Carl's Climate Letters

“The greenhouse energy effect of atmospheric rivers”

This is a subject I will be writing about for awhile, and not so much about the greenhouse effect of precipitable water (PW). This is a conceptual upgrade, one that I am excited about. Atmospheric rivers (ARs) are better defined than PW, yet still not completely defined in a normal scientific sense. There is no widespread agreement on exactly what constitutes an AR, and no thorough investigation of its properties. A few years ago most references to ARs placed emphasis on massive size. Nothing less than an AR could produce massive amounts of rainfall, usually requiring the river to travel across a large stretch of open ocean before making landfall and disgorging its contents. Anything else that traveled high in the atmosphere and produced ordinary rainfall, or any other kind of precipitation, had to be distinguished by different and less colorful terminology. I think AR is steadily becoming more inclusive of high-altitude streams of water vapor, plus condensation products, of all shapes and sizes. The next step will be the full development of a comprehensive set of properties that characterize these streams. It will be hard to keep ignoring the evidence that they have a greenhouse energy effect, which happens to be quite substantial

The term AR should end up representing something very special in the natural world. Its contents are special, likewise its behavior high in the atmosphere, and so also its properties. It begins life with water vapor going up, and comes to an end as precipitation coming back down. It never does stop moving. Each river is a separate and distinct unit, a sort of society that could have its own name. New members keep being added for a time while others are dying off, and finally the whole thing disbands. As long as the river endures its members are packed together in close association, from which a unique set of properties can emerge. The greenhouse effect that emerges is definitely unique in comparison with all other greenhouse effects that are recognized by science, but that is not a reason for ignoring its reality.

Every AR contains some amount of water vapor, a regular member of the greenhouse gas family. The logical implication is that every AR is capable of having a greenhouse effect to some extent, even if it is only rudimentary. This simple fact can serve as a good starting point for investigation that may lead o an understanding of greater possibilities. The possibility that components of a river other than water vapor may also have a greenhouse effect is not something to be ignored by an investigator who has an open mind. There are tools available for making measurements that provide inescapable evidence. I have displayed and demonstrated these tools many times in these letters when making reference to the PW that ARs are made of. The same tools work just as well when applied directly to ARs. ARs and PW are not always the same thing, but they are so in the case of PW that has been created by an AR and exists at a high altitude as part of an existing AR.

Carl

I’ve found a new and better way to describe the greenhouse energy effects of precipitable water (PW). PW is a term that has little cognizance and stimulates little interest. You seldom hear it used by the media and scientists do not spend much time talking about it among themselves, or making it the subject of a journal study. There is no quick and easy way to define the meaning, because the wording is far-fetched to begin with. Most of what we call PW is not water, as such, and much of it does not even precipitate according to the common usage of that term. There is no consistency of PW’s components except that all of them are based on H2O in one state or another.

Yes, PW does have a greenhouse effect all its own, one that is large and easily demonstrated, but science has not become interested in investigating its importance. So maybe it’s time for me to switch gears, and right now should be a perfect time to do so. Instead of PW, I’m going to start writing about “the greenhouse effect of atmospheric rivers (ARs).” This term has recently gained new meaning, and new life. It has begun to appear more and more in the daily media, where it is used to describe sources of precipitation that are actually happening and have real public interest. Scientists who are weather-oriented are writing up studies for journal publication and so so are some scientists who are climate-oriented. An AR is an individual “thing,” and each one has a life of its own, temporarily occupying its own space and time. There are signs of growing population of ARs, as well as trends of increasing size plus tendencies to change locations, affecting where they unload their precipitation. These are matters that have real importance in climate science because of the way precipitation is deeply integrated with other climate features.

