Why should we be concerned?

There’s been quite a lot of discussion recently about representing global warming as equivalent to 4 Hiroshima bombs per second. Making Science Public had a recent post by Warren Pearce called more heat than light? Climate catastrophe and the Hiroshima bomb, arguing that the Hiroshima analogy may actually be counterproductive. I am starting to see why the analogy with something as horrific as Hiroshima may not be ideal and also that the comparison isn’t necessarily that appropriate. We’re accruing energy steadily, which is not the same as 4 atomic bombs going off every second. Having said that, this does seem to be another example of where the intent of the comparison is being ignored (i.e., we’re accruing energy at quite a remarkable rate) and the narrative is being defined by those who are (knowingly or not) misleading others with respect to the realities of global warming.

A reasonably common response to the atomic bomb analogy is to point out that ocean temperatures have (on average) risen by only 0.09oC since 1970 (or that they’re only rising by 0.035oC per decade at the moment). The implication is that this is so small that we really shouldn’t be worried. The issue though is that the information used to determine this change in ocean temperature is exactly the same as that used to determine the atomic bomb analogy. They’re both quantitatively correct, but one seems big and scary and the other seems small and insignificant. They can’t both be right. It can’t both be something to be concerned about and nothing to worry about. So, which one is it? Well I think we should be concerned, so I thought I would explain why below.

Global warming is about energy. We undergo global warming if the amount of energy entering the climate system (Solar, geothermal) is greater than the amount that is leaving. If global warming is occurring (as the evidence suggests) then the energy in the climate system will increase at some rate – there will be an energy imbalance. The rate at which it increases is determined by three main factors; the luminosity of the Sun, the surface temperature of the planet, and the composition of the atmosphere. There are some additional factors, such as changes in albedo, but these mainly depend on one of the three factors mentioned earlier, so I’ll ignore them here. The energy imbalance can reduce if the Sun were to get fainter – unlikely and even if it did, our continued addition of CO2 means that any reduction in solar luminosity would simply delay the warming by a few years, rather than stop it altogether. The atmospheric composition could change – this is indeed happening but in the wrong direction given that we’re continuing to add greenhouse gases. Or, the surface temperatures can rise so that the amount of energy leaving the system increases until it balances the amount of energy entering the system. So, it is reasonable to argue that the most likely manner in which the energy imbalance will be reduced is through a rise in surface temperatures.

At the moment it appears that most of the excess energy associated with global warming is entering the oceans. Some seem to think that this could continue indefinitely because the ocean is a massive heat sink. Well, there are a number of issues with this. Firstly, if surface temperatures continue to rise slowly (or pause) then the energy imbalance will grow as atmospheric CO2 concentrations increase. The oceans will have to absorb an ever increasing amount of energy. Secondly, they can’t simply continue to absorb almost all the excess (and even now they aren’t absorbing it all). As ocean temperatures rise, this will increase sea surface temperatures, the difference between the incoming energy and outgoing energy will reduce, and the oceans will absorb a decreasing fraction of the excess. This means that the fraction heating the land and atmosphere will increase and surface temperatures will start to rise more rapidly.

Another way to look at this is that the entire system is tending towards some kind of equilibrium. There may be periods when the energy in one part of the system increases faster than in another, but overall, if global warming is happening, the system will tend towards a state when the energy in all parts of the system will have increased. Furthermore, the longer the “pause” lasts the more CO2 we’ll have added to the atmosphere and the greater the energy excess will become. Hence, the rate at which surface temperatures rise in future is likely to be faster if the pause is long than if it were short.

To try and quantify the effect, one can consider the heat content of the land and atmosphere. Together, they have a mass of approximately 1019 kg and a specific heat capacity of 1000 J kg-1 K-1. This means that it would take 1022 J to increase the temperature of the land and atmosphere by (on average) 1 K. Hence if only 1% of the current energy excess were to heat the land and atmosphere, temperatures would rise by 0.1oC per decade. If the fraction were larger, it would be faster. I don’t know about the rest of you, but I find the idea that (on average) temperatures could rise by 0.1oC per decade or faster somewhat concerning.

