To illustrate this Question, I use “official” IPCC data1.
The above graph shows the smoothed year by year contribution of each greenhouse to climate warming. It is unclear how the heating influence is calculated, but one gets a general idea of their prevalence and increase..
Once greenhouse gasses are emitted, their amount in the atmosphere declines geometrically (by a similar percentage every year). The amounts remaining become very small, but never completely disappear. A lifetime for each gas is widely publicized, but almost no one understands what that means — so policies will not reflect reality. The lifetime of a greenhouse is not until it has almost disappeared, nor is it the half-life of the gas (the number of years when half of it disappears). Rather it is when the amount of gas is 37% (roughly) of its original value. (I spent hours digging for a definition, before thankfully finding a young man’s blog. He took six months to find the answer.)2
Global Warming Potential (GWP) is a widely published number that is the long-term effect of a weight of gas emitted. Traditionally this is shown compared to CO2. To calculate the global warming potential of a pulse of gas you add together the amount of gas that exists in each year of the gas’s total life, then multiply that by the gas’s absorbing power3. So take methane, for example, if in the 20 year warming potential is 84, in years 20-40 it may be roughly 42; in years 40-60, 21; in years 60 to 80, 10; and in years 80 to 100, 5. Averaged over the whole 100 year period we get 32, which is not far from the official 28. For methane and other short life gases the 100 year warming potential is a very deceptive measure, because essentially it takes the effects of the first few years and spreads them over 100 years, when after the first few years they have a virtually immeasurable effect.
Almost all publications use the 100-year Global Warming Potential. However, to study what has happened in the past 20 years, or what happened between 1980 and 2000, or what will happen in the next twenty years, we need to use the 20-year Global Warming Potential. If you believe, as I now do, that warming in the short term will accelerate warming in the medium and long term, then it is essential to use the 20 year warming potential (if it were available, a 10 year warming potential).
| In molecules parts per billion molecules in atmos-phere ~2022 / millions of metric tons emitted per year | Lifetime (when about 35% of the gas remains. | Immediate Radiative Efficiency in watts per square meter (equal number of molecules) | Relative weight of a molecule of the gas compared to a molecule of CO2 | Comparative Immediate Radiative Efficiency in watts per square meter by equal weight = immediate warming potential | 20 Year Global Warming Potential (by weight) | 100 Year Global Warming Potential (by weight) | |
| CO2 | 411,000 37,000 | very long 100 – 1000 | 1.37 | 1 | 1 | 1 | 1 |
| Methane | 1893 135 | 12 | 330 – 420 | 2.74 | 120 -156 | 84 | 28 |
| Nitrous Oxide | 336 10 | 114 | 300 | 1 | 300 | 273 | 273 |
| Ozone | 337 | short | 300 | NA | NA | NA | NA |
The IPCC numbers for the immediate radiative efficiency of methane by weight are the lower of the numbers above. The 20 year and 100 year Greenhouse Warming Potentials are the IPCC’s. Published statistics of greenhouse gas emissions are by weight.
I haven’t included the multitude of halogens (chlorine containing) gases in the chart; they often have extremely high warming potentials and very long lives; today they have very low concentrations, and many are banned. It is obvious that if these are emitted in quantity, we will have run-away global warming. For that reason, all countries have taken steps to switch to halogens of lower global warming potentials, and to limit all halogen emissions. One must hope that rogue production will be eliminated, and that the steps to recycle or destroy used refrigeration gases will be observed.
Nitrous Oxide (N2O — laughing gas) will be especially hard to limit, because there are many natural sources and because there is no easy and economical way to reduce nitrogen fertilizer use or limit its growth in a world short of food. An excellent description of the problem is found here: https://www.bbc.com/future/article/20210603-nitrous-oxide-the-worlds-forgotten-greenhouse-gas.
I discuss ozone (O3), methane (CH4)), and carbon dioxide (CO2), in separate questions.
- https://www.epa.gov/ghgemissions/overview-greenhouse-gases ↩︎
- https://climateer.substack.com/p/methane-lifetime. The number e is the base of natural logarithms; 1/e = 36.8 ↩︎
- This laborious calculation can be replaced by the calculus formula for an area under a geometrically declining curve. ↩︎