Sea ice again

Posted by William M. Connolley on November 21, 2007
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So, the question was, how much effect does removing the Arctic sea ice make to the rest of the world? In particular, what is the direct effect, ignoring all the feedbacks that would occur on SST and so on? Happily, an atmosphere-only GCM can answer this question, and happily I had one to hand, viz HadAM3.


Happily also it runs fairly fast nowadays – about 20 years/day on 8 opterons – so here are results from 12 years of run, compared to 50 years of control run. The difference between the two is that I’ve wiped out the Arctic sea ice (in all seasons). It gets replaced with sea at (I’d guess) -1.8 oC – considerably warmer than the sea ice there previously.

So what happens? Not all that much. Here are some zonal mean pictures, by season: top annual, bottom june-july-august (ie NH summer) and middle NH winter. Unsurprisingly the winter changes are largest; perhaps surprisingly the summer changes are small. But then the control run ice is probably not far off freezing in summer anyway. There are 50 black lines for each year of the control run, and 12 blue lines for the anomaly.

i-77086c5d40d8d156708ddbd4e230ec92-ppn.zm.png

The only really obvious change is over the Arctic basin itself. And then a map of the mean change, again by season. I’ve plotted the fractional change – ie, (anomaly-control)/control. Don’t get too carried away analysing individual coloured blobs – many of them are probably not stat sig. It may as well run out for a few more years before being done for sig.

i-98495dbdf021d6a1162f799f1ba62a7d-map-ppn.png

Disclaimer: all this was run off fairly quickly and there may be errors. If you see anything obvious, do let me know.

[Update: 20+ years now, and things settle down a bit. See here for another map pic. The US stuff disappears; but the drought in Australia proves robust :-) ditto some of the stuff over Africa -W]

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Comments

  1. #1 ice
    2007/11/21

    i don’t get it:

    how come you don’t have more evaporation in summer over open artic waters ? wouldn’t you expect more precipitation, then ?

    [Probably because the sfc T doesn't change that much. It was neear zero anyway (I've just checked). You aren't going to tell me that water at 0 oC evaporates more than ice, are you :-)? -W]

  2. #2 Gavin
    2007/11/21

    I don’t think this tells you what you think it does. Since SST elsewhere is fixed (right?), the potential for bigger changes elsewhere is minimal. However, if you use a slab model (with variable SST), you need to think about a way to melt the ice consistently within the energy budget of that model (increased ocean heat convergence into the Arctic might do it, or a big change in albedo). Then you of course get into the problem of whether the reason why sea ice disappears is related to it’s larger scale effects.

    However, in your run you could diagnose the potential for larger effects by examining the anomalous net surface heat budget over the ocean – anomalies would indicate the likely sign of any feedbacks.

    [I agree, there would ineveitably be further affects from the removal of the sea ice. However, what I really wanted to see was what difference *just* removing the sea ice made. People have made all sorts of assertions of how it would affect drought in America, etc etc. I am surprised by how little global effect it has -W]

  3. #3 Steve Bloom
    2007/11/21

    IIRC the model result showing a connection between western U.S. drought and Arctic sea ice was not a straightfoward cause-and-effect, so that bit seems to be a little bit of a strawman.

    And of course, what everyone really cares about (I think) is what effect there will be on the GIS, and for that I suppose a much more elaborate exercise including all the feedbacks is needed. Given the recent need to catch the models up with Arctic sea ice reality, I suppose it’ll be awhile before anything along those lines sees publication.

  4. #4 Hank Roberts
    2007/11/21

    > what difference *just* removing the sea ice made

    This reminds me of the way Lubos calculates climate sensitivity — doubles CO2, with no other change, and says that shows climate sensitivity is very low, I think?

    Hope you can rerun as Gavin suggests to compare results.

  5. #5 bigTom
    2007/11/21

    I would think that with a fixed boundary SST temperature, that the main effect would be in the amount of heat absorbed by the arctic ocean. Without letting that effect SSTs elsewhere I wouldn’t think you’d see much of an effect. I think it does illustrate one conjecture however, the extra heating due to the albedo change will be initially buffered by the Arctic ocean. I suspect that effects on the rest of the system will take awhile as the oceans adjust to the new boundary conditions.

  6. #6 Gareth
    2007/11/21

    It’s difficult to resist commenting on your coloured blobs…. there’s a dry area over the SW USA, and it looks like something fairly dramatic’s going on over the Sahara.

    If I read what Masters and Ostro are saying correctly, they are suggesting that current changes in the Arctic are already affecting (and can be seen in) NH weather. It would be useful if you could somehow tweak a model to look at what impact the transition to a summer ice-free Arctic (with winter freeze up) has on NH atmospheric systems. This means you have to get the overall heat budget right, as Gavin points out.

    If Masters’ conjecture is correct, and the loss of summer ice means longer, warmer NH autumns and late winter onset, arguably this amounts to abrupt climate change. The impacts may take a few seasons to show, but the effects on ecosystems and agriculture are unlikely to be minor. A friend in Sweden, commenting on last winter, told me that severe frosts arrived late (January), but before significant snow cover (in Feb), which caused a lot more damage than normal. Snow cover usually arrives before the coldest part of winter.

  7. #7 Eli Rabett
    2007/11/22

    As I recalls, the water has some salt content, so the melting point is below 0 C. That being the case the sat vap pressure above the ice is lower than that above the water. The vp will be a few mbar and the difference a few tenths. As far a global effects, how about the extra greenhouse gases H2O and CO2 that get loose??

  8. #8 Bryan Lawrence
    2007/11/22

    So I would expect there to be some sort of Rossby wave scale response (yeah, ok, it is close to the pole, so I need to think about that, but anyway) which you wouldn’t see in zonal means … basically you have introduced a major temperature anomaly, and there must be *some* response. Your job is to find it :-), which may not be easy since wavelike responses may require some statistical analysis to locate.

    [There are indeed responses in MSLP - in fact there are sig changes all through the tropics. But they are tiny -W]

  9. #9 Benjamin Franz
    2007/11/22

    It turns out Jacob Sewall and Lisa Sloan at UC San Santa Cruz also did some simulations back in 2004/2005

    Sewall, JO and Sloan, LC, Disappearing Arctic sea ice reduces available water in the American West, Geophysical Research Letters, v. 31, L06209, doi:10.1029/2003Gl019133, 2004.

    Sewall did a second paper in 2005: Sewall, JO, 2005, Precipitation shifts over western North America as a result of declining Arctic sea ice cover, the coupled system response, Earth Interactions, v. 9, p. 1 – 23.

  10. #10 Adder
    2007/11/23

    Eli, salt also reduces sat vapor pressure of water. At the freezing point the vapor pressures of ice and salt water are equal.