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O Say can you See Ice…

Filed under: — gavin @ 6 November 2017

Some concerns about continued monitoring of sea ice by remote sensing were raised this week in Nature News an article in the (UK) Observer: Donald Trump accused of obstructing satellite research into climate change. The last headline is not really correct, but the underlying issues are real.

What is this about? Since the late seventies, there have been almost continuous observations of polar sea ice by passive microwave sensing on multiple polar-orbiting satellites. This is the preferred technique since microwaves from the surface can penetrate clouds (which are abundant in the polar regions) and can be detected during the day and night – again, important for the wintertime at the poles.

The current workhorse satellites for this measurement are the (aging) DMSP F-series (managed by the UASF). There are two currently operational for sea ice retrievals, F-16/18, which are 14 and 8 years old respectively. Another, F-17 is still in orbit, but may not be usable on its own for sea ice (. The design lifetime was nominally 5 years. A replacement satellite, F-19 failed completely in October, but in fact had not been useful for sea ice since February 2016. The last satellite in the series (F-20) was built two decades ago and kept in storage, but was decommissioned finally in November 2016 after a decision in Congress to no longer fund it in the FY16 budget. This was after the election, but before the inauguration of the Trump administration.

[Note: A comprehensive (though not always up-to-date) resource on satellite capabilities (OSCAR) is available from WMO if you want to navigate this for yourself.]

While many instruments can be used to detect sea ice, the continuity required for long-term climate monitoring makes it vital that the different products are cross-calibrated and have similar characteristics to be useful. The closest instruments to the those on the DMSP satellites are the radiometers (MWRI) on the Chinese Feng Yun-3 series of satellites. Unfortunately, again because of Congress, NASA collaborations with China are restricted and since the sea ice work at NSIDC is funded by NASA, that might prevent this source of data being used in the US (though presumably non-US colleagues would not have this problem).

Another possibility is the Japanese satellite GCOM-W1 which has a more advanced AMSR2 instrument (in space since 2012, and has also passed it’s design lifetime), but the merge of this data with the DMSP satellites is still a work in progress. This is being used for the Bremen University sea ice maps though.



Differences in views from passive microwave instruments (SSMI vs. AMSR2) via Arctic Roos.

Unfortunately, the next scheduled passive microwave sensor to be launched is not until 2022 on the European Space Agency’s 2nd Generation MetOp satellite, and will need a year’s overlap with an existing satellite to be optimally calibrated. Thus the likelihood of a gap in the record developing before then is very high.

Other measures of sea ice will be possible – IceSat2 is launching next year will have an active laser altimeter to measure sea ice height, satellites with visible or infrared capabilities are able to see ice when it’s not dark or cloudy, but cross calibration into a homogeneous record will be hard.

To be clear, people have been warning about this looming lack of capability for a while – in April and May 2016, as well as more recently. Unfortunately, new satellites and new instruments take a long while to develop, build and launch, and possibly we’ve been taking them for granted.

That probably needs to stop.

Note. Thanks to Walt Meier at NSIDC for chatting about these issues with me.

14 Responses to “O Say can you See Ice…”

  1. 1
    Mac Sean says:

    Does anyone know, what rough percentage of the absent IceSat data, is the IceBridge project able to provide, while we wait for IceSat2 to come online?

  2. 2

    What about the microwave sensors on the new JPSS series, which launches (very early) Friday morning? Can that data be calibrated and used to fill the future gap? And thanks for the post.

    [Response: Apparently not. This is a good summary of the SSMI algorithms: http://onlinelibrary.wiley.com/doi/10.1002/2017JC012768/full and you’ll see that they use the 19 and 37 GHz channels with both vertical and horizontal polarization. The JPSS-1 ATMS instrument has channels at 23.8 and 31.4 GHz with only horizontal polarization (The same instrument is flying on SUOMI-NPP). While you can certainly see the sea ice in these channels, the mixing algorithm would be different and I imagine that you still have some calibration issues. Maybe some one with more experience can chime in? – gavin]

