Natural variability

I wrote a little while ago about the Dutch advice to the IPCC. It was quite an interesting document as it appeared quite critical of the IPCC. In particular it suggested that the IPCC should, more explicitly, address natural climate change, rather than focusing on human-induced climate change. This was lept on by some skeptics as an indication of a major problem with the IPCC. Collin Maessen, however, wrote to the Dutch Meteorological Institue who reponded and basically said that the were simply recommending that the IPCC should more explicitly address natural climate change and make it part of their remit. Nothing particularly major then.

However, the issue of natural variability is often raised by those who are skeptical of human-induced climate change. They claim that it appears to be ignored and that climate scientists (and the IPCC) have not explained why what we’re experiencing today cannot simply be due to some kind of natural variability. Now, I don’t believe that scientists are ignoring it, but the skeptics may have a point. It does appear as though the issue of natural variability has not been addressed – in the public debate at least – particularly clearly and it would seem sensible to address it more explicitly in future. I do, however, think that there may be a perfectly good scientific reason why it has typically not been addressed particularly clearly in the public realm. The reason is that it really can’t explain (or be the reason for) what we’re currently undergoing.

Let me see if I can explain why. Bear in mind that I’m a physicist and not a climate scientist. Also, these are just some of my musings so I may well overlook something, or make some silly mistake. The first thing to realise is that the Earth gets most (virtually all) of its energy from the Sun. The surface temperature will tend towards a value at which we are radiating as much energy back into space as we receive from the Sun. We’ll call this the equilibrium temperature. There are 3 primary factors that determine what this equilibrium value is. One is the amount of energy we intercept from the Sun (the Total Solar Irradiance, TSI); another is our albedo (what fraction of this energy we reflect directly back into space); the third is the composition of our atmosphere. Why is our atmosphere important? Well, the energy we get from the Sun is primarily in the visible band and our atmosphere is largely transparent at these wavelengths. Apart from the fraction reflected, the rest passes through our atmosphere to be absorbed by the surface (and some by the atmosphere). The Earth is much cooler than the Sun and so re-emits at longer wavelengths. Our atmosphere is not transparent at these wavelengths and so much of this is absorbed and essentially trapped by the atmospheric greenhouse gases. This causes the surface temperature to rise until it reaches a value at which as much is escaping into space as we receive. There are a few other factors which Tom Curtis explains very clearly in this comment, but these other factors aren’t particularly important.

That’s the first thing to realise. Another thing to realise is that the heat content of the atmosphere and surface is quite low. The atmosphere has a mass of 5 x 1018 kg. The mass of the surface is tricky to determine (as far as I can tell) but is quite low. Let’s assume, for simplicity, that the total mass of the atmosphere and surface is 1019 kg. The specific heat capacity is 1000 J kg-1 K-1. This means that to increase the average temperature of the surface and atmosphere by 1oC (1 K) would require 1022 J. The rate at which the surface loses energy (J s-1) is
L = 4 π R2E σ T4,
where RE is the radius of the Earth, σ is the Stefan-Boltzmann constant, and T is the average surface temperature. If T = 290 K, the surface is losing energy at a rate of 2.04486 x 1017 J s-1.

Let’s consider a hypothetical situation where some natural event increases the average temperature of the surface and atmosphere by 0.1oC. This will increase the heat content of the atmosphere and surface by 1021J. Since this changes the surface temperature so that T = 290.1, the rate at which the surface loses energy is now 2.04768 x 1017 J s-1, 2.821 x 1014 J s-1 faster than when T = 290 K. This means that it will lose this excess energy (1021 J) within about a month. According to my calculation, this doesn’t really depend on how big the perturbation is. Even if the temperature is increased by 0.5oC, it will still only take about a month for the temperature to return to its equilibrium value. The same is true if the temperature were to drop slightly. It would return to equilibrium very quickly.

So, why is this relevant? Well there’s a few things we can conclude. Since the mid-1800s the surface temperature has increased by about 1oC. This can’t simply be a slow recovery towards equilibrium because as I’ve shown above, that should happen quickly. It can’t take a century. It also can’t be a series of natural events that just happen to have warmed the atmosphere and surface because these would have to have happened almost every week in order for the energy from the previous event not to have been lost into space before the next event occurs. For example, it can’t be due to ocean cycles such as ENSO events. These can indeed bring energy from the ocean to the surface where it can heat the atmosphere and land, but they only occur every few years and there are both heating (El Nino) and cooling (La Nina) phases. Furthermore, if it were due to ENSO cycles we would expect the ocean heat content to be dropping as the energy were transferred from the oceans to the atmosphere. Instead, it is rising.

