Posts Tagged ‘Hanna’

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A new round of Antarctic ice alarm

March 28, 2015

The alarm of a catastrophic meltdown of the Antarctic cycles up and down every year or two.  A journal article says the rate of melt is increasing, the popular press picks up on it and breathlessly warns about huge sea level rises sinking coastal cities around the world. We are told that x number of gigatonnes of ice per year are being dumped off the continent and wreaking their havoc on the world.   Then another study says “not so fast,” the mass losses aren’t that great after all.  Or, some crazy old skeptics ruin all the fun by recklessly bringing some logic to the discussion.

Today we have “Volume loss from Antarctic ice shelves is accelerating” (Paolo, et. al., Science, 2015).  The abstract warns us

“Overall, average ice-shelf volume change accelerated from negligible loss at 25 ± 64 km3 per year for 1994-2003 to rapid loss of 310 ± 74 km3 per year for 2003-2012.”

310 km3 per year (roughly the same as 310 gigatonnes per year) is pretty high compared to most other estimates. So you will probably see many references to this number because the bigger and scarier the more the press likes it.  But for the more sober minded, consider the following comparison of ice loss estimates from “Ice sheet mass balance and climate change” (Hanna, et. al., Nature, 2013)

Various estimates of ice mass change in the antarctic

Various estimates of ice mass change in the Antarctic

How does the recent Science paper compare?  If we place it on estimate plots from Hanna’s paper it would look like this..

Ice sheet mass balance and climate change - Hanna - Nature - 2013 v4

The Paolo Nature paper is an outlier.  But lets take them at their word.  They say that the Antarctic, on average, shed about 300 more Gigatonnes of ice per year during the 2003 to 2012 period than during the 1994 to 2003 period.  Where did all this ice go?  In to the oceans, of course.  That is why we have the great sea level rise scare.

So it follows that the sea level should have been rising faster during the 2003 to 2012 period than during the 1994 to 2003 year period.  How much faster?  Well, every gigatonne of water dumped into the oceans raises the sea level by about 2.78 microns. So 300 gigatonnes of extra water per year would raise the sea levels about an extra 840 microns a year, or about an extra 0.84 mm per year.  We are told that satellite data indicates that the global sea level is rising about 3 mm per year.  0.84 mm per year is a significant fraction of 3 mm per year, so such a rate increase should really stand out in the sea level rise data..

Well, here is some of that satellite sea level rise data…

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This discussion has been about ice that is moving from the land to the sea and raising the sea level.  But let’s take a quick moment to look at the sea ice that surrounds Antarctica.  While this ice does not contribute to changes in the sea level, it does say something about the conditions in that area.

seaice_anomaly_antarctic - Cryosphere Today 150328

Do you see a trend?  I see a trend.  And I know there are variety of “just-so stories” to explain away this trend, but I am unconvinced.

Conclusion

Between 1994 and 2003 the average sea level rise rate was 3.77 mm/yr, according to satellite data (University of Colorado).  If the Antarctic were depositing an average of about 300 more gigatonnes of water in the ocean per year in the following years (2003 to 2012), then the average sea level rise rage from 2003 to 2012 should have increased by about 0.84 m/yr, to 4.61 mm/yr.

Instead, the average sea level rise rate from 2003 to 2012 dropped to 2.66 mm/yr.

The claim of a huge rise in ice loss from the Antarctic over this period is quite implausible.