    [Further Response: This paper discusses some of the calibration issues with using the AMSU sensors to detect sea ice https://www.researchgate.net/profile/R_Ferraro/publication/224167872_A_New_Sea-Ice_Concentration_Algorithm_Based_on_Microwave_Surface_Emissivities-Application_to_AMSU_Measurements/links/55fad6e508aec948c4af37fb/A-New-Sea-Ice-Concentration-Algorithm-Based-on-Microwave-Surface-Emissivities-Application-to-AMSU-Measurements.pdf The bottom line seems to be doable, but with greater seasonal biases than SSMI associated with surface emissivity changes (associated perhaps with deep snow, melt ponds etc). – gavin]

  3. 3
    Sou says:

    Your report of this is appreciated, Gavin. You’ve answered a number of questions that occurred to me, and probably to other people as well. Thank you.

    The fact that there may be a gap in sea ice records is disheartening. The fact that resources will need to be diverted to minimise the loss of data is also a shame. Those resources could have instead been devoted to other research.

    This is an issue on which it would be tremendous if there could be more effective international collaboration and financing and planning ahead, given the lead times involved. (At the government level, not just the scientific level.)

  4. 4
  5. 5
    Adam Lea says:

    “You might also want to mention this comment: http://www.drroyspencer.com/2017/11/trump-wrongly-blamed-for-destroying-sea-ice-satellite/

    A quick look at that site reveals this:

    “This website describes evidence from my group’s government-funded research that suggests global warming is mostly natural, and that the climate system is quite insensitive to humanity’s greenhouse gas emissions and aerosol pollution.”

    http://www.drroyspencer.com/global-warming-natural-or-manmade/

    This, along with the usual “alarmist” labels flying about, and the dumb coments, makes me very skeptical about the truth of anything on that site.

  6. 6

    #4–

    Yes, that’s a good post from Dr. Spencer, and seems pretty consistent overall with what Gavin said, both in the original post and in the responses at #2. Just don’t waste your time with the comments!

  7. 7
    Eric Swanson says:

    #2 – Regarding Gavin’s comments on the sea-ice concentration algorithms, it’s well known that melt ponds on the surface of the sea-ice impacts the microwave data. Since the concentration calculation provides an average for each pixel seen by the instrument, the addition of melt ponds to the surface gives the appearance of more open water, i.e., lower concentration. Here’s an open source article discussing the situation:

    The impact of melt ponds on summertime microwave brightness temperatures and sea-ice concentrations

    The authors note:

    “A satellite brightness temperature measurement of a mixed scene is therefore composed of contributions from the open water, i.e., cracks, leads, melt ponds, and from the (snow covered) sea ice. This has two main consequences for a sea-ice concentration product computed from such coarse-resolution satellite measurements. The sea-ice concentration in the presence of melt ponds is likely to be underestimated – because melt ponds are seen as open water.”

    It should be noted that melt ponds also impact the microwave brightness temperature measurements by the MSU/AMSU instruments which have been used to monitor atmospheric temperature changes. An increase in melt ponds, in addition to any increase in open water, results in a reduction in surface emissions, which will appear as a cooling trend in the resulting data. Given that there is a well documented negative trend in measurements of sea-ice “area” from the various measurements, this would imply that the MSU/AMSU data is an unreliable indicator of climate change over regions with polar sea-ice.

  8. 8
    Alastair McDonald says:

    Eric,

    Do you think that the low global sea ice extent shown here:

    https://ads.nipr.ac.jp/vishop/#/extent (click on Global
    could explain the high October temperature recorded by Roy Spencer?

  9. 9
    Nichol says:

    I think the world seems much too dependent on the USA and NASA for these environmental observation satelllites. The USA isn’t anymore a trustworthy hegemon we can simply rely on to keep an eye out for the world. They’ve become more and more unreliable. This means more redundancy is necessary if we’re to avoid gaps, with more satellites from more countries, and the data shared as widely as possible. Make Earth Great Again.. working together.

