As I understand it, moisture condenses at mountain tops to form the mountain glaciers that provide water. The mountain topography doesn't change much in the span of a few decades, so if there's more moisture in the atmosphere due to rising global temperatures, then it seems like there should be more water to condense on these mountains. But somehow, there's even less mountain ice, why?
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17$\begingroup$ Because ice doesn't like warm temperatures? $\endgroup$– David RicherbyCommented Jan 28, 2018 at 11:51
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4$\begingroup$ To be more accurate, the atmospheric moisture doesn't condense on top of mountains, it falls as snow (or rain if the temperature isn't cold enough). If the temperature during the summer isn't warm enough to melt all the snow that falls during the winter, it remains unmelted. If this continues for a number of years, you have the start of a glacier. OTOH, if the summer temperature is warm enough to melt all the snow, and a bit extra, any existing glaciers shrink. $\endgroup$– jamesqfCommented Jan 28, 2018 at 20:57
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3 Answers
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Indeed some glaciers are growing and gaining mass due to increased precipitation (in part due to climate change and enhanced atmospheric water content). But that's the case only for glaciers in very cold places, like East Antarctica, where most glaciers appear to be growing at increasing rates (1,2). But unfortunately West Antarctica is pretty much a desert; the total precipitation on average is less than in the Sahara Desert. That is usually the case in very cold places. The capacity of air to hold dissolved water decreases exponentially at lower temperatures. Therefore cold glaciers (the ones that are most often growing) account for a small amount of the total global ice mass change.
In contrast, temperate glaciers (i.e. not so cold) constitute the majority of the most well known mountain glaciers (outside Antarctica and the High Arctic). These in many cases are receiving more precipitation, but the increases in accumulation are in most cases offset by the increase in temperature, which have a double impact:
- More melting due to higher temperatures
- A larger fraction of the precipitation falls as rain instead of snow, therefore promoting melting instead of accumulation.
That last point is not that intuitive, but as explained in this question and answer, warmer air carrying more moisture will develop water droplets quicker, therefore dropping most of the extra moisture at lower elevations. As a consequence, the rain-driven melt in the lower areas of the glacier will overcome the a moderate increase in the accumulation at higher elevations.
Also, there is a positive feedback kicking in against the glaciers: The more the glacier melts, the lower its surface will be, therefore it will be exposed to warmer temperatures leading to further melting, leading to more surface lowering, exposing it to even higher temperatures, producing even more melting and so on.
It is also important to note that glaciers grow due to snow accumulation. It is unclear to me if that's what you meant by "moisture condenses at mountain tops".
Finally, it worth noting that glaciers have been receding since the last ice age about 20,000 years ago (as we have learned from Oxygen isotopes on foraminifera deposits and glacier deposits), the recession slowed down the last ~6,000 years slowly approaching to a steady state. However, recent warming has just accelerated the process a bit, enhancing the extent and speed of the natural deglaciation, but we have to acknowledge that the glaciers were already out of balance and shrinking way before the industrial revolution.
answered Jan 28, 2018 at 4:47
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$\begingroup$ While your post is well thought, you keep talking about the ocean glaciers and deserts despite that the question is clearly about the ice on mountains. $\endgroup$– John JoeCommented Jan 28, 2018 at 7:51
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2$\begingroup$ @JohnJoe - This answer does address temperate glaciers. Only the first paragraph covers polar areas. Regarding deserts, it's important to realize that the two largest deserts in the world are Antarctica, followed closely by the Arctic, each of which is more than 50% larger than is the Sahara. $\endgroup$ Commented Jan 28, 2018 at 13:43
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1$\begingroup$ @JohnJoe, I've added a sentence to clarify that the temperate glaciers I talk about most of the time, correspond to the mountain glaciers you refer to. $\endgroup$ Commented Jan 28, 2018 at 14:47
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$\begingroup$ Okay, but if it's simply a matter of snowfall, then why does snow always uncoincidentally happen to fall on the mountains? It has to have something to do with the actual height of the mountains themselves. $\endgroup$– John JoeCommented Jan 28, 2018 at 22:40
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2$\begingroup$ @JohnJoe Yes, temperature drops with elevation, so mountains are colder and precipitation is more likely to fall as snow. Also, air cools as the wind force it to rise over the mountains, enhancing precipitation (a.k.a. Orographic precipitation). Mountains high enough will always have glaciers, and in them the glaciers always grow (it is call the accumulation area) but when the glacier creep down to warmer temperatures it do start melting. A glacier will find balance between accumulation and melting. But the extra precipitation you point is not enough to counteract the increase in temperature. $\endgroup$ Commented Jan 29, 2018 at 1:18
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More moisture in the atmosphere also means less orographic lifting is required to reach saturation. This should lead to rain or snowfall at lower elevations, where it would melt more rapidly and not contribute to snowpack.
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$\begingroup$ Good point. That is seen in high mountains near the sea (like mount Logan), where middle elevations sometimes get more snow than the top. $\endgroup$ Commented Feb 1, 2018 at 22:20
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Mountains are a little unusual in terms of water vapor capture. Temperature drops with elevation so some mountains have ice-cover even in summer and/or even in warm latitudes. Mountain ranges tend to block air-flow and if there's a prevailing wind direction, it's not uncommon for for the other side of the mountain to be desert. This is called a rain shadow. Death Valley is an example of a rain shadow with mountain ranges on both sides. The Atacama desert is another.
