It sounds like the above argues convincingly for firestorms, but not necessarily for significant soot ejections into the stratosphere given firestorms? Relatedly, from the Key Points of Robock 2019:
Between 3 February and 9 August 1945, an area of 461 km2 in 69 Japanese cities, including Hiroshima and Nagasaki, was burned during the U.S. B-29 Superfortress air raids, producing massive amounts of smoke
Because of multiple uncertainties in smoke injected to the stratosphere, solar radiation observations, and surface temperature observations, it is not possible to formally detect a cooling signal from World War II smoke
These results do not invalidate nuclear winter theory that much more massive smoke emissions from nuclear war would cause large climate change and impacts on agriculture
Iām not sure I follow this argument: almost all of the above were serious fires but not firestorms, meaning that they would not be expected to effectively inject soot. We did not see 100+ firestorms in WW2, and the firestorms we did see would not have been expected to generate a strong enough signal to clearly distinguish it from background climate noise. That section was simply discussing firestorms, and that they seem to present a channel to stratospheric soot?
Later on in the article I do discuss this, with both Rutgers and Lawrence Livermore highlighting that firestorms would inject a LOT more soot into the stratosphere as a percentage of total emitted.
Iām not sure I follow this argument: almost all of the above were serious fires but not firestorms, meaning that they would not be expected to effectively inject soot. We did not see 100+ firestorms in WW2, and the firestorms we did see would not have been expected to generate a strong enough signal to clearly distinguish it from background climate noise.
Agreed. I pointed to Robock 2019 not to offer a counterargument, but to illustrate that validating the models based on historical evidence is hard.
Later on in the article I do discuss this, with both Rutgers and Lawrence Livermore highlighting that firestorms would inject a LOT more soot into the stratosphere as a percentage of total emitted.
I think the estimates for the soot ejected into the stratosphere per emitted soot are:
From Los Alamos, 6.21 % (= 0.196/ā3.158), which I believe is implied by the results of Reisner 2018 (see Table 1 of Reisner 2019). I estimated it from the ratio between the 0.196 Tg of soot ejected into the stratosphere, and 3.158 Tg of emitted soot in the rubble case. Even for a fuel loading of 72.62 g/ācm^2, it is 6.44 % (= 1.53/ā23.77).
From Colorado and Rutgers, 80 %, as supposed in Turco 2007. āWe adopt a baseline value for the rainout parameter, R (the fraction of the smoke emission not removed), of 0.8, following Turco et al. (1990)ā. From the header of Table 2 of Turco 1990, āthe prompt soot removal efficiency is taken to be 20% (range of 10 to 25%)ā, which does correspond to R = 0.8 (= 1 ā 0.2).
I am not sure Lawrence Livermore estimates the soot ejected into the stratosphere per emitted soot. From Wagman 2020, āBC [black carbon, i.e. soot] removal is not modeled in WRF, so 5 Tg BC is emitted from the fire in WRF and remains in the atmosphere throughout the simulationā. It seems it only models the concentration of soot as a function of various types of injection (described in Table 2). I may be missing something.
Los Alamos: Even for a fuel loading of 72.62 g/ācm^2, it is 6.21 % (= 0.196/ā3.158).
So basically, no matter how much fuel Los Alamos puts in, they cannot reproduce the firestorms that were observed in World War II. I think this is a red flag for their model (but in fairness, it is really difficult to model combustionāIāve only done computational fluid dynamics modelingācombustion is orders of magnitude more complex).
So basically, no matter how much fuel Los Alamos puts in, they cannot reproduce the firestorms that were observed in World War II
For the high fuel load of 72.62 g/ācm^2, Reisner 2019 obtains a firestorm:
Of note is that the Constant Fuel [72.62 g/ācm^2] case is clearly in the firestorm regime with strong inward and upward motions of nearly 180 m/ās during the fine-fuel burning phase.
So, at least according to Reisner, firestorms are not sufficient to result in a significant soot ejection into the stratosphere. Based on this, as I commented above:
It sounds like the above [The case for concern: Firestorms, plumes and soot] argues convincingly for firestorms, but not necessarily for significant soot ejections into the stratosphere given firestorms
For reference, this is what Reisner 2018 says about modelling combustion (emphasis mine):
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. Further, because the current version of FIRETEC assumes BC production to be inversely proportional to oxygen depletion (no soot model was employed), that is, all the carbon in the fuel participated in the reaction and was turned into BC, the estimates, which represent upper bounds for the given fuel loadings, are higher (worst case [from Reisner 2019, āour BC emission factor is high by a factor of 10ā100ā]) than they would be if a detailed chemical combustion model was used for soot production. Although FIRETEC does not presently include this capability, it does have the ability to simulate combustion of fuel and fire spread though heat transfer, while other fire-modeling tools, such as WRF-FIRE (Coen et al., 2013), employ prescribed fire spread approximations typically based on wind speed and direction.
So my understanding is that they:
Are being pessimistic with respect to ignition probabilities, and production of soot from fuel.
Modelled the combustion of fuel, but not the chemical reaction describing the production of soot from fuel.
Minor correction to my last comment. I meant:
Even for a fuel loading of 72.62 g/ācm^2, it is 6.44 % (= 1.53/ā23.77) [not 6.21 %]
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 wouldnāt 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/ācm2 scenaro is still very low. They claim the fire is in the āfirestorm regimeā, but it again doesnāt 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 Alamosā 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 Rutgerās 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/ācm2 scenaro is still very low. They claim the fire is in the āfirestorm regimeā, but it again doesnāt 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/āagainst the 72.62 g/ācm^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/ācm^2, Reisner 2019 obtains a firestorm
Thanks for the correction. Unfortunately, there is no scale on their figure, but Iām 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 donāt think they model that.
