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David Gould
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That sounds reasonable. One of the things that I did when modelling was not allow the summer volume to drop below zero. This led to a linear trend in the maximum volume, which seems more realistic. I would point out that the main thing that I am arguing is that for what we currently observe there is no need to speculate that feedbacks are somehow accelerating ice melt in some fashion. I would suggest that at this point there is not much evidence that feedbacks, positive or negative, are occurring (or if they are all they are doing is making the linear temperature increase a steeper one and thus accelerating slightly the quadratic decline.) Feedbacks, both positive and negative, will occur in the future. But there is not much to suggest that they are playing a major role in ice dynamics at present.
Toggle Commented Mar 20, 2012 on March 2012 Open Thread at Arctic Sea Ice
Andrew Xnn, I will point out that the most sophisticated models project that the ice will be gone at the end of the melt season by around 2070, which appears to be contradicted by current observations. Most scientists looking at this believe that 2030 is a more reasonable time frame, with some looking at much earlier dates.
Looking at the NSIDC extent data, there is a strong correlation between melt and freeze - the larger the melt of year N, the greater the freeze in year N+1. I have not checked for autocorrelation or anything like that, but the r^2 value is 0.77 and the slope is 0.87, meaning that for every increase in melt by 1 million square kilometres means a corresponding increase in freeze by .87 million square kilometres. Note that this still sees an increasing gap between melt and freeze. It would thus seem to me that the effects of rapid re-freeze are already built into the extent data. So no cavalry there. (And, yes, extent data does not show meltout until around 2030. But we all agree that volume is the main game here - just looking at the extent data for clues as to behaviour). Based on this, the data predicts that 2012 will see the third largest freeze in the NSIDC record - close to 9.8 million square kilometres of new ice. It is predicted, however, to still be the lowest March extent.
Andrew Xnn, Yes, but that rebound did not affect the polynomial decline. The 2007 was a drop below the trend line - the rebound just brought us back up to it. It should be pointed out that Tietsche's thesis is that the Arctic can rebound from *anomalous* declines and that there is likely to be no tipping point. I do not think that the polynomial decline is anomalous.
I am still unsure that there could suddenly be evidenced a counter process to the melt that has not been seen up until now. Extent/area has declined dramatically over the last few years, leaving a lot of open water that was not there 20 years ago. No let up in the decline has appeared. If a further, say, 20 per cent of the remaining extent went, getting us to an extent of around 3.7 square kilometres, would that extra one million of open water suddenly and dramatically slow the decline when the four million new square kilometres of open water did not slow it at all, but instead saw it speed up? Given that the exponential curves show ice vanishing very soon, a very large effect has to come into play very quickly ...
William, While I can see your point, there is an issue with it. If you were measuring the rate of melt of an ice cube in the sun, then you could take the rate of melt for the ice cube as a whole or you could discount the rate for the outer few mm and base your projection on the fact that none of the inner part of the ice had melted, showing that the rate of melt was zero. And after ever little bit of further melt that intruded on the centre part, you could simply arbitrarily reduce the size of the bit that you were using to measure the rate of melt. You could keep doing this until just prior to the final bit melted, predicting that there was going to be no further melt, as the centre bit had not melted one bit during its whole time in the sun. So: my question to you would be what bit would you use to determine the likely future melting rate and why? If it is the Arctic Basin, isn't that simply an arbitrary decision? Why not choose everything north of 81 degrees? Or 82 degrees? Or 83 degrees 58 minutes and 21 seconds? In a few years, there will be no ice below 81 degrees. So why include that region when making predictions for the future?
Re no-one thinking that the melt will continue parabolically, I do - to a point. The reason I think that it will continue is this: if the amount of ice melted each year increases linearly while the amount of ice the freezes each year remains constant, the plotted graph is an parabola (2nd order polynomial). And there is a reason to think that ice melt will increase linearly: the temperature rise is basically linear. At what point will this change? My guess is that it will change when there is basically open water (in other words, zero ice)at the end of summer. Heat will escape, meaning that the temperature rise will slow, changing the dynamic.
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Sep 9, 2011