Rain shadows aren't part of your question, but the point is that Mountain ranges are especially good at pulling water vapor from the air and in cold enough climate, or if the mountains are tall enough, they're very good at creating localized glaciers that ebb and flow with the seasons.
I disagree with the sentence in the other answer of glaciers have been receding for the last 15,000 years. While somewhat true, given that you asked about mountain glaciers, that's a tricky assumption to make. Sea-level studies suggest that glacial melt, significantly slowed down to a very gradual sea level rise by about 6,000-6,500 years ago. If we assume (if I may speculate), a steady but gradual increase in East Antarctic ice, there has been some steady glacial melt globally over the last 6,000 years, but not a huge amount and whether mountain glaciers contributed to that is unknown.
Getting to your question, When the weather is warmer and there's more water vapor in the air, the amount of water that mountain ranges catch should increase on average. That's 100% accurate, captured water only becomes glacier (or snow or frost), when the temperature is below freezing.
As the other answer points out, it's a combination of a period of time below freezing and the amount of water vapor in the air that enables the mountain glaciers to grow. But you also have to look at absolute humidity. Air tends to be high relative humidity over oceans but over land there's much greater variation.
Wind generally blows in an easterly direction across the US, the US' western most mountain ranges are the Sierra Nevadas and the Cascades, and as noted above, mountains pull water-vapor from the air, so the western most mountain ranges capture a lot of the water-vapor who's source is the Pacific ocean.
Past those mountains, the air needs to fill up again with water vapor, sometimes called recycled water-vapor or secondary source water vapor. This secondary source comes mostly from rivers and plant transpiration, but as CO2 levels rise, plant transpiration decreases, so you have a double whammy effect, air that can hold more water, and less water vapor returned to the air. This means, by the time the air reaches the Rocky Mountains, there's often less, not more water vapor in the air, combined with a shorter winter season, and the glaciers can shrink significantly.
Regions that rely on secondary sources of water vapor, such as rivers and transpiration, can be especially vulnerable to climate change. Add to that, a decrease of snow melt from shrinking mountain glaciers upstream and you have a potentially serious water shortage issues . . . but I digress, as your question wasn't about water shortages.
No two mountain ranges are alike, so the details should be studied individually for each mountain range or region, but specifics such as where the wind that blows past that mountain gets it's water vapor from is very important, in addition to any variation in the time that mountain glacier spends above or below freezing. That warmer air holds more water is true in general, but it's not globally true as a result of climate change. Some parts of the Earth are becoming dryer as a result of climate change.
There's no shortage of articles that indicate that mountain glaciers are rapidly shrinking. Mountain glaciers that rely on secondary sources of water vapor are the most vulnerable.
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$\begingroup$ I agree that the recession of glaciers have slow down dramatically in the las 6,000 years compared to the previous 6,000. But during the las few hundred years (that is what matters for this question), all proxies (geomorphology, dendrocronology, historical photography, moraine dating, etc) point to a generalized overall recession of mountain glaciers worldwide. That said, I your answer don't explain the recession of glaciers in the Sierra Nevada and the Csscades. I think the transpiration reduction due to increased CO2 is a minor effect. $\endgroup$ Commented Jan 31, 2018 at 5:32
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1$\begingroup$ @CamiloRada generalized overall recession or a rapid recent recession? Mountain glaciers grew during the little ice age, in Europe at least. I also said temperature was part of the equation, just not the entire answer. It's well documented that climate change leads to drought. Drought means less water in the air to form mountain glaciers. Each range needs to be looked at individually, there's not one answer for all. And Transpiration is a larger effect than you might think. 10% of water vapor is from Transpiration. water.usgs.gov/edu/watercycleevapotranspiration.html $\endgroup$– userLTKCommented Jan 31, 2018 at 12:39
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$\begingroup$ Interesting, I didn't know it was that important. But I still don't think it is one of the main controls. For the recession, if you smooth out events like the little ice age and look at mountain glacier mass over the last 6,000 years you still have a constant trend of mass loss according to my understanding. $\endgroup$ Commented Jan 31, 2018 at 17:44
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$\begingroup$ Mountain glacier runoff also, isn't linear. Think of glaciers as money in the bank. When it gets warmer, the glaciers shrink but initially there's more run-off, but that's a temporary surplus. When they shrink enough, back to a new equilibrium state, the runoff is lower than it was before the temperature change. . . . anyway, I'll look for some links to back up and/or improve my answer. $\endgroup$– userLTKCommented Feb 2, 2018 at 13:36
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$\begingroup$ Perhaps an additional point to be made here. Along most of the US west coast, the Sierra Nevada/Cascades are the second mountain ranges encountered by wind from the Pacific. The Coast Ranges are the first. However, they're only about half as high, so most of the precipitation falls as rain rather than snow. $\endgroup$– jamesqfCommented Mar 13, 2018 at 19:44