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什么因素解释了山地冰川退缩的变化?

众所周知，世界上大多数山地冰川都在退缩，但通过20世纪80年代和今天的一些LANDSAT图像，我能够看到在同一时间段内退缩了几公里的冰川，以及同一区域内几乎没有变化的其他冰川。

我想知道是什么变量造成的?基岩岩性?降水差异呢?山坡吗?等。

回答

冰川的行为确实存在很大的变异性，甚至在相邻的冰川之间也是如此。如果你仔细观察，你甚至会发现前进的冰川就在快速后退的冰川旁边。有很多因素可以解释这种变化，我将列出最重要的因素:1。**地形控制**:气候变化对邻近冰川的影响不同。如果风、空气湿度或温度发生变化，由于地形的不同，相应的降水变化或太阳照射在山体的不同区域也会有所不同，因此对每个冰川的影响也不同。一个典型的说明性例子是下图所示的地形降水。这解释了为什么山脉迎风一侧的冰川可能比另一侧更大。现在，在温度和空气湿度(通常同时出现)增加的情况下，相关的降水增加可能更有利于一侧的积雪积累，而温度或多或少会以同样的方式影响两者。导致背风面冰川比迎风面冰川后退得更快。[![enter image description here][1]][1] 2. **Topographic controls**: All glaciers flow downhill, and the speed of such flow depends on the resistance the bedrock, valley walls, moraines, etc. A glacier that is in steady state (i.e. not advancing nor receding) is in balance between snow accumulation in its upper reaches and melting in its lower areas. Even if there is no changes in accumulation or melting, this balance can be broken if the speed of the glacier changes. If the ice is taken faster from its cold head to the warm front, that will increase the melting and more snow would be needed to offset the additional melting. Most of the resistance to flow is provided by constrictions of the valley walls or protrusions of the bed. As in the figure below (from [here][2]), taking a floating ice tongue as example (a.k.a. ice shelf) where most of the resistance to flow comes from the bed protrusion below the "ice rise", in this case such protrusion is called a "pinning point". Now, going back to your question: Even if two glaciers that are receding at the same rates, eventually one could recede behind a constriction, therefore its speed will increase, pushing it further out of balance, so it will start loosing mass faster than its neighbor and at some point that will show up as a more pronounced shrinking in images like the ones you were studying. In the case of ice shelves (like the one in the figure), loosing contact with a pinning point can lead to the collapse and breakup of large sections of the floating tongue in very quick and dramatic events. [![enter image description here][3]][3] 3. **Reaction time scales**: Neighbouring glaciers can have huge differences in mass. And that means that they don't react with the same speed environmental changes. In a very small glacier, you will see advance after a few years of high accumulation. However, in a very large glacier, a few years of high accumulation doesn't make much of a difference. And the glacier can still recede after such events, perhaps due to a few decades of low accumulation that happen a hundred years ago. 4. **Area is not volume**: What you see in the images are changes in area not volume. So even if two glaciers are loosing the same amount of mass per year. If one is thinner than the other, you will see that the thicker one shrink less that the other. 5. **Hypsometry**: It refers to the distribution of values of surface elevation of a glacier. For example if two of the glaciers you are observing span from 1,500 to 2,500m of elevation, but one of them have 90% of its surface above 2,000m of elevation, and the other only a 30%. You will observe that a change in temperature that push the average freezing line from 1,800m to 2,000m will affect the second glacier much more than the first, because it will suffer a larger increase in the fraction of its surface exposed to melting temperatures. 6. **Surging glaciers**: Some glaciers undergo what is call a [glacial surges][4]. They show a cyclical behavior in which they advance very quick over few years and then recede slowly over decades to then start over again. I won't go in detail about this process that is very complex and still poorly understood. But in the images you observe, they might be some variability also to the presences of glacier surges. Other factors could be more complex weather redistribution patterns, bed topography, bedrock lithology, basin divide migration, frontal wasting (calving) for lake and marine terminating glaciers, etc. [1]: https://i.stack.imgur.com/7JXsN.jpg [2]: https://www.gvsu.edu/rmsc/interchange/z2011-september-connections-563.htm [3]: https://i.stack.imgur.com/qL91x.png [4]: https://en.wikipedia.org/wiki/Surge_(glacier)

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