Comments 1 to 9:

1. joe - at 05:57 AM on 26 April, 2017

   A stream flows through the toe of Kaskawulsh Glacier in Kluane National Park in the Yukon. In 2016, this channel allowed the glacier’s meltwater to drain in a different direction than normal, resulting in the Slims River water being rerouted to a different river system. (Dan Shugar)

   A team of scientists on Monday documented what they’re describing as the first case of large-scale river reorganization as a result of human-caused climate change.

   A more accurate statement is that the river is reverting back to its natural drainage basin,  which existed prior to the glacier blocking its natural flow.  The retreat of the mendenhall glacier revealing tree stumps from the early mwp indicates a much warmer period in the region and that the north bound flow of the river has most likely only been a fairly recent occurance.

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2. nigelj at 12:08 PM on 26 April, 2017

   Joe @1, I don't see how you can say that this rivers natural drainage basin existed back in the mwp, because this is arbitrary. I could equally say it's natural drainage basin was back before the  mwp where things were probably different yet again, but this is just as arbitrary. Rivers change their courses long term.

   I think the only meaningful definition of natural drainage basin would be "before humans substantially changed things" for example by agriculture and hydro power etc, or by agw global warming, depending on specific rivers. This would mean natural drainage basins are far more recent than the mwp.

   The real point is we are causing the glacier to melt through burning fossil fuels, or are at least this is a dominant cause. And its happening at a fast rate compared to previous warming periods like the mwp. And its altered river flows.

   The glacier has indeed revealed some tree stumps from very roughly around the mwp. However studies of the mwp find that for Europe as a whole, it was rather weak with about half a degree of warming, in the northern hemisphere only, as below

   (LINK)

   The mwp was also rather short, and was clearly not enough to seriously raise sea levels long term. The recent agw warming is driven more by greenhouse gases, and is at a much higher rate, and likely to lead to long term sea level rise. 

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3. joe - at 23:50 PM on 26 April, 2017

   Joe @1, I don't see how you can say that this rivers natural drainage basin existed back in the mwp, because this is arbitrary. I could equally say it's natural drainage basin was back before the mwp where things were probably different yet again, but this is just as arbitrary.

   NigelJ - How can I say what is the river's natural drainage basin? - Its basic topography

   How can I say what is the rivers natural drainage basin during the MWP? Based on known facts.  First we know that the most of if not all the glaciers in the region made significant advances during the LIA (LINK)

   Secondly, we know that mwp was most likely warmer in the region as indicated by the exposed tree stumps dating from the mwp ( which you acknowledge) from the retreating mendenhall glacier. You may also note that the age of the tree stumps indicate a reasonably long warm period. 

   I think the only meaningful definition of natural drainage basin would be "before humans substantially changed things" for example by agriculture and hydro power etc,

   Are you attempting to argue that topography that has existed for millineums is not relevant to what any hydrologist would demonstrate is the natural drainage basin?  

   The glacier has indeed revealed some tree stumps from very roughly around the mwp. However studies of the mwp find that for Europe as a whole, it was rather weak with about half a degree of warming, in the northern hemisphere only, as below

   The mwp trees stumps that you acknowledge are just one of the many of the pieces of evidence that the region for the slim river was most likley warmer during the mwp and thus  the slim river most likely drained south which is its natural drainage basin.  The river most  certainly followed its natural course as recently as 2k-3k years ago. 

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4. Tom Curtis at 02:16 AM on 27 April, 2017

   joe @3:

   "Secondly, we know that mwp was most likely warmer in the region as indicated by the exposed tree stumps dating from the mwp ( which you acknowledge) from the retreating mendenhall glacier."

   With respect, we do not know that at all.  Firstly, glaciers exhibit a lagged response to temperature.  Consequently, in a period of very rapid temperature rise as at present, the equilibrium glacial terminus may be significantly up slope of the observed glacial terminus.  So while the tree stumps at Mendenhall Glacier give reason to think the temperature whose equilibrium terminus coincides with the current terminus, all else being equal, was less than that in the MWP in that region.  But that may have been the temperature 10 or 20 years ago in a rapidly warming region.