I think of ARs as especially exciting for reasons of a different kind. One is because they are composed entirely of PW, a changeable mixture of H2O molecules, with no exception. Next, ARs are relatively large in size, making the amount of PW they contain a significantly large portion of all the PW held throughout the atmosphere. Third, location. ARs are exclusively located at high elevations in the the atmosphere, and specifically appear over total portions of each hemisphere outside of the tropical belt. I had already found that varying amounts of PW existing in these very same global locations have a significant temporary effect on surface temperatures below, depending on movement into positions that add to the greenhouse effect of ambient PW that exists close to the surface.

It now seems likely that were it not for the outreach of ARs there would be almost no PW, or plain water vapor alone, in these high altitude locations. By way of comparison, the PW content of ARs has extraordinarily high concentrations by any standard, imparting considerable leverage to the total greenhouse impact upon addition to the amounts below. Huge temperature anomalies, recorded daily, are a common result.

The highly concentrated PW in an AR is what establishes the potential for intense rainfall over continental interiors. The intermittency of intense rainfall is easily explained by the separation of individual ARs, their erratic courses of travel, and the inconsistency of condensation patterns. The greenhouse energy production of the PW content of an AR exhibits comparable behavior but without so much inconsistency—production is constantly tied to whatever amount of content is in place. From the point of view of any surface the greenhouse effect is intermittent mainly because of the separate and erratic flowing motion of all ARs, which have no such thing as a fixed riverbed.

I am confident that climate scientists will one day recognize a need to understand the special characteristics and impact of the greenhouse energy effects that are everyday properties of an AR’s PW content, alongside its precipitation effects, and of at least equal importance.

Carl

While looking at the weather maps yesterday I found an opportunity to measure the temperature effect of heavy rainfall in an extremely hot region during the peak of summer, with the sun at a high angle. Central Australia meets those qualifying solar conditions very well right now’ It is also experiencing a perfect setup of unusual differences in weather conditions in regions that are close to each other and would normally be very similar in temperature and other weather categories. One result of the current differences is clearly indicated on the temperature anomaly map:

With the help of magnification, I am seeing a lowest reading of 11C on the cold side and another 11C on the warm side, for a total difference of 22C, or 40F, for regions that are normally about the same. Let’s see how this looks on the temperature map:

Everything checks out well, with an average reading of +18C in the spot of the coldest anomaly and +42C in the warmest spot, for a difference of 24C. On a normal day at this time of year they should both have an average within a degree or two of 30C if all else were equal in the way of weather-making conditions—which is clearly not the case on this occasion. We’ll first check to see if there are enough differences in overhead precipitable water (PW) content to have that kind of effect on temperature comparisons tied to variations in greenhouse energy production:

Something is not right. When the warmest and coldest temperature locations are lined up just right with the PW values at the same locations I get a PW reading of 47kg on the cold side and 27kg on the warm side—completely the reverse of normal expectations. Normally, a 70% higher PW reading would be a factor that adds around 8 degrees in relative temperatures, but here we are apparently subtracting 24 degrees. Is this a graphic mistake, or is something unusual going on that has an effect even more powerful than PW content?  We need to look for clues in the clouds and precipitation map:

There it is, an area of heavy rainfall that almost exactly matches up with the area covered by the cold anomaly in the map at the top. The warm anomaly area, on the other hand, does show cloud cover, presumably light, but not a bit of rainfall. The cloud cover alone does not have a cooling effect large enough to prevent the warm anomaly that goes with the observed relatively high PW reading over a normally dry desert region. And yet, a PW reading that is even higher still is accompanied by temperatures that have turned much, much colder. How does this make sense?

I have no complete description of this day was like in a remote part of Australia, but I do have personal memories of midsummer days in the US when the skies turned pitch black and rain came down in torrents for an unreasonably long time. This combination was capable of quickly producing temperatures of around 60F when normal would have been in the 90s. The major decline in solar energy reaching the surface must itself be great enough to cause a large part of the swing in temperatures. In addition, I can how the continuing stream of thick and heavy raindrops should cool the air at the surface even more, assuming that those drops were created at elevations of normally cold air to begin with and never had much of a chance to warm up.  The prime requirement for a phenomenon like this is an atmospheric river that is heavily loaded with liquid or icy states of PW. It still has a greenhouse effect, but on balance the albedo effect plus ice-cold rain win out by a wide margin. We don’t see anything like it in winter that I can remember.

