One of the nice things about physics (well, I like it) is that you can often quantify things by making basic back-of-the-envelope calculations. Maybe a classic example of this is David MacKay’s book about renewable energy called Sustainable energy – without the hot air. It’s a masterclass in how to use simplifying assumptions and basic physics to try and understand various physical processes.Another example might be the standard Trenberth-like energy flux diagrams. It’s a nice simple illustration that tells us quite a lot about the basic greenhouse effect. If we consider the surface, we know that it receives something like 160Wm-2 from the Sun and has an average temperature of around 288K. Given this temperature, we know it must be radiating around 390Wm-2. We also know that it must lose some energy via non-radiative processes (thermals and evaporation); maybe another 100Wm-2. Hence it is losing – on average – a bit less than 500 Wm-2, but only receiving – on average – about 160Wm-2 from the Sun.
We also know that although surface temperatures can vary (day/night, seasons,…), if we average across the whole globe and over a long enough time interval, it is pretty steady (well, until we started adding GHGs to the atmosphere, that is). This tells us that it must – on average – be receiving as much energy as it loses. Since it is only receiving about 160W-2 from the Sun, it must be receiving – on average – about another 330Wm-2 from somewhere else. This is essentially the greenhouse effect; radiatively active gases in the atmosphere block outgoing long-wavelength radiation, returning some energy to the surface, and causing the surface to warm up to a higher temperature than would be the case were there no such gases in the atmosphere (or, no atmosphere).
We also know that the planet as a whole is in approximate thermal equilibrium (well, again, before we started adding GHGs to the atmosphere) and that we absorb – on average – 240Wm-2 from the Sun. Therefore, we must be ultimately radiating 240Wm-2 back into space. Since it is the atmosphere that is blocking energy from being radiated directly from the surface to space, one way to think of this is that there is some effective radiating layer in the atmosphere from which we lose as much energy into space (240Wm-2) as we gain from the Sun. However, as illustrated by the Trenberth energy flux diagram, it’s not quite that simple; some does come directly from the surface and some from within the atmosphere. We also know that – in reality – more complex physical processes (such as convection and evaporation) play an important role in setting temperature gradients in the atmosphere. However, we can still get a good idea of what’s happening by considering these fairly simple illustrations and calculations.
We can also use this to understand what will happen if we add more greenhouse gases; it makes the atmosphere more opaque to outgoing radiation and raises the effective radiative layer to a higher altitude. This causes temperatures below this layer to increase so that the amount of energy being radiated back into space once again matches the amount of energy being received from the Sun. It is simply an enhanced greenhouse effect.
The above is actually a rather lengthy and convoluted way to introduce something I encountered recently. I came across a blog post that critiques Peter Ward’s ozone depletion theory. Peter Ward’s basic idea is that CO2-driven warming is wrong and that what is causing global warming is the depletion of ozone. His basic idea (which is wrong) is that ozone absorbs ultra-violet (UV) radiation, that there is much more energy in the UV than the infrared (IR), and therefore that the warming is driven by changes in the UV flux driven by changes in ozone. His basic error is that even though a UV photon has much more energy than an IR photon, this does not mean that there is much more energy in the UV than in the IR (you also need to account for the number of photons in each wavelength band)
What I found interesting is that Peter Ward made an appearance in the comments and we had a rather lengthy exchange of views. It was quite a pleasant exchange and only became somewhat tetchy towards the end. However, Peter Ward was completely unwilling to quantify his alternative theory and claimed that the standard methods for determining energy fluxes (as in the Trenberth-like energy flux diagram) are simply wrong – apparently because the energy of a photon is (which he – incorrectly – kept claiming was the energy per square metre).
Although a little frustrating, I found this discussion quite fascinating. Someone is proposing an alternative to a well accepted theory, but won’t quantify their alternative and suggests that a lot of very basic physics is simply wrong; physics that has been extremely successful for a very long time. This is also physics that virtually every university in the world teaches its undergraduates and that has been used extensively in the development of advanced technologies that many of us use every day; are we just getting it right by chance?
To me, if you’re going to suggest an alternative to something that is well accepted, you have to be willing to actually show how it works quantitatively; you can’t just hand wave. This is especially true if your alternative requires that some very basic things, that are accepted by virtually everyone else, are fundamentally wrong. If you can’t – or won’t – quantify your alternative, then the chances of you being correct is pretty small. If your alternative idea also requires that well-accepted ideas that can quantitively match what we observe/measure are wrong, then the chances of you being correct becomes negligibly small. Given this, the conclusion of the post where I encountered Peter Ward’s ideas is almost certainly correct
his theory is garbage.