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SMMR/SSM/I  brightness  temperature  record  with  some  care.  The  tricky  part  is 
comprehensive inter comparison and validation. Few groups appear to have the motivation or 
resources  to  perform  truly  comprehensive  inter comparison  and  validation  studies,  which 
would be a necessary part of deciding which one algorithm (or combination of algorithms) is 
best for climate monitoring purposes. 
It is also necessary to critically assess the passive microwave record in the context of 
extended  GCOS  climate  monitoring  principles  (in  terms  of  its  stability,  homogeneity  and 
continuity). Given that I have had trouble finding out whether or not there are stability issues 
with the passive microwave data, I suggest this sort of information could be better presented. 
Climatically speaking, the 25 years of the passive microwave is insufficient by itself to study 
decadal variability, so homogeneity over the whole data record must also be assessed. In order 
to do this, we need a good understanding of how charts are and were created and exactly how 
this relates to a passive microwave concentration field. 
A  lot  of  historical  chart  data  for  the  Arctic  have become  available  over  the  last  five 
years or so, which is a major step forward. There has been less improvement in the Antarctic, 
but there are fewer data to find and those that are around are likely point observations which 
will need digging out of archives and assembling or reconstructing. The first meeting of the 
JCOMM  Expert  Team  on  Sea  Ice  agreed  to  prepare  historical  information  for  the  Southern 
Ocean  and  a  report  is  due  this  autumn.  In  the  meantime,  AARI  have  useable  data  in  their 
archives  for  the  1950s  and  1960s  and  I  believe  NSIDC  plan  to  digitise  these  shortly. 
Assuming  the  Expert  Team  survey  identifies  some  potentially  fruitful  data  sources,  OOPC 
should encourage funding for digitization of historical Southern Ocean sea ice. 
The usefulness of passive microwave retrievals in summer time is seriously limited by 
the  instruments'  inability  to  see  through  melt  water  and  wet  snow  on  top  of  the  ice.  More 
work  on  finding  ways  around  this  should  be  encouraged  (by  approaching  funding  bodies). 
This research is best done by remote sensing experts. However, the OOPC should first check 
the  status  of  the  EUMETSAT  Ocean  and  Sea  Ice  SAF  project  as  they  appeared  to  be 
intending to go in the right direction in this regard. 
Uncertainties in passive microwave sea ice concentration retrievals are estimated in a 
limited fashion for some algorithms and given in the literature, so encouragement should be 
given  to  derive  comprehensive  fields  of  uncertainty.  The  inability  of  most  algorithms  to 
retrieve thin ice should also be reflected in these estimates, as this is important for monitoring 
of  total  ice  extent.  However,  complete  and  meaningful  uncertainty  estimates  will  not  be 
available  until  a   best   (combination  of)  algorithm(s)  is  identified,  the  stability  and 
homogeneity  of  the  record  is  assessed  and  we  can  retrieve  more accurate  concentrations  at 
times  of  melt.  It  is  also  necessary  to  encourage  production  of  error  estimates  for  historical 
chart  data  sets,  so  these  can  be  meaningfully  compared  with  satellite  retrievals  and  their 
differences understood. 
Progress  will  likely  be  best  made  by  liaising  with  the  lead  author  team  on  the 
Observed  Cryosphere  chapter  of  IPCC  4AR  when  it  is  announced.  OOPC/IPCC  could  use 
their  combined  influence  to  try  and  encourage  focused  work  (aimed  at  solving  the 
aforementioned  problems,  which  I  see  as  the  main  challenges  facing  any  progress  in  being 
able  to  say  anything  new  about  sea  ice  change  in  the  next  IPCC  report)  from  data  set 

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