So, we can conclude that the change in surface temperature since the mid-1800s has to have been accompanied by a corresponding change in our equilibrium temperature. The equilibrium temperature must be about 1oC higher today than it was in the mid-1800s. Are there any natural processes that could do this? Well, one obvious candidate is the Sun. It clearly influences our climate and plays a key role in setting our equilibrium temperature. The TSI does indeed vary. There is an 11-year solar cycle, but this shouldn’t produce a century-long increase in surface temperatures. Furthermore, it’s only associated with a small change in surface temperature. There are much longer cycles and indeed the TSI did rise during the first half of this century. However, this too was quite a small change (and so should only have produced a small change in surface temperature) and it’s been falling since about 1970. The Sun alone can’t explain what we’re currently undergoing.

What about volcanoes? They do indeed influence our equilibrium temperature, but tend to have a cooling effect. They eject aerosol particles into the atmosphere which slightly increases the albedo, directly reflecting more sunlight back into space. Also, these aerosol particles tend to precipitate out and so only influence the equilibrium temperature for a few years.

What about water vapour? It is indeed a greenhouse gas and so if the water vapour concentration has increased it would act to increase our equilibrium temperature. One problem with this is that the amount of water vapour that the atmosphere can hold depends on the atmospheric temperature. Any excess should precipitate out quickly. Also if some natural event could increase the atmospheric water vapour concentration so as to produce a 1oC surface warming in a century, it would be difficult to explain why our past climate appears to be so stable.

Could the albedo have changed? There seems to be no evidence for this. This would probably require that there was a change in the area of polar ice and snow cover and this hasn’t been observed. Maybe more correctly, it didn’t appear to start changing significantly until about 1960 and the changes we’re seeing now are almost certainly anthropogenic. Also, if it isn’t anthropogenic, what would be the driver. Ice doesn’t just simply decide to melt.

Clouds? Clouds can play a role in changing our equilibrium temperature, but can both heat and cool. They can influence the albedo and reflect more incoming radiation (cooling) but also can trap more outgoing long-wavelength radiation (heating). I will accept that the role of clouds is quite uncertain, but it does seem unlikely that they could explain the surface warming seen since the mid-1800s.

One could argue that maybe they’re all linked. Maybe a small increase in TSI can slightly increase atmospheric temperatures. This can allow more water vapour into the atmosphere which further increases temperature. This could lead to more clouds and to the melting of polar ice and snow which then changes our albedo. Maybe, but there are some fairly fundamental problems with this. TSI is thought to be about 0.25 Wm-2 greater today than it was in the mid-1800s. An increase of 1oC in surface temperatures means that the surface is radiating about 5.5 Wm-2 more today than it was in the mid-1800s. These linked effects would therefore have to essentially amplify a small change in TSI so as to produce a large change in surface flux. The amplification (or feedback) factor would need to be about 20. This is extremely high and, again, would make it hard to explain why our past climate appears to have been so stable. If it’s so sensitive to small changes in solar flux, surely we would have detected this in the paleoclimatological record.

Anyway, this has all got rather long. As I said at the beginning, these are just some of my musing and I am no expert at climate science. I do think that addressing natural variability more openly is a good thing. I also understand, I think, why this may not have happened in the past – it can’t really explain what we’re currently experiencing. I hope this post makes some kind of sense and – as usual – happy to take comments from anyone, but in particular from those who can correct any of my mistakes/misunderstandings.

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26 Responses to Natural variability

  1. In a sense I assumed above that the temperature at which the Earth was radiating into space was the surface temperature. This isn’t strictly correct as technically the amount of energy we lose into space is set by the temperature at which the atmosphere has an optical depth of 1 at that wavelength. This means that, externally, the Earth appears to have a temperature of -18oC (255 K) and possibly I should have done my calculation using this temperature, rather than using the surface temperature (290 K). It, however, doesn’t really make any difference to what I was trying to illustrate above.

