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.