   Secondly, all else is not equal.  Specifically, the glacial terminus is the result of a lagged equilibrium between precipitation on the glacier above the snow line, and temperature below the snow line.  Current precipitation in the area is high, having risen from a low base (for the region) over the last century or so.  That would have slowed the retreat due to temperature alone for glaciers in the region, and for some specific glaciers may have reversed it.  In contrast, from approx 900 to 1300 AD, precipitation in the region fell - which would have resulted in a glacial retreat even in the absence of any increase in temperature.  The reduced glacial length of Medenhall Glacier shown by the tree stumps are as likely to be a consequence of those changes in precipitation as changes in temperature.  (Precipitation data from Lowe et al 1997)

   Thirdly, glacial responses to temperature and precipitation changes within a region can vary significantly within a region due to shifting microclimates.  Medenhall Glacier is in British Columbia, sufficiently distant from the Kaskawulsh Glacier, which is in Yukon, that the glacial record of Medenhall cannot be treated as directly relevant to Kaskawulsh.  Kaskawulsh glacier has itself been shown to have advanced between the MWP and the LIA by tree ring and C14 data on woody debris, but that debris has been found next to currently existing vegetation, in some cases in trees with trunks thicker than the debris (See fig 3 of Reyes et al 2006).  The debris has been carried downhill from its initial location, but it cannot be directly inferred that it originally grew uphill of the current terminus, let alone the current equilibrium terminus line.

   Finally, a temperature reconstruction for the summer temperatures in South West Canada has been inferred from known glacial advances in retreats in the area (see fig 12 of Menounos et al 2008).  This shows peak temperatures over the last 1000 years around 1020-60 and 1400 AD, with 1940 temperatures (20 year mean) being 0.3 C cooler than those peaks.  Temperatures in the region have risen by more than 0.5 C since the 1940s:

   

   That is, based on multiple regional proxies, it is more likely that curren temperatures are slightly warmer than the MWP peak than that they are slightly cooler; but the data is not sufficiently robust to say definitively which was warmer or cooler.

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5. nigelj at 08:39 AM on 27 April, 2017

   Joe @3, thank's for your comment.  I was indeed thinking of drainage basins in the very long term. 

   However I take your point, but I just think so what? I'm not sure what you think is so compelling about the river returning to a previous state / drainage basin.

   The more important thing is we are melting the glacier, and altering the course of the river in that process. This could possibly cause us problems either in this specific case, or other cases, and regardless of whether it goes back to some previous state or not. It's also showing the impact of agw climate change, and is just another potential heaadache being caused by agw climate change.

   Nothing personal, your comments were interesting and raised various issues. I didn't know about the tree stumps etc.

   But I have the same reaction to the tree stump issue. Basically so what? It's well known that during the mpw some specific locations  warmed more than others. The more important point is the mwp was rather weak overall, and climate change really "is" like a hockey stick according to all the studies I have seen.

   But anyway Tom Curtis has cast very genuine doubt about whether you can conclude the region of the Mendenhall glacier was particularly warm.

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6. Tom Curtis at 10:49 AM on 27 April, 2017

   nigelj @5, I think your dispute with Joe about the natural drainage basin is terminological rather than real.  Topography can be defined by the shape of the ground, or the shape of the ground plus permanent ice.  Which we use is a matter of convenience, which in turn is a matter of just how permanent the ice is on human time scales.

   The perfect example to illustrate this is the Antarctic Peninsula.  If you look at the level at which the ice grounds, ie, the "natural topography" according to Joe, then clearly the Peninsula is actually an island (or series of islands).  Nobody, however, is making an attempt to have the Peninsula renamed, or various regions of West or East Antarctica renamed as seas.  Because the ice is sufficiently permanent, we take the surface of the permanent ice sheets to define the topography of Antarctica, so any such change in nomenclature would be absurd.