So, my basic summary of why we should be concerned is as follow

  • Evidence suggests that the energy in the climate system is currently increasing at about 1022 J per year.
  • If atmospheric CO2 concentrations continue to rise while surface temperatures “stall”, the energy excess will increase.
  • The most likely way in which this energy excess will be reduced is through an increase in surface temperatures.
  • Currently most of the energy excess is entering the oceans, but this cannot continue indefinitely as the fraction being absorbed by the oceans will decrease as ocean temperatures rise.
  • At some point in the future surface temperatures will have to continue rising both because it is the only way to reduce the energy excess and because oceans cannot continue to absorb all the excess indefinitely.
  • The longer the “pause” the faster the subsequent rise in surface temperatures is likely to be.

Anyway, those are my reasons for why we should be concerned, irrespective of whether we view global warming as 4 atom bombs per second or as ocean temperatures rising at 0.035oC per decade. It is, I believe, why the recent leaked portion of the upcoming IPCC document apparently says that at least 95 percent likely that human activities – chiefly the burning of fossil fuels – are the main cause of warming since the 1950s. It really is very difficult to explain the current warming any other way. There is a chance that we’ve misunderstood something and hence noone is claiming that they’re 100% sure, but at least 95% is very certain. I realise that I’ve missed some details in what I’ve said above and that some of it may be slightly simplistic, but I do think it is roughly consistent with the current scientific thinking.

I do have a challenge for commentators. I have an open comment policy at the moment, so you’re free to say whatever you like. However, I would appreciate it if people actually tried to address the science rather than making some unsubstantiated comment about errors in the data or instruments, the trustworthiness of climate scientists, or that this is all some left-wing government conspiracy to control the world. You’re free to believe whatever you like and comment as such, but I can’t spend my time engaging in discussions about something for which there is no evidence.

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35 Responses to Why should we be concerned?

  1. Latimer Alder says:

    Heat does not distribute itself on a whim . It is guided by temperature (and entropy). If (and its an unproven ‘if’) i is going into the deep oceans, that is because the oceans are cooler. It will take a very long time before the oceans have equlibrated with the atmosphere and atmopsheric warming proceeds again.

    You say ‘oceans cannot continue to absorb all the excess indefinitely.’ Maybe not indefinitely but give their huge thermal mass and big temperature differential (c 10C), I’d reckon we’re good for about 2,500 years.

    And the simple thing that makes Hiroshima such a lousy and stupid analogy is that people know nothing about the heat output of the bomb. They do know that it went BANG, demolished the city by blast and killed people by radiation.

    Yet another attempt to exaggerate the warmist case by association with something really nasty. I’m just amazed at the puerile mentality of anyone who thought it was going to be anything better than an own goal. The public are not stupid and increasingly resent being treated as though they are. 10:10 was disaster. this little better.

  2. Rachel says:

    I don’t have a problem with the Hiroshima bomb analogy. It was used on people like me when I was living in Christchurch following the Christchurch earthquake. None of us saw the destruction that the people living in Hiroshima would have seen when we looked out our own windows but it still helped us to understand the amount of energy that was unleashed on our city. I don’t ever remember anyone complaining about this metaphor. People are only complaining about this being used in climate science because there are so many people battling to undermine the science itself and to create doubt in the public. Arguing over whether this metaphor is appropriate or not just further shadows the real issue – climate change is real, humans are the cause and we need to urgently do something about it.

    The climate scientists who are happy to make their views known to the public and to governments are the ones we should be listening to and the story as far as I can tell is one of concern and urgency.

  3. That’s close, Latimer, but at least you tried to address the science. So you think we have 2500 years. The system doesn’t have to equilibrate, it simply has to reach a state where it’s in a quasi-steady state (i.e., the energy flow into, through, and out of the system is in some kind of equilibrium). There are 2 main issues with your 2500 years is fine claim. One is simply that at the current rate of increase, CO2 levels will be so high that the energy imbalance will be significantly higher than it is today. The other is that even a small change in sea surface temperature can change how much energy is absorbed by the oceans. If you consider that F = σ T^4, then dF = 4 σ T^3 d T. So if you assume T = 263 K and dT = 0.1 K, then an increase of 0.1K in sea surface temperature can reduce the energy difference by 4 x 10^21 J per year. This will change what fraction is going into the oceans and hence change the fraction heating the land and atmosphere. So, given the current laws of physics, it’s hard to see how we can maintain the status quo for a long period of time.