  2. I have a little remark about what the KNMI said to me. They said the IPCC is addressing natural variability in their reports, what they want to see is that their mandate (as expressed in their principles document) should mention this. This is just a recommendation to bring that document in line with what the IPCC is already doing.

    Also variability is being addresses in the media, although in a very simplified way. It’s a very complex subject that has to be simplified to make it short and easy to understand (there’s a lot of nuance). However, this often gets distorted by the usual suspects to make it seem that natural variability has a bigger effect than it has. And to push the uncertainty meme.

  3. Thanks. Yes, you’re quite right. I had intended to write this more in the way say (i.e., that the recommendation was to make sure the mandate more properly reflected what the IPCC actually does) but, as I read it back, I didn’t quite get that across properly.

    I agree that variability is complex and I hope I haven’t mangled it too much in this post. As you say, it is being addressed, but the usual suspects then cherry-pick a few statements to make it seem that natural variability could be much more important than it actually is.

  4. I’m currently on my lunch break so I haven’t had the chance yet to fully read your post; I just skimmed it. When I get home I’ll fully read it and then I can say if you mangled it just enough or too much. 😉

  5. Thanks, constructive criticism always welcome 🙂

  6. Tom Curtis says:

    I think the possibility of significant natural variability is often understated by defenders of climate science. That is because:

    1) Distribution of skin temperature (ie, the mean temperature of radiation to space) is also a factor in the mean skin temperature required for equilibrium. The more unequally distributed the skin temperature, the greater the radiation to space at a given mean global skin temperature. That, however, does mean that natural temperature fluctuations with a significant latitudinal component such as the North Atlantic Oscillation can change the GMST for their duration without changing the energy balance (though I suspect only by a small amount).

    2) The various feedbacks are feedbacks on surface temperature rather than on forcing directly. Further, positive feedbacks reduce the increase in outgoing longwave radiation for a given increase in temperature. Together, if the net climate feedbacks are greater than the planck feedback, a small increase in GMST will result in feedbacks tending to result in further warming. If the climate sensitivity for a doubling of CO2 is 3, for example, a natural variation that causes a 0.33 C increase in temperature will induce a 1 C increase in temperature (three times the planck response) for the duration of that variation. If the variation is simply a fluctuation, the fluctuation will still be stronger, and longer lasting than in the absence of net positive feedbacks.

    3) Some areas of the planet appear to have stronger feedbacks than other areas, a factor largely dependent on the arrangement of continents. If this were not so, it would be impossible for mere changes in the location of peak July Insolation by latitude to instigate transitions between glacial and interglacial states of ice ages. In particular, the North Atlantic appears to such a location in part because the warm current bringing significant humidity to high latitudes results in a stronger ice/snow albedo feedback than in the North Pacific. Further, the Northern Hemisphere is in general more sensitive in that regard than the Southern Hemisphere because there is little landmass in the Southern Hemisphere sufficiently southerly to both have snow fall on it, and to have the snow periodically melt.

    Together, these considerations suggests some comments I have seen on natural variability are far to dismissive. They do not support the claims of AGW “skeptics” however, in that for natural variability to be significant, climate sensitivity must be large, with the consequence that the temperature response to a given forcing such as the doubling of CO2 must also be large. The “skeptics” who argue that climate sensitivity must be small because natural variability exists are contradicting themselves.

  7. Tom, thanks again for really good comments. I realise that my “analysis” is simplistic, dealing with global averages and not really taking into account possible variations that could influence things. So, I agree that dismissing natural variability is not only wrong but counter-productive in the sense that it makes it seem as though advocates of AGW are trying to hide something.

    Your comment about positive feedbacks is a good one. I had tried to cover that a little in this post but didn’t really address that any natural variation should introduce some kind of feedback so that the net effect is amplified (in fact, I suspect, I didn’t quite appreciate that subtlety when writing this). As you say, however, for this to explain how natural variability could be responsible for what we are currently experiencing would require a large climate sensitivity which would then beg the question of why the effect isn’t much bigger than we currently see given the concurrent rise in CO2 concentrations.

  8. Just thought of something else that would be worth clarifying with you. You were suggesting in your comment that for natural variations to explain what we’re currently experiencing would require a large climate sensitivity. I think this is roughly consistent with what I was saying towards the end of my post – that it would require a large amplification factor (or feedbacks). My understanding is that one of the argument against this (apart from trying to explain it from basic physics I guess) is that it would imply that our past climate history should have been far less stable than it appears to have been. Is this interpretation roughly correct?

