
HFC-23 12,400 0.000024 ppm 0.3 0.05%
SF6 23,500 0.00001ppm 0.25 0.04%

Fluorinated gases typically have a 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. Although 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 replaced, and it is theoretically possible to remove old refrigeration equipment from the stream of waste (bonuses on discarded refrigerators?). When governments banned certain chlorofluorocarbons in the 1980s, the world watched with relief as the ozone hole in the atmosphere shrunk to a less dangerous size.

Among other greenhouse gases, sulfur hexafluoride wins something of a prize with a GWP of 23,500. We were recently asked with some concern about the dangers of SF6, which is used in high-voltage switches. There are a lot of them around, right? Maybe, but at this point the concentration of SF6 in the atmosphere is 11 parts per trillion, and its global warming impact is only 0.04% of the total.

But although the contribution of these other gases to global warming is relatively small, they have the potential to increase over time and therefore deserve attention. The good thing is that fluorinated gases are relatively easy to remove from the atmosphere, either by keeping them contained or by using alternatives.

CO2, N2O and methane are another story. These 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 that point we will most likely also have to actively remove CO2 from the atmosphere with technologies that are still undeveloped.

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One of the stories going around “Here is the REAL culprit of global warming” is that the biggest problem is water vapor. Yes, water vapor is actually the most abundant greenhouse gas, but it has very little to do with human activity. The water content of the atmosphere is actually determined by warming, not the other way around. Human-induced increases in CO2 concentrations lead to warming, which can then lead to increased atmospheric water vapor and accelerate warming.

The urgency to reduce global warming increases every day. We need to focus precisely on the most important drivers of global warming and how to target them. A basic understanding of the relative importance of the above gases is an important part of this focusing task!

Paul Stancioff, PhD., is professor emeritus of physics at UMF. Cynthia Stancioff rephrases everything he writes. E-mail: [email protected] or [email protected] Previous columns can be found at https://paulandcynthiaenergymatters.blogspot.com/.