Los Alamos claims that they are being pessimistic, but then end up with very low soot conditions compared to observations.
They claim that firestorms are difficult to form with 15kt weapons. If their logic holds this may be accurate, due to the circle of blast damage nearly overlapping with the circle of fire damage (see the map above), but that wouldn’t be the case as weapons get larger (see the 100kt + weapon circles). This makes their conclusion less relevant for larger exchanges.
Their claimed soot lofting in the 72.6 g/cm2 scenaro is still very low. They claim the fire is in the “firestorm regime”, but it again doesn’t seem to meet observations (of lofting post Hiroshima for example, with the photo above). This also contradicts other modeling as well as the few observations we have of firestorms: both Rutgers and Lawrence Livermore model that soot would be far more effectively lofted than their model.
My point from the article is that:
There is uncertainty, BUT:
Some of the Los Alamos critiques may not apply at larger weapon sizes and larger exchanges.
Los Alamos’ modeling may or may not be correct, we have credible reasons to be concerned about soot lofting from multiple other sources (both observations and models), and there are questions on some of their key model outputs.
If Los Alamos is correct in all of their modeling, and it all holds for larger exchanges, then there would likely be no climate shock. If any of the points raised above hold, there is a threat, even without the most pessimistic of Rutger’s projections.
Los Alamos claims that they are being pessimistic, but then end up with very low soot conditions compared to observations.
Which observations do we have? I did not find the source for the 0.02 Tg emitted in Hiroshima you mentioned. I suspect it was estimated assuming a certain fuel load and burned area, which would arguably not count as an observation.
Their claimed soot lofting in the 72.6 g/cm2 scenaro is still very low. They claim the fire is in the “firestorm regime”, but it again doesn’t seem to meet observations (of lofting post Hiroshima for example, with the photo above).
I cannot read how much soot is in the photo, so I do not know whether it is evidence for/against the 72.62 g/cm^2 simulation of Reisner 2019.
Therefore: nuclear winter is a threat.
I very much agree it is a threat in expectation. On the other hand, I think it is plausible that Los Alamos is roughly correct in all of their modeling, and it all roughly holds for larger exchanges, so I would not be surprised if the climate shock was quite small even then. I welcome further research.
For the high fuel load of 72.62 g/cm^2, Reisner 2019 obtains a firestorm
Thanks for the correction. Unfortunately, there is no scale on their figure, but I’m pretty sure the smoke would be going into the upper troposphere (like Livermore finds). Los Alamos only simulates for a few hours, so that makes sense that hardly any would have gotten to the stratosphere. Typically it takes days to loft to the stratosphere. So I think that would resolve the order of magnitude disagreement on percent of soot making it into the stratosphere for a firestorm.
I think the short run time could also explain the strange behavior of only a small percent of the material burning at high loading (only ~10%). This was because the oxygen could only penetrate to the outer ring, but if they had run the model longer, most of the fuel would have eventually been consumed. Furthermore, I think a lot of smoke could be produced even without oxygen via pyrolysis because of the high temperatures, but I don’t think they model that.
Los Alamos 2019 “We contend that these concrete buildings will not burn readily during a fire and are easily destroyed by the blast wave—significantly reducing the probability of a firestorm.”
According to this, 20 psi blast is required to destroy heavily built concrete buildings, and that does not even occur on the surface for an airburst detonation of 1 Mt (if optimized to destroy residential buildings). The 5 psi destroys residential buildings that are typically wood framed. And it is true that the 5 psi radius is similar to the burn radius for a 15 kt weapon. But knocking down wooden buildings doesn’t prevent them from burning. So I don’t think Los Alamos’ logic is correct even for 15 kt, let alone 400 kt where the burn radius would be much larger than even the residential blast destruction radius (Mike’s diagram above).
Los Alamos 2018: “Fire propagation in the model occurs primarily via convective heat transfer and spotting ignition due to firebrands, and the spotting ignition model employs relatively high ignition probabilities as another worst case condition.”