Carl

One of my favorite science writers is Bob Berwyn, whose posts appear every few days in the Inside Climate News website (which also has many other good reporters besides Bob.)  Berwyn is special because of his uncanny knack for emphasizing scientific points that are commonly overlooked by other writers, and even by scientists themselves.  In today’s post he makes a point about global temperature averages that I think is quite original, in the sense that no one else has written about it using the same kind of terms.  2021 was in many ways a rather cool year, but it ended up as the sixth warmest on record. The Southern Hemisphere as a whole had a few bad heat waves but otherwise reported minimal gains most os the time compared with the averages from three decades back. A medium-scale La Nina event has had an important cooling effect on ocean surfaces in the SH and done nothing to bring up averages in the NH.  So what did bring the total global average up so much? 

As Berwyn points out, with an assist from climate scientist Jonathan Overpeck, it was largely the effect of a large number of specific and unusually powerful heat wave extreme events that occurred, most of which were over continental lands in the north. His article, entitled “Last Year’s Overall Climate Was Shaped by Warming-Driven Heat Extremes Around the Globe,” is found at this link:  https://insideclimatenews.org/news/14012022/heat-extremes-2021/. Here are a few salient quotes:  “Earth’s annual average temperature checkup can mask a lot of the details of the climate record over the previous year, and 2021 showed that deadly heat-related climate extremes happen, even if it’s not a record-warm year……Last year, the climate was metaphorically shouting to us to stop the warming, because if we don’t, the warming-related climate and weather extremes will just get worse and worse…..July 2021 ended up being the single hottest month for Earth since measurements started….. 1.8 billion people in 25 countries—about a quarter of the world’s population—experienced a record-warm annual average in 2021.” (July was especially troubled by a series of, major heat waves.)

James Hansen has also just published his review of global temperatures for 2021, which contains a number of other points of interest plus a prediction about 2023:  https://mailchi.mp/caa/global-temperature-in-2021.  Hansen reminds us again of the extraordinary theme that was quietly announced in a monthly report last summer, “Accelerated warming of the past seven years requires an explanation.  The big jump above the trend line (Fig. 1) is not caused by the ocean exhaling heat. On the contrary, ocean heat content and Earth’s energy imbalance increased markedly. As discussed in July Temperature Update: Faustian Payment Comes Due[5] last August, that accelerated warming seems to be caused by a decrease of human-made aerosols.”  Hansen goes on to say he , based on does not think greenhouse gas concentrations have increased enough to cause such a spike in warming over the last seven years.

I think Berwyn and Hansen both make valid points, but there is one more factor involved that both of them have overlooked—the greenhouse energy effect of precipitable water (PW). This effect has been enhanced in recent years, such that it adds relatively more and more to the total amount of energy absorbed by the surface. It acts independently as an accelerator, based on the accelerated activity of atmospheric rivers (ARs) in the upper part of the atmosphere. Its effectiveness increases, for one thing, because PW is what ARs are made of, and AR volumes are increasing. PW has the same effect as water vapor, but fewer limitations in effective amounts of concentration when the upper atmosphere is employed as a conduit. Poleward extensions of the distribution of these rivers adds to the effectiveness of a given concentration of PW. When high level concentrations are created in unfamiliar quantities over regions in the mid to upper latitudes they produce the amount of extra greenhouse energy required for the formation of heat waves below. They also contribute to the amount of energy that ends up being stored in the ocean depths.