  4. I too didn’t see an issue with the analogy, partly because I did see it simply as an energy comparison rather than a comparison with a horrific event. I can, however, see the argument as to why it might not be appropriate, but I haven’t quite convinced myself that it isn’t. I am concerned, as you suggest, is that it’s another example of where the narrative gets hijacked and rather than focusing on what the analogy is trying to illustrate, we focus on the analogy itself. It seems like a very common strategy.

    I totally agree with your latter point. We really do need to start listening to climate scientists and not to those who continually try to claim that climate scientists can’t be trusted and that this is all some kind of massive conspiracy.

  5. Rusty Parker says:

    The dose makes the poison.

  6. This could be a very funny and subtle comment. If so, it’s gone over my head unfortunately.

  7. Lars Karlsson says:

    An analogy that most people might understand is “microwave ovens relative to area” (or “hair dryer” or ..). 1.7 W/m2 corresponds to one 750 W microwave oven every 440 m2, or in each 21 m x21 m square of Earth’s surface.

  8. Yes, to the climate system the discussion on Hiroshima is inconsequential.

    Comparing an earth quack to an atomic bomb is a much better analogy. Both are localised in space and time.

  9. There’s been quite a lot of discussion recently about representing global warming as equivalent to 4 Hiroshima bombs per second. … Having said that, this does seem to be another example of where the intent of the comparison is being ignored (we’re accruing energy at quite a remarkable rate

    Does 4 atomic bombs really tell us the the energy increase is remarkable? Only if you know what the natural variability in the energy content of the climate system is. It is hard to imagine how big the Earth is, that makes it just as abstract to imagine what the influence of an atomic bomb is.

    It does make a nice tweet. However, I am not sure whether the short term attention is worth it. Especially to get attention for something as well known as climate change.

  10. The problem with most of the analogies I’ve seen are that they don’t always get across that we’re talking about the energy that remains, not the total amount energy entering the system. So, for example, I’ve read people say “0.5 W m-2 – so what? That’s like a small LED every square metre”. That may be true, it is like a small LED per square metre, but emitting energy that never leaves the system. It seems that getting people to recognise that the amount of energy is continually increasing is quite tricky because in an absolute sense, there are quantities that are much bigger – the total amount of energy we receive from the Sun every year is 1000 times greater than the energy excess.

  11. Lars Karlsson says:

    ..or an old-style 75 W light bulb every 6.6 m x 6.6 m square…

  12. I did give the claim that it was remarkable a little thought when I wrote it. My meaning was that if one considers paleoclimatological data it appears that our recent warming is unprecedented. Therefore, it seems that the current rate at which we are accruing energy is remarkable in that sense. So, what I was meaning by that statement was that the rate of increase is quite remarkable, however you choose to describe it. I wasn’t meaning that comparing it to 4 atom bombs per second makes it remarkable, although that presumably was the intent when that comparison was made.

    As I said in the post, I am starting to see and appreciate the criticisms of the comparison and I wasn’t using it here in this post to make it seem remarkable. I was really just referring to it to make the comparison between one analogy that seems “scary” and another that does not, despite them actually presenting (or being based on) the same information.

  13. Lars Karlsson says:

    One could maybe say that the additional amount of energy would be sufficient to run one microwave oven in every 21 m x 21 m square of Earth’s surface, day and night, year after year…

  14. That’s an interesting one. Something like that may actually work. Describing it in those kind of terms may well make it more accessible.

  15. Layzej says:

    For large values the standard American unit for:
    1) data is Libaries of Congress
    2) area is football fields
    3) energy is nukes.

    There is no reason this needs to be different for climate science.

