In the EPA EF reconsideration document there is a section on p62 where they attempt to make the argument that the CO2 endangerment finding would also apply to direct water vapor emissions to the atmosphere, which is (according to them) obviously absurd. But both claims are bogus.
First off, the definition of pollutant in the Clean Air Act (CAA) clearly does include CO2 as well as water vapor. This was the point litigated in Massachusetts v. EPA in 2007:
An air pollutant is defined as any substance, or combination of substances, including physical, chemical, biological, or radioactive matter, that is emitted into or otherwise enters the ambient air and may reasonably be anticipated to cause or contribute to air pollution.
A Hazardous Substance is further defined as one “that can cause or may reasonably be anticipated to cause adverse health or environmental effects“.
So there are two factors to assess. First, is the substance emitted into the air? (Yes, for both CO2 and water vapor). Second, might it be reasonably anticipated to cause adverse effects? (This is precisely the point of the Endangerment Finding process!). Thus it is not self-evidently absurd that water vapor emissions might be regulatable under the CAA, but the issue is whether there is any evidence that these emissions might plausibly have adverse effects.
It’s worth listing some pertinent comparisons between CO2, water vapor and a criteria pollutant like SO2 (which oxidises to SO4), to see the differences:
| Substance | CO2 | SO4/SO2 | Water Vapor (H2O) |
| Perturbation timescale(s) | > 1,000 years | ~ 2 weeks | ~ 10 days |
| Increase over background since 19th Century (%) | > 50% | ~350% (Greenland, 1980) | ~ 4% (since 1979) ~ 9% (estimate since 1900) |
| Anthropogenic direct emissions | ~ 36 GtCO2/yr | ~ 130 MtSO2/yr (1980) | ~ 21 GtH2O/yr |
| Anthropogenic sources | Fossil fuel combustion, deforestation | Sulfur in coal, biomass burning | Irrigation, combustion |
| Attribution of anthropogenic direct sources to atmospheric increase | ~ 90% | 100% | ~4% |
| Impact of climate feedbacks | ~ 10% (ocean/soils etc.) | 0 % | ~ 96% (impact of T on saturation vapor pressure) |
| Adverse effects of increase | Increased heat waves, sea level rise, etc. | Acid rain, public health, agricultural yield | More intense rainfall, enhanced global warming |
Basically, direct emission of water vapor has only a minimal impact on the increasing water vapor burden (which is the opposite of the case for CO2 and SO2/SO4) (less than 1% of the WV increase of ~9% is plausibly related to direct emissions). Indeed, it’s clear that the vast majority of the increase is due to climate change itself (warming the atmosphere and increasing the saturation vapor pressure). Thus even if direct WV emissions were reduced to zero, that would be counteracted by a mere 0.05ºC change in global temperature.
We are not environmental lawyers, so the criteria are likely more subtle than presented, but the case for direct water vapor emissions being a hazard is very weak. If one was interested in reducing the harms associated with increasing water vapor in the atmosphere, one would obviously start with CO2 and the other greenhouse gases that are driving up the temperatures, not with the almost negligible effect of direct WV emissions. However, it is not absurd that one might consider this, but in practice it is not a very sensible idea.
The bottom line however is that the impact of direct WV emissions are totally immaterial to whether CO2 emissions are a hazard and so this discussion in the EPA proposed rule is merely distraction.
I believe that both the 2009 finding and the later aircraft finding discuss water vapor, as do the responses to comments in those documents. Pulling language directly from EPA’s own records may have additional weight in the courtroom
My high-level understanding is that, to first order, the concentration of water vapor is a function of temperature – that is why WV is said to be a feedback and not a climate forcing. Thus the only way we realistically *could* regulate WV as a GHG is via existing regulations on other climate-warming GHGs like CO2 that do act as forcing. Is that accurate?