Carl

Results of a major study covering the interaction between rising CO2 levels and tropical rain forest productivity have been published.  The study was conducted over a period of 21 years, with a good many significant variables taken into account.  The main objective was to learn more about the potential loss of tropical biomass due to temperature increases and whether or not the steady rise in CO2 levels was beneficial to its overall rate of growth.  Evidence was collected establishing a new level of accuracy for answering both of these questions.  The study was published by AGU, the journal of the American Geophysical Union.  The best explanation of its purpose, conduct and results are provided by a science writer for Eos, a public relations outlet for the AGU, at this link:  https://eos.org/research-spotlights/drop-in-rain-forest-productivity-could-speed-future-climate-change.

In summary, “The new research provides further evidence that as nighttime temperatures continue rising and as more daytime hours exceed the optimum temperature for photosynthesis, productivity will decline. The authors warn that tropical forests could soon enter into a positive feedback loop that accelerates both global warming and tropical forest decline. As forests become less productive because of rising temperatures, they will soak up less carbon dioxide, which in turn will lead to more warming. This cycle could pose a major threat to the survival of these highly biodiverse ecosystems.” 

The study itself has open access and is clearly expressed, at this link: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021JG006557.  Quoting from the Abstract, “Together these findings raise the prospect of declining tropical-forest productivity as global warming intensifies. This would in turn speed up the rate of increase of CO₂ in our atmosphere, a positive feedback to warming.”  In the Discussion we read, “These findings from a high-fertility tropical rainforest are reason to strenuously question model projections of sustained productivity increases for tropical forests…..our findings argue for refining the climatic metrics used in the global models.”

Information like this is valuable except for its lack of any hard data that would be applicable on a broader scale. Timing is also an issue that is not well resolved. About all that climate models can do to “refine the climatic metrics” of this knowledge in its current state is to eliminate any assumptions of a contrary nature that are now being incorporated as faulty estimates. These are possibly not very large compared with potentially real but unknowable consequences over time. Similarly, the IPCC can do very little with the information other than to issue a general warning that its carbon budget aimed at maintaining the 1.5C target possibly needs to be tightened up, but can’t say how much. I have seen a number of other studies from time to time that fall into this same category. They may or may not be followed up with further research of similar quality that can add viable new metrics to the search for proper conclusions.

Carl

It’s midwinter at the North Pole and midsummer in the south. I want to save a record of how they compare on the weather maps, hoping to do so again in July when the seasons have reversed. The shape of the upper level air pressure configuration is the key determinant of all activity in the upper atmosphere, with an endless number of potential options, so that is the image we’ll start with. The south comes first because everything about it is nice and orderly:

Notice the compact shape of the blue zone and the tight fit of the green zone around it. This enables the formation of a single, strong jetstream wind tracking most portions of the closely aligned outer borders of the two zones:

A number of atmospheric rivers (ARs) are roaming around in the upper atmosphere, all of them having a common propensity to flow straight into the polar zone along with the precipitable water (PW) they are made of. They have a problem doing so because of the strong jet streams that are effectively blocking their way, so only a few fragments actually break through. Most of the polar zone atmosphere therefore stays relatively dry:

As long as the polar zone air stays dry most of the surface will stay cold. Everything within the light blue shading is below freezing, even though midsummer is the peak of the heating season. The cold surface helps to sustain the current shape of the upper level air pressure gradients, which may be able to stay in place about like they are now for the rest of the summer, and then start to expand outward next winter.

In the north we have a very different story. The blue and green zones of air pressure gradients are large in total area, but far from compact:

As a result the jet winds that follow the borders of each of these zones tend to be irregular in positioning and inconsistent in strength:

ARs, for the most part, are currently not very successful in their efforts to break through this complex pattern of jet winds, with one notable exception. The strongest jet stream of all, moving from south to north on the western side of the Atlantic, is not in the kind of position that normally will block AR movement. Instead, this stream is literally transporting a large AR over a very long distance, from its warm water origin as far as Scandinavia, where the combination breaks down and sends some of its remnants over the Svalbard region:

The track followed by this combination is vividly expressed on a map showing a long and unbroken stream of rainfall:

Temperatures have stayed below freezing for practically all land surfaces contained within the gradients marked off by the borders of the air-pressure green zone:

