Climate Letter #2051

“Atmospheric rivers”are suddenly making headlines in the daily news, thanks to a particularly colossal event happening on the US west coast. We can look forward to some interesting coverage all week as reporters begin pressing top scientists for more complete explanations. I spent some time this weekend making sure I was up to date on what was being said in the latest studies published in the leading science journals. There was not one word of mention about the possibility of any linkage to water vapor’s greenhouse effect, which I fully expected, but otherwise there were some pleasant surprises, including interesting answers to a few questions I could not hope to answer with the limited kind of research capabilities I have. Today’s letter will dig into three of these studies. One thing I look for is how scientists are now defining the basic meaning of the term,” atmospheric rivers.” How does it compare with the meaning of the phrase I have always depended on, namely, “high-altitude concentrated streams of precipitable water (PW)?” Is their current focus on just the biggest events, or does it include the lesser ones as well? I think the trend now may be to include everything, which means I should be able to start using the same term in a more confident way, and will be happy to do so.

As for new information, I have always felt frustrated by having no ability to gauge the total amount of water vapor able to enter the upper level wind system of each hemisphere on any given time frame, or as a general trend.  There are a number of climate models at work looking for exactly the same information, because it would help the sponsors make estimates of how much precipitation to expect along the course of each river before it came to an end.  The models seem to support the idea that significant increases can be expected in the future, and that these increases would not necessarily be tightly regulated by the same laws or rules that govern the rates of increase of water vapor inhabiting the air closer to the surface.  That’s an unexpected surprise, if it holds up by way of further confirmation.  From my point of view, if the total weight of water vapor (or PW) in the upper level of the atmosphere increases so will its power of greenhouse energy generation, unit for unit.  And remember, once the effect is in place and working, at that high level, and the rivers are in motion, their impact on surface temperatures will tend to increase, through logarithmic leverage, if and whenever the PW material is transported closer to either pole, which means to any higher latitude.  Now I will show links to these three studies and point out some highlights, using italics and some extra commentary.

1.  From the journal of the American Meteorological Society, Sept. 2021 https://journals.ametsoc.org/view/journals/clim/aop/JCLI-D-20-1005.1/JCLI-D-20-1005.1.xml:  “Atmospheric rivers (ARs), narrow intense moisture transport, account for much of the poleward moisture transport in midlatitudes…..Under climate warming, the idealized model produces robust AR changes similar to CESM large ensemble simulations under RCP8.5, including AR size expansion, intensified landfall moisture transport, and an increased AR frequency…..In addition, the latitude of AR frequency maximum shifts poleward with climate warming…..AR changes in a warming climate can be understood as passive water vapor and cloud tracers regulated by large-scale atmospheric circulation….”  (Re the last point, I would have added the observation that jetstream wind strength and positioning has a significant influence on many aspects governing the pattern of circulation.)

2.  From the journal Nature, Oct. 2021, which has extremely limited access but still offers some key points https://www.nature.com/articles/d41586-021-02681-6:  “Climate change hasn’t yet had a noticeable influence on atmospheric rivers, narrow air streams that carry huge amounts of moisture towards the poles. But these ‘rivers in the sky’, which bring downpours to mid-latitude coastal regions, could become markedly more extreme if atmospheric greenhouse-gas concentrations continue to rise steeply.”  (Note the reference to causation, which these authors ascribe to warming created by many greenhouse gases rather than just CO2 alone.  I would call that a much-needed break from the normal orthodoxy that students have always been taught and endlessly repeat.)

3.  From the journal Nature Climate Change, Oct. 2021 https://www.nature.com/articles/s41558-021-01166-8:  “Atmospheric rivers (ARs) are filamentary conduits of intense water vapour transport in the extratropics, accounting for the majority of poleward moisture transport in the mid-latitudes…..little to no change in mean AR characteristics in 1920–2005 due to opposite but equal influences from industrial aerosols, which weaken ARs, and greenhouse gases (GHGs), which strengthen them…..simulations project steep intensification of ARs in the coming decades…..as the influence of GHGs greatly outpaces that of industrial aerosols….. future AR changes are dynamically and thermodynamically driven, highlighting the need to conceptualize AR change beyond the scaling of humidity with warming.  (Again, many greenhouse gases, not just CO2, are said to warm the surfaces that provide the evaporation of water vapor specifically headed for the upper level wind system.  The last point suggests that the condensation rules employed by the Clausius/Clayperon equation perhaps do not work as well in the upper level as they do close to the surface—another highly unorthodox view that I can easily endorse.)

The unorthodox views embodied in these last two reports, both from a publisher that prefers authors having top-level credentials, are by themselves enough to effectively decouple the traditional bonding of all of the (supposed) strength of water vapor to that of CO2. The powers of CO2 should instead be treated independently, exactly like those of methane and the other well-mixed gases. They all make a contribution to the water vapor feedback due to increased evaporation. Water vapor could then be given its own rightful place in the tables of radiative forcing, with a separate set of error bars. The greenhouse effect of water vapor separated into the content of atmospheric rivers has yet to recognized. At least we know it’s up there, and we have been told here that it may be different from surface humidity in some type of scaling. How so? In my view this body of water vapor cannot avoid having a greenhouse effect, and maybe some day a few intrepid scientists will make the same discovery. Upon doing so, they will also see how unpredictable this effect is, apart from a near-certainty that it will be adding more warming to current forecasts.

Carl