  16. As I’ve said a number of times, I didn’t see the problem because – as you indicate – I simply saw it as an energy comparison and not a comparison with a horrific event. So, in most of my writing I’ve used the term “atom bombs” rather than “Hiroshima bombs”. However, I can see the argument against using Hiroshima specifically. There is also the issue of entropy that Tom Curtis has pointed out. We’re accruing energy steadily and globally which is not the same as 4 atom bombs going off every second. Apart from mentioning it in this post and comparing it to the calculation of how much ocean temperatures have changed, I’ve largely decided to stop using it (or, rather, I’m going to avoid using it in a blind, this is a big number, type of way).

  17. Rusty Parker says:

    The solution to pollution is dilution.

  18. Fragmeister says:

    Or six 75W bulbs per tennis court. And the surface of the Earth could have 109 billion tennis courts. Might want to check my maths (figures are approx) and not sure it helps but I give it as a thought.

  19. Latimer Alder says:


    Do the sums. We know (or believe we know) all the variables, How long do we have until the heat starts warming the atmosphere again? What’s your best guess?

    Of course that all assumes that we know why it stopped in the first place. Which seems to be a bit beyond anyone’s intellect at the moment………could it be that we don’t understand climate as well as we like to delude ourselves we do?

    On the topic of ‘listening to climate scientists’.

    Your problem is not that people don’t listen to climate scientsists. That ship has sailed, mes braves. Climatologists had approaching twenty years of unfettered and unchallenged access to governments and media.They were heard with awed respect. ‘Trust Me, I;m a Climate Scientist’ was the password into the corridors of power. The IPCC as an organisation was even awarded the Nobel Peace Prize. In the UK we passed the Climate Change Act. Kyoto was signed and implemented with huge input from climate scientists. They had the ear of all the big decision makers.

    And that is the nub of the problem. People did listen to climate scientists, and have learnt the hard way that none of their confidently apocalyptic predictions have come to pass. As soothsayers they are a busted flush. And the general public have got bigtime climate fatigue. The chances of the magic phrase ever having its power again are zero. Obscure departments like DECC might give them a cup of coffee and a bikkie every now and then, but the big guns are no longer interested. The overplayed their hand fifteen years ago and their influence has never recovered.

    So dream on of a return to the glory days if you will. But the days of the hegemony of climatology are long past.

  20. BBD says:

    People did listen to climate scientists, and have learnt the hard way that none of their confidently apocalyptic predictions have come to pass. As soothsayers they are a busted flush.

    A blatant strawman. Climate scientists point to what is likely to happen later this century and thereafter if CO2 emissions continue unabated. This kind of cheap rhetoric is irritating, Latimer. It smells of demagoguery.

  21. Rattus Norvegicus says:

    The only problem I see with using nukes as a unit (and I see your point layzej) is that it provides an easy entry point for denialists to complain about alarmism. Over at HotWhopper someone suggested using the amount of energy generated by humans as a yardstick. That seems like a pretty good way to put it, not as dramatic as nukes, but it still gets the point across.

    Besides, the Hiroshima bomb was pretty puny by today’s standards :-).

  22. BBD says:


    May I draw your attention to what our host wrote above:

    I do have a challenge for commentators. I have an open comment policy at the moment, so you’re free to say whatever you like. However, I would appreciate it if people actually tried to address the science rather than making some unsubstantiated comment about errors in the data or instruments, the trustworthiness of climate scientists, or that this is all some left-wing government conspiracy to control the world. You’re free to believe whatever you like and comment as such, but I can’t spend my time engaging in discussions about something for which there is no evidence.

  23. Tom Curtis says:

    BBD, it smells of flat out lies, is what you mean.

  24. toby52 says:

    Tom Curtis has made me uneasy about the atomic bomb analogy, but I think it could be prefaced with – if you took all that heat energy diffused every second and packaged it, the resulting package would be equal to the potential of 4 atomic bombs … and get the message across.

    The message is that it is a lorra, lorra energy.

  25. BBD says:

    Tom Curtis

    I concede the point at once. No further argument.


  26. Rachel says:

    Ok, so I think I must have missed what Tom Curtis said about the atomic bomb analogy. Can someone provide the link for me?

  27. It’s a comment Tom made a couple of days. It was one of the main reasons I too have become slightly uneasy about using the analogy.

