May 11, 2005

Humanity's Greenhouse Footprint

Writing about web page http://cdiac.esd.ornl.gov/pns/current_ghg.html

Let's tackle the 'humanity not responsible for most of the greenhouse gas' thing, now.

First port of call – the actual figures.

Carbon dioxide % contribution: 25.3%
Methane % contribution: 60.2%
N2O % contribution: 14.8%
Ozone (not in ozone layer) % contribution: 26.5%

These aren't small numbers. And there are currently no competing theories to explain them except human factors – many of these figures are unprecedented for 400,000 years, a time period which encompasses grapes in England, ice skating on the Thames and the last mini-ice age.

Now, we need to tackle the claim that the above doesn't matter. The gist of this particular argument is that water vapour allegedly dominates greenhouse gases, so these don't have an effect.

There is some partial truth in this – depending on who you ask, water vapour accounts for between 88% and 98% of the overall effect. But this does not affect the case for climate change. (This should be unsurprising, since these figures are from IPCC, who have built it into their models.) Why not?

As should be obvious by now, it's because our models are more complicated than what the public imagines. Firstly, there is a non-linear aspect to this – small perturbations can have a greater impact than you think. After all, our modern, human temperature scale is entirely subjective – a disastrous change of 5 C is only a tiny wobble if we look at things from the scale of a planet which would be really really cold without any greenhouse effect. But there is a broader argument to be made. What is significant is not just the power and volume of greenhouse gases of each type, but also their longevity. This determines how long the gas stays in the atmosphere, and also how quickly it returns to an equilibrium.

Carbon dioxide has a variable lifetime. To fast acting sinks are dominant, then 5 years. If slow acting sinks are effective (and this is increasingly the case due to rainforest damage and so on), then up to 100 years. Methane has a lifetime of 12 years. N2O has a lifetime of 114 years.

Water vapour has a lifetime of 1 week.

What does this mean? It means that any excess or deficiency is quickly balanced out. We can see this with things like rain, and snow. It means also that water vapour cannot have a forcing role on the climate. Unlike the other greenhouse gases, which can pool and accumulate, water vapour doesn't hang around long enough to have a real effect. For that, you would need a steady, long term influx of water vapour, and no such source exists, either natural or man made. (Hmm, human factories come close, but we can mostly ignore this for now…)

This limits water vapour to a reactive greenhouse gas. I.e. it is implausible for it to directly cause any climate change, but it is perfectly plausible for something to change the atmosphere, and so change the water vapour equilibrium, which can then have an effect on the climate. The question, then, we should be asking is what this reaction is.

The result is well known to meteologists. Warm air can hold more humidity than cold air. And sustained increase in temperature, such as that caused by carbon dioxide greenhouse effects, then, shifts equilibrium to more water vapour in the air, which has a further greenhouse effect. The role of water vapour then, is that of a positive feedback. It makes climate change worse. That's why our climate models are spot-on.

So the human effect on the greenhouse gases do matter.

QED

References:

American Geophysical Union
Carbon Dioxide Information Analysis Centre
William Connolley, British Antartic Survey


- 5 comments by 1 or more people Not publicly viewable

  1. I call into question in particular the suggestion that all 25% of that carbon dioxide figure you gave is man-contributed. For a start, since 1750 there has been a natural increase in temperature, in addition to the so-called greenhouse contribution. This increase in temperature affect sea temperatures. A little known fact is that much of the world's CO2 is stored in sea-water, and a rise in temperature reduces the water's capability to retain CO2, so the oceans give off CO2 as they warm. While I don't have exact figures to hand, I think the contribution of natural warming to CO2 levels in this manner is significant. I thought a figure of 15% human CO2 contributions was more on the mark.

    11 May 2005, 23:02

  2. The post was fine until this:

    "That's why our climate models are spot-on."

    Weather, as you point out, is inherently chaotic, so no model we can come up with can possibly model all the factors involved. This doesn't invalidate the model, but it's an important point to bear in mind when looking at any point of either sides case.

    Also, a large factor in global temperatures is the fluctuation of the Sun's energy. We don't know enough about it to accurately model its effects or to do anything about this variation.

    11 May 2005, 23:10

  3. I call into question in particular the suggestion that all 25% of that carbon dioxide figure you gave is man-contributed.

    Sure. But that's where our long term records come in. The 25% figure is arrived at by taking the difference from the MAXIMUM carbon dioxide level over ice age cycles. In short, the maximum that we can expect from natural warming effects alone. Unless we insist that there is some special, non-human effect in place over the last 100 years that we don't see in the last 400 thousand, (which seems, in the absence of contrary evidence, to be absurd) then the conclusion is inescapable. That 25% must be from human factors, even if we are just talking indirect triggering of ocean carbon sinks. It's there because of us.

    Weather, as you point out, is inherently chaotic, so no model we can come up with can possibly model all the factors involved. This doesn't invalidate the model, but it's an important point to bear in mind when looking at any point of either sides case.

    Sure. No model is perfect. But in a finite set of possibilities, the model that is best is the one which we agree with. Models involving human-caused climate change simply kick the ass of models that assume such effects are irrelevant.

    Also, a large factor in global temperatures is the fluctuation of the Sun's energy. We don't know enough about it to accurately model its effects or to do anything about this variation.

    Yes and no. Yes, we don't know WHY solar energy varies. Not very well, anyways. But we have detailed records on this, and so we know HOW solar energy varies, and how much of an effect it has. Much has been made of our current solar energy levels are high. Yes, right. But they aren't at an unprecedented level like carbon dioxide levels are. We can look back over history to the last time solar activity was like this, and we find that climate change is nothing like what it is today. Solar activity has in fact reached a peak, and is now falling. But while temperature dipped a little, it is still rising. On the basis of even the simplest intuition, something is wrong.

    And in the end, combined models simply do give accurate predictions, and recreations. It is deeply unlikely that this is a fluke.

    11 May 2005, 23:48

  4. The claimed figures seem to be (from your source) the total increase since 1750. The idea that all of the increases of these gasses since 1750 are entirely down to human action is patently absurd.

    11 May 2005, 23:51

  5. 1750 corresponds to a maxima in the natural cycle of these gases, according to the long term sources I gave. The idea that all these increases are entirely down to human action is conclusively borne out by the available data.

    11 May 2005, 23:57


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