I think they ignore secondary ignition, e.g. from broken natural gas lines or existing heating/cooking fires spreading, the latter of which is all that was required for the San Francisco earthquake firestorm, so I don’t think this could be described as “worst case.”
Unfortunately, there is no scale on their figure, but I’m pretty sure the smoke would be going into the upper troposphere (like Livermore finds). Los Alamos only simulates for a few hours, so that makes sense that hardly any would have gotten to the stratosphere. Typically it takes days to loft to the stratosphere. So I think that would resolve the order of magnitude disagreement on percent of soot making it into the stratosphere for a firestorm.
Actually, I think they only simulate the fires, and therefore soot production, for 40 min:
HIGRAD-FIRETEC simulations for this domain used 5,000 processors and took roughly 96 h to complete for 40 min of simulated time.
So you may well have a good point. I do not know whether it would be a difference by a factor of 10. Figure 6 of Reisner 2018 may be helpful to figure that out, as it contains soot concentration as a function of height after 20 and 40 min of simulation:
Do the green and orange curves look like they are closely approaching stationary state?
I think the short run time could also explain the strange behavior of only a small percent of the material burning at high loading (only ~10%). This was because the oxygen could only penetrate to the outer ring, but if they had run the model longer, most of the fuel would have eventually been consumed. Furthermore, I think a lot of smoke could be produced even without oxygen via pyrolysis because of the high temperatures, but I don’t think they model that.
In that slower burn case, would the fuel continue to be consumed in a firestorm regime (which is relevant for the climatic impact)? It looks like the answer is no for the simulation of Reisner 2018:
Note that because of low wind speeds and hence minimal fire spread, the fires are rapidly subsiding at 40 min (not shown) [the time for which they simulated the fire].
For the oxygen to penetrate, I assume the inner and outer radii describing the region on fire would have to be closer, but that would decrease the chance of the firestorm continuing. From Reisner 2019:
It is our contention that the distance between the inner and outer radii is key to the development [and I guess continuation] of a possible firestorm. Obviously, as the inner radius approaches the outer radius, the firestorm probability goes to zero, whereas if the inner radius approaches the center of the detonation, the risk increases due to a large burnable area.
Reisner 2019 also argues most soot is produced in a short time (emphasis mine):
Another important point concerning these simulations is that the rapid burning of the fine fuels leads to both a reduction in oxygen that limits combustion and a large upward transport of heat and mass that stabilizes the upper atmosphere above and downwind of the firestorm. These dynamical and combustion processes help limit fire activity and BC production once the fine material has been consumed (timescale < 30 min [from Reisner 2018, the fire was simulated for “40 min”]). Hence, the primary time period for BC injection that could impact climate occurs during a relatively short time period compared to the entirety of the fire or the continued burning and/or smoldering of thicker fuels.
I do not think they model pyrolysis. Do you have a sense of how large would be the area in sufficiently high temperature and low oxygen for pyrolysis to occur, and whether it is an efficient way of producing soot?
I think they ignore secondary ignition, e.g. from broken natural gas lines or existing heating/cooking fires spreading, the latter of which is all that was required for the San Francisco earthquake firestorm, so I don’t think this could be described as “worst case.”
Good point! It is not an absolute worst case. On the other hand, they have more worst case conditions (emphasis mine):
Figure 3 shows the initial vertical profiles of wind speed and potential temperature used for the HIGRAD-FIRETEC simulations. The wind speed profile was chosen to be high enough to maintain fire spread but low enough to keep the plume from tilting too much to prevent significant plume rise (worst case). Wind direction is set as 270° (west-to-east, +x direction) for all heights, with no directional shear, and a weakly stable atmosphere was used below the tropopause to assist plume rise (worst case).
Actually, I think they only simulate the fires, and therefore soot production, for 40 min. So you may well have a good point. I do not know whether it would be a difference by a factor of 10. Figure 6 of Reisner 2018 may be helpful to figure that out, as it contains soot concentration as a function of height after 20 and 40 min of simulation. Do the green and orange curves look like they are closely approaching stationary state?