  9. I think you are underestimating the potential of natural variability in that you are neglecting the oceans. Without oceans, temperature anomalies would indeed decay very quickly, but the thermal inertia of the oceans allows natural variability to persist (and for there to be “warming in the pipeline”).

  10. I was kind of ignoring the oceans, but not really. The point is that our equilibrium temperature is largely set by TSI, albedo, atmospheric composition. If the surface temperature is above equilibrium, it should decay back to equilibrium very quickly – matter of months. That the ocean is a large heat sink, shouldn’t really matter. On the other hand, if we are below equilibrium then they can play a role (and I didn’t discuss this, I will acknowledge). If all the excess energy simply acted to heat the land and atmosphere, then it would take a very short time to retain equilibrium. However (and this is what I suspect you’re referring to) the oceans can absorb a large fraction of the excess energy and hence slow down the surface and atmosphere warming. However, for this to explain the roughly 1oC rise in surface temperature observed since about 1880, would require that the oceans absorb something like 99.9% of the excess energy. Observations suggest that it is more like 95%. So, yes, if the surface temperature is below the equilibrium value, the oceans could slow down the surface warming by a factor of 20. Hence it might take a few years to retain equilibrium, but it can’t take a century (unless we’re all wrong).

    I guess one could make a bit weaker and suggest that the warming that we’re interested in is the warming that’s happened since 1970, but even that would require that the oceans absorb 99% of the excess energy, which is still higher than that predicted by observations.

  11. Actually, maybe you could clarify a little more. I thought I had kind of addressed part of what you’re suggesting. If the a temperature spike decays in a matter of months, then for oceans to play a long-term role, they’d have to be releasing energy on a short timescale (so that they can add to the previous pulse, rather than simply replacing it). It’s possible I guess, but then you have the problem of explaining why they’re releasing energy while the ocean heat content is still growing.

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  13. Tom Curtis says:

    Yes. In fact past climate history fairly tightly constrains climate sensitivity in that, if it were very high we would oscillate between snowball earth conditions, and conditions to warm for the existence of life. On the other hand, were it very low we would not have been able to escape the two or three snowball earth episodes that happened in the past, and significant temperature oscillations would not exist. Given this, climate sensitivity estimates below (at a rough guess) 1 C per doubling and above 10 C per doubling are pretty much ruled out.

  14. Tom Curtis says:

    Richard, if the Earth’s temperature is in approximate equilibrium relative to current energy inputs, then the surface temperature will be in approximate equilibrium with ocean temperatures at all depths. In this condition, thermal inertia from the ocean will work against climate fluctuations, tending to restore surface temperatures to those in equilibrium with the thermal storage in the ocean, ie, those near radiative equilibrium.

    In contrast, if there is a change in forcing so that the surface temperature is not in radiative equilibrium, but is in thermal equilibrium with the ocean, then the thermal mass of the ocean will tend to slow the shift away from that original thermal equilibrium, and hence slow the surface temperature response to the radiative forcing.

    A temperature variation driven by a fluctuation in ocean currents partly escapes this logic, and the AMO is a prime candidate for such an ocean current driven temperature oscillation. However, based on the Central England Temperature index, the AMO prior to industrialization has a much shorter period and much smaller amplitude than is currently proposed for it. Further, the currently proposed period and amplitude match closely the regional radiative forcing of the North Atlantic. Given that, while I am open to the idea of the AMO as a significant factor in global temperatures, I would want to see the analysis done on an AMO index defined as SD normalized NA SST/SD normalized NA regional forcing. It is only by thus eliminating the impact of regional forcing (which undoubtedly influence NA SST) that we can truly discover if there is an AMO, and if so, what influence it has.

  15. Interesting. Showing my ignorance again, but I can see how the ocean can slow the rise in surface temperatures if the surface temperature is below equilibrium. It’s less obvious to me what role it can play if the surface temperature is above equilibrium, unless what is happening is that the energy from the surface is going into the atmosphere, back down into the ocean and then back out to heat the atmosphere and land.

