The Greenhouse Effect

I’ve ended up in a rather silly discussion/debate on Twitter about the Greenhouse Effect. I realise it’s rather pointless, in that those who dispute it are very unlikely to accept its reality. However, it’s still interesting to think about how to construct an argument, and it gave me an opportunity to highlight some papers and articles that might be of interest to others. I thought I might repeat some of it here.

One of the arguments being made is that noone has demonstrated that CO₂ can provide the backradiation that heats the surface. Firstly, it’s not simply CO₂, it’s emission back to the surface from the lower atmosphere. Also, given an average surface temperature of around 288K, we know the surface must be radiating almost 400 Wm². It also loses just over 100 Wm² through evapo-transpiration and thermals (see energy balance figure on the right).

However, the surface receives less than 200 Wm² from the Sun. Hence, to be in energy balance, there must be an additional energy flux, with a magnitude of over 300 Wm². This is the back-radiation from the lower atmosphere. If it wasn’t there, the surface would be losing much more energy than it was receiving and would rapidly cool. This clearly is not happening, hence there must be some backradiation. I should add, that it has also been measured.

Another argument is that the effect of CO₂ is now saturated. This is an old argument that has been addressed a good number of years ago. It illustrates a rather fundamental misunderstanding of the Greenhouse effect. What’s not always well understood is that convection in the lower atmosphere (troposphere) is important. Convection plays a role in the atmospheric temperature decreasing with increasing altitude; it gets colder as you go up in the atmosphere.

The presence of greenhouse gases in the atmosphere prevents all of the energy from being radiated to space directly from the surface. Instead some of the energy is radiated to space from within the atmosphere. Since the system will tend to energy balance, and since temperature decreases as you go to higher altitudes, this means that the surface has to be warmer than it would be in the absence of an atmosphere; the Greenhouse effect.

Also, as illustrated very nicely in the Figure on the right, taken from this Realclimate post, adding more greenhouse gases to the atmosphere increases the altitude at which energy is radiated to space. Again, since the temperature decreases with altitude, this would then require that the lower atmosphere and surface would have to warm to return the system to energy balance; the enhanced Greenhouse effect. This is what we’re doing now with our emissions of CO₂, and other greenhouse gases.

A final claim is that most of our recent warming is due to increases in absorbed solar radiation, which contradicts what’s expected from the Greenhouse effect. This is a somewhat subtler point but, again, illustrates a misunderstanding of how the Greenhouse effect actually works.

A somewhat simplistic view of the Greenhouse effect is that adding greenhouse gases to the atmosphere reduces the outgoing longwave flux, which then recovers as the system warms back to equilibrium. However, as I explain in this post, there are both short- and long-wavelength feedbacks. The outgoing longwave flux actually recovers quite quickly. However, the shortwave feedback (mostly due to changes in clouds) leads to an increase in absorbed solar radiation, which causes the system to keep warming, increasing the outgoing longwave flux to above the level it had before the greenhouse gas concentrations increased.

So, in a sense, this continued warming is due to an increase in absorbed solar radiation, but this doesn’t mean that this wasn’t caused by the initial increase in greenhouse gases in the atmosphere. It doesn’t somehow contradict what we expect from the Greenhouse effect.

Anyway, I think it’s useful to think about these things even if arguing about it on Twitter is mostly pointless. A key point, which I’ve probably highlighted before, is that if someone thinks they’ve encountered some simple, and obvious, reason why a well accepted scientific argument is flawed, maybe they should first check that they properly understand the scientific argument they claim to have overthrown.