Wow—only 40 minutes—my understanding is actual firestorms take hours. This graph is for the low loading case, which did not produce a firestorm. The lines do look similar for 20 and 40 minutes, but I don’t think it’s the case we are interested in. They claim only the fine material that burns rapidly contributes, but I just don’t think that is the case with actual firestorms. The 2018 was with low loading, and most of the soot is in the lower troposphere (at least after 40 minutes), so the question is when they actually did find a firestorm, what is the vertical soot distribution? For Livermore, it was mostly upper troposphere. Los Alamos did recognize that they were not doing latent heat release even in the 2019 simulation. I think this is quite important, because it’s the reason that thunderstorms go to the upper troposphere (and sometimes even stratosphere). It’s been a while since I took geophysical fluid dynamics, but the argument that the initial plume would stabilize the atmosphere seems off to me. If we look at the example of night in the atmospheric boundary layer (lower ~1 km), the surface cools radiatively, so you get stratification (stable). But when the sun comes up, it warms the surface of the earth, and you get thermals, and this upward convection actually destabilizes the boundary layer. Now it is true if you have a fire in a room that the hot gases can go to the ceiling and stabilize the air in the room. But if they are arguing that the plume only goes up a few kilometers (at least for the non-firestorm case), it seems like in those few kilometers, the potential temperature would be more equalized, so overall less stability. Even if that’s not the case, the plume has hardly even reached the upper troposphere, so there would be hardly any change in stability there. In addition, if the simulation is run over hours, then new atmosphere could come into place that has the same old stability. So I think the Livermore results are more reasonable.
I feel there are a few things here:
Los Alamos claims that they are being pessimistic, but then end up with very low soot conditions compared to observations.
They claim that firestorms are difficult to form with 15kt weapons. If their logic holds this may be accurate, due to the circle of blast damage nearly overlapping with the circle of fire damage (see the map above), but that wouldn’t be the case as weapons get larger (see the 100kt + weapon circles). This makes their conclusion less relevant for larger exchanges.
Their claimed soot lofting in the 72.6 g/cm2 scenaro is still very low. They claim the fire is in the “firestorm regime”, but it again doesn’t seem to meet observations (of lofting post Hiroshima for example, with the photo above). This also contradicts other modeling as well as the few observations we have of firestorms: both Rutgers and Lawrence Livermore model that soot would be far more effectively lofted than their model.
My point from the article is that:
There is uncertainty, BUT:
Some of the Los Alamos critiques may not apply at larger weapon sizes and larger exchanges.
Los Alamos’ modeling may or may not be correct, we have credible reasons to be concerned about soot lofting from multiple other sources (both observations and models), and there are questions on some of their key model outputs.
If Los Alamos is correct in all of their modeling, and it all holds for larger exchanges, then there would likely be no climate shock. If any of the points raised above hold, there is a threat, even without the most pessimistic of Rutger’s projections.
Therefore: nuclear winter is a threat.
Thanks for clarifying!
Which observations do we have? I did not find the source for the 0.02 Tg emitted in Hiroshima you mentioned. I suspect it was estimated assuming a certain fuel load and burned area, which would arguably not count as an observation.
I cannot read how much soot is in the photo, so I do not know whether it is evidence for/against the 72.62 g/cm^2 simulation of Reisner 2019.
I very much agree it is a threat in expectation. On the other hand, I think it is plausible that Los Alamos is roughly correct in all of their modeling, and it all roughly holds for larger exchanges, so I would not be surprised if the climate shock was quite small even then. I welcome further research.
Thanks for the correction. Unfortunately, there is no scale on their figure, but I’m pretty sure the smoke would be going into the upper troposphere (like Livermore finds). Los Alamos only simulates for a few hours, so that makes sense that hardly any would have gotten to the stratosphere. Typically it takes days to loft to the stratosphere. So I think that would resolve the order of magnitude disagreement on percent of soot making it into the stratosphere for a firestorm.