  16. We know that natural variability can be large. Within the last ice age, there were numerous rapid warming event recorded in Greenland. These are almost certainly unforced, the product of internal variability. During warm phases, there are also rapid cooling events, which might be forced by glacial lake outbursts, but might also be unforced.

    You won’t get this type of behaviour if you consider the climate with a 0-d energy balance budget, as natural variability is caused by regional processes (like the AMO) plus feedbacks that give them larger scale significance.

    The importance of unforced variability within the Holocene, away from critical thresholds, is less clear. My guess is that at century scales it is less important than forced variability. One way to get a handle on this is to compare forced and unforced climate model runs, perhaps over the last thousand years. The last-millennium CMIP5 runs should be ideal for this.

  17. As I think I made clear, I’m a physicist, not a climate scientist, so am happy to acknowledge that I’m slightly (very) out of my depth when it comes to paleoclimatology – or maybe even modern climatology. What I was trying to do here was just some basic, back-of-the-envelope calculations to illustrate how it seems difficult – based in some basic physics – for natural variations to explain what we’ve observed globally. Now, I don’t dispute (or even know enough to dispute) that there were rapid, local warming events in the past. I can see how some form of natural variation could move energy around the climate system and produce a phase of rapid warming in some location (and presumably cooling somewhere else). What I’m having trouble seeing is how this could explain a long-term (century) rising trend in global surface temperatures (and also a long-term rising trend in ocean heat content – although I didn’t really address that in this post).

  18. Tom Curtis says:

    I must have worded something poorly. If ocean heat and SST are in a near equilibrium state which is being perturbed by forcing, the ocean’s heat capacity will slow a rise in temperature for a positive forcing, and slow a fall in temperature for a negative forcing. If the forcing, in either case, returns to the original condition, the ocean will initially accelerate the fall (rise) in SST until the temperature falls below that dictated by the ocean’s new OHC (which will have changed) after which it will act to retard the fall (rise).

  19. chris says:

    One has to be careful in addressing the specifics in these discussions since some of the natural variability (e.g which richard telford refers to) has quite specific origins, may be relatively localized, and may arise due to a particular configuration (e.g. glaciation or glacial-interglacial transition) that is poised to respond rather dramatically to relatively small changes in forcings or phenomena associated with internal variability.

    That’s the case with the large, very fast and dramatic temperature excursions recorded in Greenland cores during the last glacial and glacial-interglacial transition. These seem to be a response to periodic shutdown and restart of the North Atlantic Deepwater limb of the global ocean circulation as a result of meltwater pulses in the high NH latitudes. These are barely detectable in Antarctic cores supporting the interpretation that the net effect on global scale energy balance was not necessarily very large.

    In considering the role of natural variability in the context of the very marked global scale warming of especially the last century and contemporary period, those Greenland temperature excursions are of little relevance since they relate to an Earth climate configuration that is vastly different to the current one. We expect/hope that the current configuration isn’t itself poised to undergo dramatic state shifts in response to internal or external forcings (although we don’t know this for sure). At least considering the Holocene the evidence supports the interpretation that the climate configuration is relatively stable and that the contribution from natural variability to Earth energy /surface temperature variability is relatively small…

  20. I think it was me, not you. I was driving home in the car when I realised that I was forgetting the difference between a change in forcing which changes the equilibrium temperature, and something like an El Nino event that simply releases energy to heat the atmosphere and land. Indeed, it’s quite clear that if the equilibrium temperature changes due to a change in forcing (either positive or negative) then the ocean heat content also has to change in order to re-establish equilibrium. In such a case the timescale will be quite a bit longer than I worked out in my back-of-the-envelope calculation above, but – unless I’m mistaken – probably not by much more than a factor of 10 – 20 (so a few years rather than a few months).

  21. Tom Curtis says:

    Is the back of the envelope calculation you are referring to the one in your first response to Richard Telford?

    If so, you are forgetting the feedbacks. The feedbacks are a response to temperature increase, not to forcing. So, what happens is you get an initial restriction in outgoing radiation due to an increase in CO2. This causes an initial temperature increase which results in some feedbacks. Those feedbacks (being net positive) result in a temperature increase which results in a further feedback on the feedback, and so on until equilibrium is reached. But thermal inertia of the ocean slows each step so that the full feedback response is not felt until decades after the initial change in forcing, and indeed, technically not until the full increase in GMST is reached. The result is that the time taken to reach equilibrium is much longer than a simple calculation of heat capacity would indicate.

