Hot Times in the Cities
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Alvia Gaskill
Sep 1, 2026, 11:32:23 AM (2 days ago) Sep 1
to Geoengineering FIPC
As many of us experience yet another week of 100-degree (F) temperatures in the hottest summer on record for the U.S., the endless procession of SAI model studies, concept papers and social science fear fantasies received here daily via the inbox offers little hope for relief.
SAI or SRM as it was once called along with little brother MCB will disrupt weather patterns and potentially have other negative side effects including killing people, none of which collectively will be as bad as the base case, the one we are living, and which is getting worse. Add the Nepal glacier collapse/mudslide to the list. We also knew or suspected all these problems 20 years ago.
These studies have provided little more than confirmation. A waste of money but more importantly a waste of time. Time that could have been better spent on field studies. The relevant modeling could have also been done concurrently with field studies.
The climatological navel-gazing has also empowered the opponents of modernity who have seized upon climate change as the reason to start things over or even end it all and geoengineering as something standing in the way. The moral hazard is simply a cover story for getting rid of people, however that is accomplished.
As SAI slowly drowns in repetitive studies with no end in sight, perhaps their next target will be surface albedo enhancement, currently relegated to the role of a second or third tier approach.
One potential new whipping boy in the category is passive daytime radiative cooling (PDRC), the description and applications described here in some detail.
The principle is that the coatings or other materials have both high reflectivity in the solar spectrum (400-2500 nm which is visible + NIR) and high IR emissivity in the so-called atmospheric window (8000-13,000 nm which is in the region of thermal radiation).
Conventional surfaces and coatings don't emit as much IR in this range. The PDRC coatings by contrast upward scatter most of the sunlight and emit lots of heat resulting in surface temperatures that are permanently below ambient including at night when radiation prevails and solar gain is zero.
Surface temperatures well below ambient of course do occur when the air cools rapidly enough like in the early morning in the fall and water condenses on surfaces like autos and their windows. But these are short-lived events and as will be noted the condensation blocks IR.
In one particular imagined global application, an area half the size of the Sahara Desert (part of the real estate in the desert cover idea I promoted more than 20 years ago) is covered with some kind of panels coated with the PDRC substances, locally offsetting 100 W/m2
and globally 1 W/m2, the same target in the SAI models.
So, if these coatings work as advertised, they are the environmentally friendly alternative to SAI or at least they can lower GHG emissions due to A/C.
There are several problems that may greatly limit applicability for any purpose.
1. The atmospheric window is only truly a window if there are no clouds and low humidity. Emission from a ground surface is also not the same as emission to space. Clouds and water vapor can reduce this considerably. However, the local environment including the interior of a building can be made cooler in spite of this.
2. Condensation and precipitation can reduce IR emission to 0. Soiling can also interfere just as with conventional white coatings and membranes. Liquid water and ice will absorb all of the IR as shown in the figure below.
3. The coatings are very expensive, in some cases $150/gallon. Granted, this is a technology that has only been around for about 10 years so scaling and other improvements can probably bring this down, but this is speculation. The fact that the slightly more expensive cooler roof shingles have never caught on is a cautionary red flag for the future adoption of this approach.
Flat white roofs with other types of coatings have seen greater use but they still are only applied on a tiny fraction of available surfaces. Restricting use to only flat roofs also means little use for residential which is where it matters most.
4. There have been very few field studies. In fact, most of the relevant references in the Wikipedia article are from a handful of Chinese research groups. Some uncoordinated recent U.S. field work is in progress as illustrated below.
The article is inaccurate in stating that the coatings don't work on steeply sloped roofs. They would. For aesthetic reasons, white coatings are not acceptable on most residential roofs. Nobody wants to live in the gingerbread house, and your neighborhood HOA doesn't want you to either.
Of some concern to me is why LBNL is not investigating these materials as the issues related to accelerated weathering common to all surface treatments are right in their wheelhouse. Lack of funds due to Elon or lack of interest?
Perhaps after the recent unpleasantness beginning in 2025 is over, we can get back to investigating real problems instead of each other. However, if we take as long to evaluate PDRC as we have SAI, there may be no applications left or even people to investigate.
As a bonus here, in the course of my look at PDRC, I ran across mention of a local study by Duke's Nicholas School where they found that trees planted in urban areas block surface radiation more than they cool the surroundings.
One of those places is near where I live and while wandering around in the heat in 2024, I saw one of their monitors affixed to the side of a power pole.
Because trees along sidewalks are also adjacent to roads, there is little soil surface around them. So unlike in a true rural area where all of the surface is soil and doesn't have as great a heat storage capacity, the side walked trees wind up blocking a lot more radiation than would a corresponding grove probably due to bleed through of heat from the sidewalks. They also block convection which is the primary heat removal mechanism during the day. At the same time, they transpire water which will remove some heat to higher in the atmosphere. Soil temperature measurements might confirm some of these conclusions. A tree is not a forest. And concrete is not soil.
To further illustrate, I took a field trip this morning to the site of the heat monitor. You can see the residue of the black tape used to hold it in place, so it was about at breathing level. Directly behind it is overgrown shrubbery next to the sidewalk. There is almost no transition zone between the shrubbery and the sidewalk showing that the heat coming off it would also be intercepted by the shrubbery. Due to the file size limit, the second photo is sent separately.
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