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Hmpitcher
Washington, DC
Economist, climatologist, demographer, statistician: have computer will travel!
Interests: long-run scenarios, economic modeling, working with data, gardening
Recent Activity
Piomas surprised me by coming out with an"interim" number for sea ice volume (through 8/25). While it is clearly well below the record, it struck me that it probably was not going to lead to the kind of numbers that would mean that next year was automatically going to be an utter disaster. Rather than do anything really sophisticated in the way of analysis, i simply downloaded the additional daily data values and built a table showing the range of the ratio of (8/1 -9/15)/(8/1-8/25). Not surprisingly, there is not a lot of additional volume loss beyond 8/25 in the historical record. Not that we couldn't get more , but the kind of very low projected value that i had in early july doesn't look to be in the cards. (2.3 or thereabouts). why the model didn't work very well is an interesting problem that i have begun to chew on. (more on this real soon now). (incidentally, after the July number came out i redid the projection and the new projection was 3.6). the current set of results, (3.1 to 3.4 ) doesn't give those of us who worry about the arctic any comfort, but it also doesn't suggest that 2013 will be the year the arctic will be effectively ice free. table showing range of final projected sea ice volume. 1.11 2.41092 0.23892 3.36008 1.18 2.56296 0.39096 3.20804 1.22 2.64984 0.47784 3.12116 col (1) is ratio of final arctic sea ice vol (proxy is value for 9/15) less 8/1 to 8/25 less 8/1 Upper and lower values taken from a histogram of all data from 1979 to 2011. colum (2) is observed change (2.171) times column (1). Column (3) is the additional loss estimated to occur by the minimum, and col (4) is the observed value on 8/25 less the projected change. ( projections are largely mechanical exercises--as compared to predictions--where in my book anyway,, the analyst should have some skin in the game). h
Commented Aug 30, 2012 on ASI 2012 update 10: (wh)at a loss at Arctic Sea Ice
hello all, (note: In the discussion below i have treated loss of ice volume as a positive number) i have followed your discussions with interest and no small amount wonder at the civility and intelligence shown by one and all. i hope to finally have something positive to add to the discussion. specifically, to the problem of projecting the minimum sea ice volume. Using the piomas data set through 6/30/12, i constructed data inputs in excel for 3/31 (day 90) for the 34 years 1979 to 2012. i did the same for 4/30, 5/31, 6/30, 7/31, 8/31, and 9/15. i then ran a variety of regression models in excel (i will move to R shortly) and found the following to work well. 7/31 on 6/31 and 5/31; ie explain july 31 as a function of june 30 and may31 levels. The results for July31, Aug 31, and sept 15 are table 1A. the equations fit so well (table 1b) that i wonder if i have inadvertently picked up some characteristic of piomas process. Table 1A dep.var. 31-Jul Coefficients Standard Error t Stat P-value Intercep -1.958376655 1.076168499 -1.819767682 0.078786697 6/30 1.304233372 0.111008976 11.74890014 9.44756E-13 5/31 -0.425587792 0.128211544 -3.319418664 0.002375901 dep.var. 31-Aug Coefficients Standard Error t Stat P-value Intercept 0.161541168 1.01483997 0.159178957 0.8745947 7/31 1.332404399 0.177414035 7.510140897 2.26132E-08 6/30 -0.365845088 0.167221772 -2.187783823 0.036611428 dep.var. 15-Sep Coefficients Standard Error t Stat P-value Intercept 0.009715172 0.130989372 0.074167638 0.941369338 8/31 1.157851368 0.052767924 21.9423333 4.97293E-20 7/31 -0.144396653 0.050174141 -2.877909827 0.0073092 Table 1b 31-Jul Adjusted R Square 0.993343819 Standard Error 0.283087527 31-Aug Adjusted R Square 0.990494797 Standard Error 0.321661296 15-Sep Adjusted R Square 0.999092654 Standard Error 0.100109097 Below i provide the estimated values and errors for the three periods for the years 2007 t0 2011. (Y^ y-hat is the estimate and Y -Y^ is the error.) table 2 7/31 Y^ Y-Y^ 2007 9.06199196 -0.09799196 2008 11.05346432 -0.003464315 2009 10.18669655 -0.412696551 2010 6.594701414 0.320298586 2011 6.106160145 0.306839855 8/31 2007 6.400591738 0.251408262 2008 8.503539746 -0.680539746 2009 7.096067803 0.199932197 2010 4.664496229 -0.018496229 2011 4.216233809 0.069766191 9/31 2007 6.417370876 0.154629124 2008 7.47200341 -0.21800341 2009 7.046065868 -0.148065868 2010 4.390589773 0.077410227 2011 4.0462504 -0.0262504 below i show the results of using the estimates to project the arctic sea ice volumes for 7/31, 8/31, and 9/15, respectively 4.9, 2.6 and 2.3 x 10^3 km cubed. table 3 proj 7/31/12 Intercept -1.958376655 6/30 1.304233372 11.163 5/31 -0.425587792 18.1 4.897641441 proj 8/31/12 Intercept 0.161541168 7/31 1.332404399 4.897641441 6/30 -0.365845088 11.163 2.603251448 proj 9/15/12 Intercept 0.009715172 8/31 1.157851368 2.603251448 7/31 -0.144396653 4.897641441 2.316690391 i have a couple of figures that i had hoped to include in this note but can't manage to get them to copy. Figure 1 would show the differences by month (ie. there would be a series for april 30 - march 31 may 31- april 30, etc). examination of this chart shows a couple of interesting things: first, the change in ice loss volume is focused in the months of June--now the largest loss month and May. Second the estimated losses for July, August and September are all well within the historical values for these months. {I do worry that the size of the June loss means that the historical relationship for July is corrupted by the fact that the remaining ice is "harder" to melt. The second chart shows the levels at the end of each month for 1979, 1991, and 2002 to 2012. one sees in this chart that the seasonal pattern is now stronger (more ice melts from day 90 to day 257) and the thing that i hadn't realized before was how long the ice would be close to the minimum level. my summary of these results is as follows: We have a significant risk of a large downward break in sea ice volume If this occurs this year, then the stage will be set for an effectively ice free arctic within the next several years. If it doesn't then we will need to work hard at the underlying micro structure of the areas where the sea ice stays high. my summary of these results is as follows: We have a significant risk of a large downward break in sea ice volume. i don't need to belabor the import of a sea ice volume below 2.5 to the group. If this occurs this year, then the stage will be set for an effectively ice free arctic within the next several years. If it doesn't then we will need to work hard at the underlying micro structure of the areas where the sea ice stays high. my research questions for a post ice free arctic would focus on the consequences of this state of affairs on the Greenland ice sheet. we are presently melting just short of 20x 10^3 cubic km of ice as part of the annual cycle. (we could figure out what the net is (at least a ballpark number and then begin to think harder about just how fast the sheet might melt) I would love it if somebody could talk the folks at Piomas into doing a mid month estimate. my gmail account is hmpitcher. If somebody can tell me how to make the charts available i will do so likewise for the excel spreadsheet (2007)
Commented Jul 14, 2012 on Polls July 2012 at Arctic Sea Ice
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