Carl

Why are so many expert climate scientists deeply worried about “catastrophic” results from current trends of climate change?  You need to have read yesterday’s letter about the results of a survey of opinions of 92 individuals who were qualified authors of the latest IPCC study.  A 60% majority of this group do not expect to see the kind of political response that is needed to stay within the IPCC’s climate change target of either 1.5C or 2.0C.  What these folks actually expect is the realization of temperature increases of 3C or more by the end of this century, which many of them describe in terms that are truly catastrophic. These are all well-trained scientists who collectively have the time to review and discuss most of the serious work being done by climate researchers everywhere, whether or not the studies are made use of when IPCC recommendations are finalized.  What do they learn that gives them such a pessimistic outlook, from reading studies that most of us have less reason to be familiar with?

I suspect that the best answer to that question would make reference to paleoclimatology, the study of Earth’s climate history in the deep past, going back many millions of years. There is a great deal of active interest in the subject, and much has been learned that is reasonably convincing, but not quite convincing enough to stand up to high standards of proof set at the topmost level of IPCC editors. Past results of paleo historians that were reported are commonly being replaced by new findings, and everything is fair game. That’s how the process works, and there is no alternative, but the study does have overall respectability as well as a considerable amount of fascination that most scientists should be intrigued by. So what is currently in their sights that all curious folks may want to know more about?

There was a study published three years ago in the PNAS journal that brought the paleo enterprise up to date and has attracted much attention—with at least 180 citations so far—entitled, “Pliocene and Eocene provide best analogs for near-future climates.”  It has open access and is mostly quite understandable: https://www.pnas.org/content/115/52/13288.  If you read the first few paragraphs, and study the temperature chart that soon follows you will want to keep going, to learn whatever is said about the results of the project—at the current state of extraordinarily rapid change, how close are we to replicating one or more of the milestones of deep climate history?  “We compare climates of the coming decades with climates drawn from six geological and historical periods spanning the past 50 My. Our study suggests that climates like those of the Pliocene will prevail as soon as 2030 CE and persist under climate stabilization scenarios.”  Without mitigation, we could rewind the climate clock by approximately 50 million years in just the next two centuries.

For those who prefer reading a brief introduction to the study, Science Daily provided a good review including direct commentaries from the two lead authors:  https://www.sciencedaily.com/releases/2018/12/181210150614.htm.  The Semantic Scholar report is a wonderful resource that provides us with links to 157 recent studies, many of them from the past year, that have cited this work while adding the proprietary results of yet more recent research.  Complete abstracts are included as add-ons to each link: https://www.semanticscholar.org/paper/Pliocene-and-Eocene-provide-best-analogs-for-Burke-Williams/bf145343564f93e06a5270dbaa3ad6532c788123.  The same service with abstracts added is also provided for each of the 67 references used by the study.

Carl

What is the likelihood that the global average temperature increase—relative to pre-industrial—will remain less than1.5C? This is arguably the most important of all questions relevant to the future direction of climate change. There are a number of different ways to answer it, as well as a number of different approaches to each of the different ways, which leads to confusion if you choose to go that route. Most public discussion makes an effort to keep things more simple by sticking to the most literal definition of global average temperatures without reference to regional differences that keep getting wider and wider. (See CL#2098, Dec. 30.) Every temperature reading, regardless of location, is treated equally. This seems quite logical as an approach, except that in reality climate change does not work that way. Under the same set of changing conditions, some places warm up rapidly, some more slowly, and some not at, or even go in reverse. That is the reality we have to deal with, and it is generally being avoided in the public discourse. The global number is still interesting and useful for many purposes.