  28. Rachel says:

    Ok, thanks. I think I mistakenly included the word “Hiroshima” in my comment. I don’t actually remember that word be used in the media reports after each of the big earthquakes in Chch. I’ve just searched again and the recent series of quakes to rattle NZ is also producing media reports using this analogy but they compare the energy to “nuclear bombs”. If the objections hold for comparisons with climate change and the energy imbalance, then shouldn’t they also hold for comparisons with earthquakes?

    The Wikipedia entry for Richter magnitude scale includes a table part-way down the page which has a TNT explosive comparison for each magnitude. It does use the Hiroshima bomb and also Chernobyl in the comparisons. It also notes that for earthquakes, the energy is released underground as opposed to bombs which are released above ground. Big difference.

    It’s possible that the Japanese do find these comparisons offensive and if so, then this would be reason enough to stop using them. Unfortunately, in my experience anyway, the Japanese rarely complain so it’s hard to know whether they do or not.

  29. Tom Curtis says:

    Rachel, I have no reason to think comparison with “nuclear bombs”, or “atomic bombs” are particularly offensive to the Japanese. Only comparisons with the Hiroshima (or Nagasaki) attacks.

    Leaving that aside, one major reason why the atomic bomb/OHC comparison is not useful is that the gain in OHC has a very high entropy, while an atomic explosion has a very low entropy. It is that low entropy that allows atomic bombs to be destructive. You can visualize this by imagining the same atomic explosion in a very low entropy environment, eg, the core of the Sun, and imagining just how destructive it would be (ie, barely noticeable).

    Unlike the accumulation of energy in the ocean, high magnitude earthquakes are also very low entropy events. Consequently, comparison with nuclear explosions in general is scientifically apt – particularly underground nuclear explosions. Provided the comparison is with a test explosion only, or even more abstractly, just a designated yield, the comparison would then be informative and unobjectionable.

  30. Rachel says:

    Thanks, Tom. I understand that now.

  31. Marco says:

    Comparison to “nuclear bombs” is useless for another reason, too, as rattus in a way alludes to: what nuclear bomb are we talking about? “Little Boy” was a factor 3000 lower in yield than “Tsar Bomba”…

  32. I agree, that is an issue. You do need to specify which “bomb” you’re comparing to and that invariably means comparing to the Hiroshima bomb, which then creates the problem of appearing to compare to a horrific event rather than to something with a specific energy.

    It seems to me that almost all analogies fail for one reason or another. I suspect the reason is quite straightforward. In most areas of science, the public are interested in learning about our current scientific understanding. When talking to certain audiences you can use quite detailed scientific terminology. To other audiences, this may not be appropriate so you use some kind of analogy to illustrate the science. The audience gains some understanding from the analogy.

    However, no analogies are perfect. If part of your audience is not actually interested in learning about the science (for whatever reason) they can spend their time criticising the analogy (sometimes justifiably) and ignoring what the analogy was trying to illustrate. That seems to be a particularly issue in climate science. So, as far as I can tell, analogies will always have problems if some of the audience simply want to find fault.

    I should add that I’m not suggesting that all who criticise analogies are trying to undermine the science. I have a lot of sympathy with those who criticise the use of the Hiroshima bomb. I’m simply suggesting that using analogies can almost always provide ammunition to those who’s prime goal is to criticise and undermine the scientific evidence.

  33. Tom Curtis says:

    Wotts, if a relatively precise benchmark is desired, I have previously suggested the Trinity device (20 kilotonne) as an appropriate benchmark for an atomic bomb unit. The Tsar Bomba (54 megatonnes) is also useful. They are also natural choices, being respectively, humanity’s first, and largest nuclear explosions.

  34. Tom, those would certainly be reasonable comparators. If I ever do feel the need to make such a comparison again, I will use one of those rather than using the Hiroshima bomb.

  35. Latimer Alder says:

    Using a bomb analogy for an earthquake is a lot nearer than using it for climate stuff.

    Both an earthquake and a bomb give up a lot of energy in a very short period of time.

    And it is the intensity of that energy release, not necessarily the total amount involved that causes the damage that people remember.

    Not the same for climate. Supposedly a very gradual process where changes occur over decades or longer. Not in milliseconds.

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