  10. 10
    Eric Swanson says:

    Alastair McDonald #8 – As you should know by now, the sea-ice changes are the result of a years long process. Whether one month is warmer or cooler has little to do with the seasonal growth and melting of the sea-ice. As for Roy Spencer’s October report, isn’t that blip a bump on the curve of Global MSU/AMSU “Lower Troposphere” (LT) data? The monthly UAH TMT and LT reports include selected results for the North Polar region and also split that into land and ocean sub-sets with the ocean representing about half of the area in the North Polar data. One would think that the North Polar Ocean data should exhibit an influence from the documented decline in sea-ice area, but there’s little difference between the two TMT trends.

    The latest UAH v6 analysis treats the MSU data differently than the AMSU data, which may add further confusion to their results. They claim that their TMT covers latitudes 60N to 90N, whereas I fail to see how the MSU/AMSU data can cover all the way to the North Pole, given the scan geometry which trends along a latitudinal direction at these highest latitudes. They bin the scan grid data into different 1×1 degree grids for each scan position, then combine these data in the vertical direction to calculate a final value for each position in the resulting grid. Since this process poleward of the highest ground track latitude (~82) results in the lowest level grids being unfilled, they must do something to make things fit. They have previously claimed that they interpolate over the pole to fill in the data, perhaps repeating this approach with v6, a process with dubious results, IMHO.

  11. 11
    Matt says:

    @Mac Sean – Operation Ice Bridge (OIB) was designed to provide critical “gap filler” data from airborne lidar in the areas deemed to be changing the most quickly between Icesat I and II. While it is an important and valuable dataset, it does not provide anything close to the synoptic view from space that DMSP and other polar orbiting imagers provide.

  12. 12
    Eric Swanson says:

    After my post #10, I thought to call my own bluff, so to speak. I used the techniques from my recent paper to analyze the RSS TLT for the Arctic. I applied the band pass filtering process to both the land and ocean time series, which allowed the visualization of the remnant annual cycle which resides in those time series. The band passed results exhibit little long term trend, as most of the trend is captured in the 25 month filtered series. However, there is an unusual annual cycle contained within the filtered results. I split the band passed series in half, with an April thru September portion for Melt season and an October thru March portion as a Freeze season. The UAH LT also exhibited a cycle similar to that in the RSS TLT, though the magnitude was less. These results support my claim that trends the satellite data for the Arctic Ocean are negatively impacted by changes in the annual sea-ice cycle.

    The RSS results showed a pronounced cooling trend in the ocean data for the Melt season and a warming trend for the Freeze season. The months of June and July exhibited the strongest cooling, while December and January showed the strongest warming. Looking at the land only data showed a different seasonal result, with week warming or cooling over the year. There are several complicating factors in this result, such as the use of a 2.5×2.5 degree grid, which would include both land and ocean in many grid boxes, as well as the fact that the ocean area includes regions which have not experienced sea-ice during the recent past. A more detailed analysis may provide stronger evidence of the impact of the sea-ice cycle on the satellite data over the Arctic.

    Analytical support for this conclusion can be found in the paper by Groody et al. (2004), Appendix A. (I hadn’t previously noticed that Groody et al. referenced my 2003 GRL paper in their Appendix A. How time flies…)

  13. 13
    Alastair McDonald says:

    Eric,

    Your second post seems to have confirmed what I suspected after your first post. You did not realise that I was referring to global sea ice, not Arctic sea-ice. The page pointed to by the URL I quoted does show a graph of Arctic sea-ice. To see a graph of global sea-ice you have to click on the global button, as I hinted.

    There is another graph of global sea ice extent here. You can see there, that there have been major changes in the global sea ice over the last year.

    Until last year the Antarctic sea ice had been increasing while the Arctic ice was decreasing, and so the global sea-ice anomaly had remained farly stable. Now, the Antarctic sea ice has suddenly decreased, which has not been offset by a slowing of the Arctic decrease.

    The anomaly in global sea ice is quite impressive though not as great as last year. Currently, my idea is that the last El Nino caused a tipping point to be passed, where the increase in the Antarctic sea ice melt has caused the Arctic sea-ice melt to slow. We know that there is a polar seesaw.

    How that effect would affect the temperature of the mid-trosphere, which the satellites measure, is question I thought you might be able to answer.

  14. 14
    Steven Sullivan says:

    “UASF”? maybe you mean “USAF”?

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