There is another problem with the basic question, at the literal level. We are now at +1.1C, and we want to keep this number below 1.5C, knowing that some amount of increase is unavoidable. The laws of nature need to be taken into account and human behavior must also be taken into account. Both of these have a great deal of potential influence and both are subject to debate, considering numerous questions about each that are difficult if not impossible to resolve. Who can we look to for the best answers? People who are trained in the physical sciences are in the best position concerning the laws of nature. They don’t all agree, but a clear majority of climate scientists who are the most outspoken on the subject keep telling us it is possible, although less than 100% certain, that we can remain under 1.5 if the public follows certain recommended practices and procedures. These optimists are often emboldened by specialists who offer additional hopefulness based on promises of forthcoming technological advancement.

And then there is the public. Which individuals have the best vantage point for predicting what the “public” will do, globally speaking, given its various proclivities and states of awareness, along with its dependence on political operatives of all different stripes who usually agendas of their own to pursue? Astute public-interest observers can be found in all walks of life, many of whom are well-trained in a wide variety of sciences. Their deliberations are worth listening to with an open mind, but we always seem to be left weighing in balance a lot of different opinions with no closure, leaving nothing but our own personal judgment.

One group of individuals is uniquely positioned and qualified with respect to gaining a deep understanding of both climate science and political reality.  These are the people who write up the contents of IPCC reports.  The reports are purposely designed with a primary intent of steering the various political operatives into courses of action that would do the maximum possible amount of good, and might actually be pursued under the rules of political reality.  These authors all become familiar with the kind of responses expected from various policymakers and the limits to which they can be pushed.  Nature magazine, a preeminent publisher of scientific studies, ran a survey of 234 currently active authors and received 92 responses.  The eye-opening results and extra commentary were reported in November in a Scientific American article that is available at this link:  https://www.scientificamerican.com/article/top-climate-scientists-are-skeptical-that-nations-will-rein-in-global-warming/.  Only 4% of the respondents believe the 1.5 target will prove to be successful up to the year 2100, while 60% expect warming to reach three degrees or more by that time.  “Their answers suggest strong scepticism that governments will markedly slow the pace of global warming, despite political promises made by international leaders as part of the 2015 Paris climate agreement…..Most of the survey’s respondents—88%—said they think global warming constitutes a ‘crisis’, and nearly as many said they expect to see catastrophic impacts of climate change in their lifetimes.”

These expert authors are all called upon to assume that the laws of nature will allow the 1.5 target can hold, if emissions are fully controlled, and would never offer an opinion to the contrary. They have been informed about innovative technologies that are in the pipeline. Their concern is squarely based on their knowledge of the ruling class, and their conclusions are highly unfavorable.

Carl

The globe is loaded with stunning temperature anomalies these days, with quite an array of hot and cold contrasts to the north of about 40 degrees of north latitude. This is typical of the mid-winter season. Note that south of 40 degrees, on land, there are almost nothing but moderate warm anomalies today. I won’t get into specifics, but urge you to go to the website on any nearby day, open the various regional maps in the upper north and toggle back and forth between temperature, precipitable water (PW), and major anomalies at specific locations to see what their relationships are like. You will be amazed by the consistency with respect to the way PW values match up. You might also check to see if jet stream positioning may be having an influence of some kind on how the hot and cold anomalies set up in relation to each other. These winds are constantly encountering and adjusting the courses of the atmospheric rivers that carry heavy concentrations of PW across the skies:

Unlike the NH, there is a real dearth of interesting or contrasting anomalies in the mid-latitudes of the SH. Australia is the one exception, and worth looking at for details. In the far west (with magnification) we see a warm anomaly in the +8-10C (16F) bracket in one area. This is a really high number in mid-summer for a spot where normal is plenty hot to begin with. Here is a closeup view:

Highs for this day are recorded at +47C (116F) and the 24-hour average for one spot, as shown on this map (again with magnification), is a most uncomfortable +42C (107F)—about like a bad day in Death Valley.

The central/southern part of Australia is a good bit cooler, with reasonable temperatures averaging just 23C (73F) and an anomaly of minus-4C. So much contrast with the north prompts us to have a look at the PW map and check up on atmospheric river (AR) activity over the continent. Sure enough, there is a strong band of PW across the north and much lower values in the center. The actual difference in values, which range from a low of 11kg to highs near 35, is easily enough to account for the difference in temperatures based on relative greenhouse effects and a relatively normal extra level of dryness in the center of the continent.