Los Alamos 2019 āWe contend that these concrete buildings will not burn readily during a fire and are easily destroyed by the blast waveāsignificantly reducing the probability of a firestorm.ā
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 doesnāt prevent them from burning. So I donāt think Los Alamosā 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 (Mikeās diagram above).
Los Alamos 2018: āFire 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.ā
I think they ignore secondary ignition, e.g. from broken natural gas lines or existing heating/ācooking fires spreading, the latter of which is all that was required for the San Francisco earthquake firestorm, so I donāt think this could be described as āworst case.ā
Unfortunately, there is no scale on their figure, but Iām 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 donāt 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 ā40 minā]). 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/āor 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/ācooking fires spreading, the latter of which is all that was required for the San Francisco earthquake firestorm, so I donāt think this could be described as āworst case.ā
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 270Ā° (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?
Wowāonly 40 minutesāmy 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 donāt think itās the case we are interested in. They claim only the fine material that burns rapidly contributes, but I just donāt 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 itās the reason that thunderstorms go to the upper troposphere (and sometimes even stratosphere). Itās 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 thatās 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.
Great post, Mike!
It sounds like the above argues convincingly for firestorms, but not necessarily for significant soot ejections into the stratosphere given firestorms? Relatedly, from the Key Points of Robock 2019:
Hi Vasco
Iām not sure I follow this argument: almost all of the above were serious fires but not firestorms, meaning that they would not be expected to effectively inject soot. We did not see 100+ firestorms in WW2, and the firestorms we did see would not have been expected to generate a strong enough signal to clearly distinguish it from background climate noise. That section was simply discussing firestorms, and that they seem to present a channel to stratospheric soot?
Later on in the article I do discuss this, with both Rutgers and Lawrence Livermore highlighting that firestorms would inject a LOT more soot into the stratosphere as a percentage of total emitted.
Thanks for the reply!
Agreed. I pointed to Robock 2019 not to offer a counterargument, but to illustrate that validating the models based on historical evidence is hard.
I think the estimates for the soot ejected into the stratosphere per emitted soot are:
From Los Alamos, 6.21 % (= 0.196/ā3.158), which I believe is implied by the results of Reisner 2018 (see Table 1 of Reisner 2019). I estimated it from the ratio between the 0.196 Tg of soot ejected into the stratosphere, and 3.158 Tg of emitted soot in the rubble case. Even for a fuel loading of 72.62 g/ācm^2, it is 6.44 % (= 1.53/ā23.77).
From Colorado and Rutgers, 80 %, as supposed in Turco 2007. āWe adopt a baseline value for the rainout parameter, R (the fraction of the smoke emission not removed), of 0.8, following Turco et al. (1990)ā. From the header of Table 2 of Turco 1990, āthe prompt soot removal efficiency is taken to be 20% (range of 10 to 25%)ā, which does correspond to R = 0.8 (= 1 ā 0.2).
I am not sure Lawrence Livermore estimates the soot ejected into the stratosphere per emitted soot. From Wagman 2020, āBC [black carbon, i.e. soot] removal is not modeled in WRF, so 5 Tg BC is emitted from the fire in WRF and remains in the atmosphere throughout the simulationā. It seems it only models the concentration of soot as a function of various types of injection (described in Table 2). I may be missing something.
So basically, no matter how much fuel Los Alamos puts in, they cannot reproduce the firestorms that were observed in World War II. I think this is a red flag for their model (but in fairness, it is really difficult to model combustionāIāve only done computational fluid dynamics modelingācombustion is orders of magnitude more complex).
Thanks for commenting, David!
For the high fuel load of 72.62 g/ācm^2, Reisner 2019 obtains a firestorm:
So, at least according to Reisner, firestorms are not sufficient to result in a significant soot ejection into the stratosphere. Based on this, as I commented above:
For reference, this is what Reisner 2018 says about modelling combustion (emphasis mine):
So my understanding is that they:
Are being pessimistic with respect to ignition probabilities, and production of soot from fuel.
Modelled the combustion of fuel, but not the chemical reaction describing the production of soot from fuel.
Minor correction to my last comment. I meant:
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 wouldnāt 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/ācm2 scenaro is still very low. They claim the fire is in the āfirestorm regimeā, but it again doesnāt 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 Alamosā 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 Rutgerās 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/āagainst the 72.62 g/ācm^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 Iām 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 donāt think they model that.
Los Alamos 2019 āWe contend that these concrete buildings will not burn readily during a fire and are easily destroyed by the blast waveāsignificantly reducing the probability of a firestorm.ā
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 doesnāt prevent them from burning. So I donāt think Los Alamosā 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 (Mikeās diagram above).
Los Alamos 2018: āFire 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.ā
I think they ignore secondary ignition, e.g. from broken natural gas lines or existing heating/ācooking fires spreading, the latter of which is all that was required for the San Francisco earthquake firestorm, so I donāt think this could be described as āworst case.ā
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):
Wowāonly 40 minutesāmy 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 donāt think itās the case we are interested in. They claim only the fine material that burns rapidly contributes, but I just donāt 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 itās the reason that thunderstorms go to the upper troposphere (and sometimes even stratosphere). Itās 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 thatās 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.