   

   The case with the Kaskawulsh glacier is interesting because the duration of the ice is significant in human terms (several hundred years) but clearly the topography defined as including the ice surface has changed significantly on a human timescale.  That makes it a matter of terminological choice as to which definition is used.  If we choose that the topography is taken as following the surface of the ice, then the natural drainage basin of the river has changed, just as much as if the cause of the change was the reshaping of the land by an earthquake.  I am not certain, but I suspect topographical maps of the area will show contour lines following the ice surface, which would show that that was the convenction we had adopted.  If we follow the convention that topology follows the land surface, then, of course, the natural drainage basin has not changed.

   The key point here is that neither choice of terminology is right, or wrong.  They are only convenient or inconveneint.  But to avoid confusion we must be clear as to which convention we are following.

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7. nigelj at 05:55 AM on 28 April, 2017

   Tom Curtis @6, yes my disagreement with joe over drainage basins was pretty much terminological. In fact for practical purposes what Joe says is all fair enough, and I was being a bit pedantic. 

   But you are right about the need to agree on definitions. In fact I would go further and say a huge ammount of disputes and problems on the net is people talking past each other, and having different interpretations of certain words or ideas, and also general lack of clarity of writing. Of course often its a time related thing.

   However what frustrated me was Joe going on about rivers returing to some drainage basin. To me it misses the point of what is going on in terms of human caused climate change, and this is what set me off!  

   Thank's for the detail on the antarctic. Interesting stuff.

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8. joe - at 08:09 AM on 28 April, 2017

   But I have the same reaction to the tree stump issue. Basically so what? It's well known that during the mpw some specific locations warmed more than others. The more important point is the mwp was rather weak overall, 

   But anyway Tom Curtis has cast very genuine doubt about whether you can conclude the region of the Mendenhall glacier was particularly warm.

   Referencing back to the tree stumps from the MWP at the mendenhall glacier, both the age of the trees and the size  should cast reasonable doubt as to both the short length and breadth of the mwp in the region.  Its a 1,000 to  1,200 year period going back from circa 1,000ad without the glacier. Given that time frame, Tom's geniune doubt explanation is closer to a plausible explanation.

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9. Tom Curtis at 12:07 PM on 28 April, 2017

   joe @8, it has been a little difficult getting the data on the Mendenhall Glacier tree stumps.  The only public record is limited, AFAIK, to an article in the Juneau Empire, and one in Live Science.  No scientific paper reporting the results has been published as yet.  Cathy Connor (the lead researcher involved) did give a 2014 conference report on the glacier, buit was not on the topic of the tree stumps.

   From the Juneau Empire, we learn that:

   "The most recent stumps she’s dated emerging from the Mendenhall are between 1,400 and 1,200 years old. The oldest she’s tested are around 2,350 years old. She’s also dated some at around 1,870 to 2,000 years old."

   Please note that they are not continuous ages.  Had the area uncovered been continuously forested from 2,350 to 1,200 years ago, the older trees would have fallen and rotted due to old age, and only trees from about 1,700 to 1,200 year old would have been found.  Rather, what has happened is that the area was uncovered 2,350, 1870-2,000, and 1,200 to 1,400 years ago.  It was likely covered inbetween times, although there may have been intervening periods in which the glacier left the area uncovered but in which the trees remained embedded in the gravel which protected them.

   The most interesting finds are tree stumps in ice caves, ie, tree stumps still technically covered by the ice, such as the example below from Live Science  (chosen to provide scale):

   

   And here is an external view from the Juneau Empire, also to show scale:

   

   And a modern 30 year old Spruce:

   

   Comparison of trunk width shows there is no reason to think the trees were exceptionally old at the time of their destruction.  Again, this shows your idea that the area was uncovered for "... a 1,000 to 1,200 year period going back from circa 1,000 AD..." is incorrect.

   Finally, here is a photo from the Juneau Empire, whose caption read:

   "Vertical tree stumps remain in some of the ice caves beneath the glacier.  Two ice cave stumps have been dated at two different date ranges - between 670 and 780 AD, and between 620-670 AD."

   

   That date range is consistent with the "1,400 and 1,200 years old" date range, ie, circa 600-800 AD, mentioned in Live Science.  The MWP proper did not start till circa 900 AD, so the regional warm period shown by the Mendenhall Glacier preceded the MWP by (at minimum) 100 years, and appears to have been replaced by regional cooling during the MWP.

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