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Greenhouse Gas Theory & Arctic Amplification

August 19, 2013
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By Paul Homewood

 

According to theory, increasing GHG’s should lead to greater warming in the Arctic than elsewhere, a process known as Arctic amplification. There are two main principles underlying this theory:-

 

1) By far the most powerful atmospheric greenhouse gas is water vapour. Water vapour shares many overlapping absorption bands with CO2 and therefore an increase or decrease in atmospheric CO2 has limited effect on the overall rate of IR absorption in those overlapping regions, if water vapour is present in sufficient quantity. In the Polar Regions , the air is dry due to prevailing low temperatures, allowing CO2 to exert a much greater influence than would be possible in warmer and moister air masses at lower latitudes.

 

2) The albedo effect. Rising temperatures will usually increase melting of snow and sea ice, reducing surface reflectance, thereby increasing solar absorption, which raises temperatures, and so on. Conversely, if climate cools, less snow and ice melts in summer, raising the albedo and causing further cooling as more solar radiation is reflected rather than absorbed.

 

Both of these effects should be most noticeable in summer, when radiation from the sun is at its greatest. Indeed, at the highest latitudes, there should little or no GHG effect during winter, when there is no sunlight at all.

So do we see any increase in Arctic summer temperatures? Let’s take a look at the last 5 years of DMI temperatures for north of 80 degrees.

 

 

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http://ocean.dmi.dk/arctic/meant80n.uk.php

 

Remember that the green, mean line is based on 1958-2002. In all five years, the temperatures during the summer months is at or below this mean, whereas it is the winter months that tend to be warmer than normal.

Back to the drawing board!

6 Comments
  1. August 19, 2013 12:03 pm

    Reblogged this on CraigM350.

  2. @Sceptic_Tank permalink
    August 19, 2013 12:31 pm

    Nice one. I wonder what the good people at WWF and Greenpeace will have to say in response.

  3. Brian H permalink
    August 21, 2013 4:25 pm

    Their theory is right, with the exception of the driver: CO2 warming is nonexistent.

  4. Scott permalink
    September 9, 2013 6:51 pm

    Hi Paul,

    A couple things. First, a critique:

    I would expect GHG warming to be most obvious during the winter because it only affects outgoing radiation and not incoming light. Thus, its warming should be most apparent when not confounded by warmth from incoming solar irradiation. Also, the absolute humidity in the winter will be far lower than in the summer, decreasing water vapor’s influence and making CO2’s effects most prominent in the winter.

    Second, some things you left out:

    Polar amplification should happen from any warming source, not just GHGs, and also in the absence of any feedbacks. Why? Two reasons. First, the heat capacity of cold air is less than that of warm air so a uniform increase in internal energy (U) across the entire globe would show up in temperature most prominently in the Arctic. Basically, dT/dU is highest there, and that is even more true in the winter. Second, the ability to emit radiative energy goes up as T^4. So a shift from 34 to 35 C in the tropics would yield a change in radiative ability proportional to 116 million K^4. But a change from -35 to -34 C in the Arctic equates to only 54 million K^4, or about 53% less. So the first reason is thermodynamic and the second is kinetic and both lead to a more warming in polar regions than elsewhere (as we measure it through temperature), irrespective of the source of that warming or any feedbacks. When most people go all nuts on polar amp, they typically haven’t thought of correcting for these issues. Estimating corrections for either of these issues individually is pretty straightforward, but estimation a combined correction factor is tougher (and I don’t know if it’s been done.) I therefore think that most people’s interpretations of what is amplification is overstated because of significant portion of the signal they’re looking at is due to the way we measure things (i.e.–not temperatures vs energies) and the expected result absent any amplification.

    Personally, I think that reduced sea-ice cover in the winter is a big negative feedback (though that’s not true for land ice/snow). CAGWers always tend to ignore that factor…

    Just my thoughts, and thanks for your work on this blog.

    -Scott

    • September 9, 2013 8:12 pm

      Thanks Scott

      Three questions (and I don’t know the answers!)

      1) Would there not be more radiation (and therefore more GHG effect) in the summer when the sun is up?

      2) There seems to be much more variability in winter temps than summer ones in the Arctic, judging by the DMI graphs. Would not this much noise make it difficult to spot any underlying trends during winter?

      3) Albedo is obviously part of the amplification effect. Would not this be most apparent in summer , not only as the sun is at max, but also as most of the comparative ice loss, compared to earlier years is during summer/early autumn?

      Paul

      • Scott permalink
        September 9, 2013 9:33 pm

        Hi Paul. I’ll answer the questions to the best of my ability.

        1.) Yes, there would be more GHG effect, in an absolute sense, in the summer. However, the relative effect–GHG vs everything else–will be stronger in the winter when you don’t have a huge source of energy–sunlight–and everything that makes it vary (clouds, albedo, aerosols, etc) causing a significant amount of noise.

        2.) The noise in the signal during the winter would make it nearly impossible to see a good signal looking at just a single year. One would have to average several years to get a better result I’d think…either that or do a long-term trend kind of analysis. But the reason the summer signal has less noise is because it’s tightly linked to the melting temperature of the ice. As you know, ice has a huge latent heat of fusion, and therefore anything near a large ice sheet will have a temperature near its melting point. Considering that other variables like differing sensors and their locations are required for the DMI analysis, I don’t think that looking at summer temperatures can give one any more than the crudest of estimates of what’s going on. In fact, I’d go as far as to say that, during the summer, the amount of ice melt drives temperature more than the other way around.

        3.) This is a tough question. The input of the albedo feedback clearly only happens during summer months. Presumably, it’s effect is lagged though while the extra energy slowly bleeds from the system. Until any extra melt (if any) caused by the albedo changes have refrozen, it will affect the temperature reading.

        Given these thoughts, I’d think that the best place to look for JUST GHG changes in the Arctic would be to look at the time of year most removed from the solar irradiation and open waters. Maybe something like the average temp from Jan/Feb? The other option would be to look at the lower troposphere data. IIRC, the LT data shows far less warming than most surface sites for the Arctic.

        Regards,

        -Scott

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