    I have a blog post that discusses this in part.

    If I have missed the point, I apologize and will attribute it to the time (5:22 AM). Don’t you love the joys of insomnia.

  22. No need to apologise. I was referring to the back-of-the-envelope calculation I did in the post itself. I simply estimated how much the energy of the atmosphere/surface would change for a given change in average temperature. I then estimate how much the total energy loss rate (J s-1) would change, and then used that to estimate how long it would take for the system to lose the sudden pulse of energy. I got a few months. I was really using it to estimate how long it would take for for the temperature to return to equilibrium after something like the 1998 El Nino event. The point I was simply trying to make was that if one wants to invoke a Bob Tisdale-like ENSO heating, then the problem is that the system should return to equilibrium so quickly that there should be no long term temperature rise.

    I realised (partly at the time and partly later) that my back-of-the-envelope calculation doesn’t work if one considers a change in forcing rather than simply a sudden pulse of energy heating the atmosphere and surface. However, if the oceans are typically absorbing 90% or so of the energy excess, then (in the case of a positive forcing) rather than it taking a few months to reach equilibrium, I assumed it would take a few years. By analogy, I assumed that it would a similar time to reach equilibrium if the forcing was negative. However, as you quite rightly point out, I’m forgetting that the feedbacks can continue and so the time can be even longer than that (I knew I’d make some kind of silly mistake – that’s why I rely on the comments.). So, although the simple calculation implies a few months, feedbacks could extend this to years or decades, but a century is unlikely unless the forcing due to the driver (CO2 in this case) is also continuing to increase.

    I guess, however, that this still doesn’t change what were discussing earlier. If we want to explain what is currently happening (~ 1oC rise is global surface temperature since 1880, increase in OHC by 3 x 1023J since 1970) via natural variations, it would require a much large climate sensitivity than paleoclimatological studies indicate.

    I shall have to read your post. Thanks.

  23. Tom Curtis says:

    OK. In that case a few months is probably correct for a simple energy pulse. It may take longer if other factors (winds, currents) are involved.

    Time to equilibrium after a change of forcing, however, is much longer than a few decades. The e-folding time to equilibrium is of the order of one to two decades ignoring the deep ocean. Establishing equilibrium between surface and deep ocean, however, can take centuries. It may not be the best paper on the topic anymore, but this is a place to start.

  24. Tom, yes I just read your post and I see what you mean. I shall have to read the paper you recommend. What I suspect I didn’t make all that clear in this post was that I was trying to write it based on the assumption that CO2 isn’t a dominant forcing, and to then illustrate that trying to explain it all using some kind of natural variations is difficult – either because temperature variations due El Nino like events should decay quickly, or because natural forcings would tend to require large feedbacks/amplifications and this isn’t really consistent with our past climate appearing to have been so stable. I think I got this roughly right, but you’re correct that it is clearly much more complicated than my back-of-the-envelope calculation might suggest – although I did indicate that that was likely in the post 🙂

  25. Skeptikal says:

    I do, however, think that there may be a perfectly good scientific reason why it has typically not been addressed particularly clearly in the public realm.

    Yes, the reason is that if the current ‘pause’ in temperature rise can be attributed to natural variability, then the previous warming could equally be attributed to natural variability. The power of natural variability to pause the previous warming would be of the same magnitude as that needed to produce the previous warming.

    Furthermore, if it were due to ENSO cycles we would expect the ocean heat content to be dropping as the energy were transferred from the oceans to the atmosphere. Instead, it is rising.

    I don’t see why it would have to be dropping. ENSO has two phases, warming and cooling. Energy is transfered both ways… ocean to atmosphere in El Nino and atmosphere to ocean in La Nina. There’s nothing stopping more heat going into the ocean than being released from the ocean.

  26. Skeptikal, in an earlier thread (here) it became clear that your understanding of energy conservation was somewhat lacking. A number of people pointed out this error, but you seem – unless I’ve missed it – to have chosen not to address this. To me, however, this is crucial. If your views are founded (as they seem to be) on a mis-understanding of one of the fundamental laws of physics, then I’m not going to commit more of my time to discussing other aspects of this topic until we can resolve this fundamental issue.

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