I think the short run time could also explain the strange behavior of only a small percent of the material burning at high loading (only ~10%). This was because the oxygen could only penetrate to the outer ring, but if they had run the model longer, most of the fuel would have eventually been consumed. Furthermore, I think a lot of smoke could be produced even without oxygen via pyrolysis because of the high temperatures, but I don’t think they model that.
Los Alamos 2019 “We contend that these concrete buildings will not burn readily during a fire and are easily destroyed by the blast wave—significantly reducing the probability of a firestorm.”
According to this, 20 psi blast is required to destroy heavily built concrete buildings, and that does not even occur on the surface for an airburst detonation of 1 Mt (if optimized to destroy residential buildings). The 5 psi destroys residential buildings that are typically wood framed. And it is true that the 5 psi radius is similar to the burn radius for a 15 kt weapon. But knocking down wooden buildings doesn’t prevent them from burning. So I don’t think Los Alamos’ logic is correct even for 15 kt, let alone 400 kt where the burn radius would be much larger than even the residential blast destruction radius (Mike’s diagram above).
Los Alamos 2018: “Fire propagation in the model occurs primarily via convective heat transfer and spotting ignition due to firebrands, and the spotting ignition model employs relatively high ignition probabilities as another worst case condition.”
I think they ignore secondary ignition, e.g. from broken natural gas lines or existing heating/cooking fires spreading, the latter of which is all that was required for the San Francisco earthquake firestorm, so I don’t think this could be described as “worst case.”
Actually, I think they only simulate the fires, and therefore soot production, for 40 min:
So you may well have a good point. I do not know whether it would be a difference by a factor of 10. Figure 6 of Reisner 2018 may be helpful to figure that out, as it contains soot concentration as a function of height after 20 and 40 min of simulation:
Do the green and orange curves look like they are closely approaching stationary state?
In that slower burn case, would the fuel continue to be consumed in a firestorm regime (which is relevant for the climatic impact)? It looks like the answer is no for the simulation of Reisner 2018:
For the oxygen to penetrate, I assume the inner and outer radii describing the region on fire would have to be closer, but that would decrease the chance of the firestorm continuing. From Reisner 2019:
Reisner 2019 also argues most soot is produced in a short time (emphasis mine):
I do not think they model pyrolysis. Do you have a sense of how large would be the area in sufficiently high temperature and low oxygen for pyrolysis to occur, and whether it is an efficient way of producing soot?
Good point! It is not an absolute worst case. On the other hand, they have more worst case conditions (emphasis mine):
Wow—only 40 minutes—my understanding is actual firestorms take hours. This graph is for the low loading case, which did not produce a firestorm. The lines do look similar for 20 and 40 minutes, but I don’t think it’s the case we are interested in. They claim only the fine material that burns rapidly contributes, but I just don’t think that is the case with actual firestorms. The 2018 was with low loading, and most of the soot is in the lower troposphere (at least after 40 minutes), so the question is when they actually did find a firestorm, what is the vertical soot distribution? For Livermore, it was mostly upper troposphere. Los Alamos did recognize that they were not doing latent heat release even in the 2019 simulation. I think this is quite important, because it’s the reason that thunderstorms go to the upper troposphere (and sometimes even stratosphere). It’s been a while since I took geophysical fluid dynamics, but the argument that the initial plume would stabilize the atmosphere seems off to me. If we look at the example of night in the atmospheric boundary layer (lower ~1 km), the surface cools radiatively, so you get stratification (stable). But when the sun comes up, it warms the surface of the earth, and you get thermals, and this upward convection actually destabilizes the boundary layer. Now it is true if you have a fire in a room that the hot gases can go to the ceiling and stabilize the air in the room. But if they are arguing that the plume only goes up a few kilometers (at least for the non-firestorm case), it seems like in those few kilometers, the potential temperature would be more equalized, so overall less stability. Even if that’s not the case, the plume has hardly even reached the upper troposphere, so there would be hardly any change in stability there. In addition, if the simulation is run over hours, then new atmosphere could come into place that has the same old stability. So I think the Livermore results are more reasonable.