Atmospheric greenhouse effect

One of the aims of this blog was to address the science behind global warming and climate change. Probably more to address issues related to global warming than climate change as I think I understand the science related to global warming better than that of climate change. Anyway, the basic idea behind global warming is that the Earth has been receiving more energy from the Sun than it is emitting back into space. There are a number of consequences of this warming; increased global surface temperatures, increased ocean heat content, and reductions in the volume of Arctic ice.

If one considers the last 50 years, it is clear that global surface temperatures have risen (you can check this post if you want to see a plot of global surface temperature anomalies since 1880). If there is an energy imbalance such that the Earth is receiving more energy from the Sun than it loses to space, ultimately the global surface temperatures must rise so as to re-establish equilibrium. If, for example, the energy imbalance was due to increased solar flux, then the rising global surface temperatures would result in a rise in the temperature at the top of the atmosphere so as to increase the net flux from the Earth so as to eventually match the incoming flux.

On the other hand, if the energy imbalance is due – largely – to increases in greenhouses gases in the atmosphere then the surface temperature will rise simply to aim to maintain the net flux from the Earth. In fact, if adding greenhouse gases to the atmosphere has been driving us away from equilibrium you would actually expect the top of the atmosphere flux to be decreasing and, hence, the temperature at the top of the atmosphere to be decreasing. The figure below shows the stratospheric temperature since 1960 (I’m not quite sure what the different lines are but think they might be different altitudes).

Stratospheric temperatures since 1960.

Stratospheric temperatures since 1960.

The figure above (taken from a NOAA website called Climate Watch) shows that the despite the surface temperatures being 0.5oC warmer today than in 1960, the stratospheric temperature is more the 1oC lower than in 1960. This means that the surface is emitting more energy today than in 1960, while the stratosphere is emitting less today than in 1960. The only way this is possible is if the atmosphere is somehow absorbing more energy today than in 1960 (or – I guess more accurately – requires more energy today than in 1960). The only logical explanation for this is that the composition of the atmosphere has changed so as to enhance the greenhouse effect and produce this increased warming. I can’t really see any alternative. It can’t be the oceans (as Bob Tidale suggests). It could be natural, but what? The main change to the atmosphere has been the addition of CO2 due to our use of fossil fuels. I know in the previous post I discussed the possibility that it could be CFCs. Well, this would still be anthropogenic but it also requires an unrealistically high amplification factor, so doesn’t really seem viable.

I guess this is just another post to illustrate how the evidence for global warming is extremely strong. The evidence for climate change may not be as concrete but it is highly unlikely that adding extra energy to our climate system is not going to change it. It could be beneficial (although very few suspect this to be true) but assuming that it will be seems – to me at least – to be an extremely dangerous and, currently, unfounded assumption.

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7 Responses to Atmospheric greenhouse effect

  1. Gavin says:

    This graph shows MSU-LS record – the lower stratosphere. However the main impact on the temperatures is the reduction in stratospheric ozone, not changes in CO2. The CO2 related changes are far more visible in the SSU records which represent higher levels in the stratosphere.

  2. Lars Karlsson says:

    It is important to remember that there are several factors affecting stratospheric temperatures, including CO2, ozone (mainly in the lower statosphere) and volcanoes (very visible in the graph).

    Here is a recent review.

  3. Is it really the ozone? I’m surprised the effect of ozone is so large. Having said that, I don’t know enough about this, so you may well be completely correct. It does seem, however, that the SSU record is consistent with the basic picture I was trying to present in this post.

  4. Indeed. The volcanoes are clearly visible. Can changes in ozone really explain the stratospheric temperature anomaly though?

  5. BBD says:

    It seems unlikely that it’s all ozone. As ever, Science of Doom is worth a look.

  6. Thanks. It does seem, though, that what I’ve presented here is a little simplistic. Higher CO2 concentrations do seem to produce lower stratospheric temperatures but the reason does seem a but more complex than simply reducing the effective atmospheric temperature as it seems to be true even if the system is in equilibrium.

  7. Pingback: Attribution | …and Then There's Physics

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