A quick look at the jetstream wind map may give us an idea about how the contrast in PW values came to be what it is. This map shows that two separate wind streams are at work here. Neither one is doing anything to prevent full passage of a strong AR from following a rounded track across the continental north:

As an aside, this same map shows a pair of wind streams deeply entering and looping around over the lower part of the Antarctic continent. It should be worthwhile checking out the upper level air pressure configuration to see if there is a relationship, which is clearly the case. This activity has resulted in a major temperature anomaly, as viewed in the closeup image near the top:

Carl

The theory of sulfate aerosol cooling, which I have recently been writing about, is one that I find very persuasive. It involves a mechanism that is critical to the process, but it is almost impossible to understand why things should be that way. The mechanism is a sort of surprise discovery, based on diligent research which found that these aerosols interact with clouds in a way that causes a brightening of the cloud-top surfaces. The amount of brightening is considerable, large enough to deflect a significant amount of sunlight that would otherwise be passing through the cloud and sending more energy to the surface, along with its warming effect. This is considered a fact, having been demonstrated well enough to overcome just about every avenue of dispute. It leads to an assumption, which has also been demonstrated, that when these aerosol concentrations are diminished the cloud tops become less bright and more sunlight passes through, raising temperatures below.

Some sulfate aerosols are produced naturally, most importantly by volcano eruptions.  Major eruptions cause major cooling effects, which typically decline at the same rate as the associated aerosols and are gone within a year or two.  Today’s sulfate aerosols are produced every day in high concentrations, due to the burning of coal and oil, and every day there is a corresponding fallout   The amount of fallout each day must inevitably be in balance with the amount that has been added over a certain number of days preceding, as determined by the natural lifetime and daily rate of decline by fallout, which is very short.  It could take three days for a daily batch or it could take ten days, which doesn’t really matter.  Every day there is a fairly consistent new load of additional aerosols emitted, more or less replacing whatever fell out that day. 

The aerosols that are currently in the atmosphere produce a certain amount of cooling effect, which is unknown for certain but can be roughly approximated. Let’s call it one degree, on a globe-wide average (regions would be either more or less), for illustration purposes. This means if we stopped burning all coal and oil today we could expect the average to rise by one full degree within just a few days. If we stopped all burning over a more extended period, like thirty years, all else being equal or accounted for, one degree of total warming would still be added, but only in small increments. Whatever else would have an effect on temperatures over those thirty years must be accounted for separately, and certainly in an open and plausible way, but I can’t see any way for those other things to alter the direct effects of aerosol decline—with one possible exception. What if these other factors are able to cause a change in the amount of cloud cover in regions where the aerosols are active? Let’s think about that. Any difference would leave those aerosols with either a greater or lesser cloud population to interact with before they fall out, and there should be consequences.

One consequence I can think of right away is that any reduction in cloud cover will permit more sunlight to pass directly to the surface, with no amount of stoppage of any kind reducing its warmth.  Aerosol cooling would be reduced, making their presence less effective, but the net effect of these factors should be an unwanted amount of greater warming.  An increase in cloud cover is more puzzling since clouds themselves have a cooling effect, but why would more clouds take away any of the cooling effect of aerosols already in place?  Having fewer cloud-free days, on the other hand, would give existing aerosols more opportunity to interact with something before quickly falling out, potentially increasing their overall effectiveness.

Arguments are being made that tend to minimize the effect of aerosol reduction on surface temperatures.  One example can be found in this explainer from Carbon Brief:  https://www.carbonbrief.org/cutting-air-pollution-would-not-cause-near-term-spike-in-global-warming.  I have questions about the validity herein, left for you to decide.  There are other arguments that I have found links to and will post later.

Carl