Beating a retreat

What does the disappearance of glaciers mean for lakes and rivers? (10 minute read)

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When I was a child, my favourite ice cream was mint chocolate chip. It was always a big treat to have an icecream when my family were out, because such exotic flavours were not available in the supermarket. I can taste it just thinking about it – the subtle mint flavour balancing the sweetness, the smoothness of the icecream against the crunch of the cone, and the bursts of chocolate. The colour sticks in my mind too – an unnatural shade of milky green. A couple of times in the last few years I’ve had fancy mint chocolate chip icecream that hasn’t been coloured, and something didn’t seem right.

This very specific ice cream is on my mind right now because I’ve been thinking about the first time I saw a glacier. It was during the 1990s and I was living in Christchurch. As I did sometimes, I took a few days holiday and went exploring the South Island. This time, I drove south from Hokitika and eventually reached the Franz Joseph Glacier. I parked my little car, walked up the track and watched people clambering over the barrier which was intended to prevent them from getting too close. I’m not exactly sure where my old photos and notes are from that time, so I haven’t pulled them out to prompt my memory. But I do remember writing down one thing in my notebook – the ice of the glacier reminded me of mint chocolate chip ice cream. There were small stones trapped in the ice which represented the chocolate chips, and the ice was reminiscent of the greenish ice cream.

When I tried to find out why, I began to doubt my memory. Glacier ice is not green, I found, but blue. It’s a remarkable shade of blue too, darker than the sky and as intense as any jewel. The colour comes as a result of the ice being compressed – most of the air is squeezed out and it forms into crystals, and the way it reflects light makes it appear blue. But when I look at videos online, I can see that the ice on Franz Joseph Glacier can appear more greenish, so I’m reassured that my memory is still reliable.

I visited Franz Joseph before I had a digital camera, so here’s a picture of glacier ice from Antarctica (image credit: Melanie Newfield)

Franz Joseph Glacier is on my mind because if I went back today, I’m not sure how much I could see. From 2012-2019, the Franz Joseph Glacier retreated 900 metres (the link takes you to a spectacular time-lapse video covering those years). I can’t find out how much it has retreated since the 1990s. I realise, now I think about it, that when I saw the glacier I was half the age I am now. The thought is like dust in my throat, catching at something inside me. Time is passing, the climate is changing, and there’s so much momentum against solving the problem. What is going to happen to our glaciers? And what does that mean for us?

A glacier is more than just a static lump of ice. The ice I saw at the end of the Franz Joseph Glacier all those years ago had not formed in that steep-sided valley surrounded by rainforest, but high in the mountains. The process begins with snowfall. Newly fallen snow is light and fluffy – we think of it as frozen water, but in fact it’s 90% air. If the snow keeps falling and doesn’t melt, the layers beneath are compacted and much of the air is squeezed out. Eventually, the snow has compacted so much that it has become ice. But the air hasn’t disappeared completely. Glacier ice is about 20% air, which is why icebergs float.

This process takes time. In more temperate climates with abundant snowfall, such as southern Alaska, the process of snow becoming glacier ice can take as little as 3-5 years. However, in colder, drier areas, such as eastern Antarctica, the formation of glacier ice can take hundreds or even thousands of years. The colder it is, the slower the transformation of snow to ice.

Although it floats, a mass of ice is still heavy. Under the force of gravity, it will move down a slope. The way that a particular glacier moves depends on a number of different factors, such as properties of the ice, the slope of the land, the type of rock beneath it and what is at the end of the glacier. Some of the movement comes from the ice deforming and changing shape – acting more like a liquid than a solid. Some of the movement comes from the whole mass of ice sliding on the land beneath it.

I know that the first time I was told that glaciers flowed like rivers, I couldn’t comprehend it. The idea just seemed too bizarre. But looking at time-lapse images of glaciers made the process more comprehensible. I’ve linked to a great example here.

As you might imagine, and as you can see from the video, ice which flows down a slope gets twisted and fractured. Cracks in the ice, known as crevasses, open up. Some can be massive – up to 45 metres deep and hundreds of metres long. Over time, snow on the surface can build up and cover the crevasses, forming a snow bridge which can be dangerously unstable when walked on.

A perfect opportunity to share a photograph of crevasses, also from Antarctica (image credit: Melanie Newfield)

In Antarctica, almost all glaciers end in the sea, where pieces break off and become icebergs. But in places like New Zealand, where glaciers flow down into warmer areas, the ice at the end of the glacier melts, forming lakes and rivers. New Zealand has a number of rivers and lakes which are fed by glaciers. Lake Pūkaki and Lake Tākapō (also known as Tekapo) in the central South Island are both glacial lakes. The Waitaki and Clutha rivers, our two most important rivers for hydro-electricity generation, are fed by glaciers. So are many other South Island rivers.

Around the world, glaciers are the source of many mighty rivers. The Ganges (Ganga) River has its origin in glaciers high in the Himalayas, as does the Indus, which runs through Pakistan. So does the Yangtze (Chang Jiang) in China, the Rhine in Europe and the Yukon in North America (there’s a story there, more on that later). Even the Amazon receives some of its water from glaciers high in the Andes.

This is not to say that all of the water in these rivers was once glacial ice. Figures aren’t easy to find, but the contribution of glaciers to the Yangtze ranges from around 10-20%, depending on the time period when it was measures. In the case of the Indus, it’s estimated that around 40% of its flow comes from glaciers, more than for any other river. Although they only contribute a part of the flow, glaciers are important, because they can compensate for dry seasons or years lower in the catchment.

Over the last few decades, glaciers have been retreating around the world, from the tropical Andes to Antarctica. While a link with rising temperatures is obvious, the precise reason for glacier retreat depends on the glacier. It’s not simply a matter of warmer temperatures increasing the rate of melting, although that does happen. For glaciers which end in the ocean, warmer waters in the ocean increase the process of melting the glacier from below, which increases the rate at which pieces of ice break off and become icebergs (this link has a great video explaining the process). For glaciers in warmer areas, higher temperatures increase melting, but it is more complicated than that. At higher temperatures, the balance between snow and rain shifts. The total precipitation may not change, but if less of it lands as snow and more lands as rain, melting of the glacier will increase.

What does glacier retreat mean? I’ve already written about the contribution of melting glaciers, especially those in Greenland, to sea level rise, so I won’t go into that in detail here (here is the link to the article). What I want to know this week is what the melting of glaciers means for the rivers and lakes which flow from those glaciers.

The simple answer is that if glacier melting increases, there will be more water flowing into lakes and rivers. But this doesn’t continue indefinitely. Past a certain point, known as “peak water”, the flow decreases. A study published in 2018 found that in around half of the catchments assessed, the peak water point had already passed, and flows were decreasing.

In some cases, these changes may not be large. The 2018 study found that for many catchments, the difference would be less than 5%. In some catchments, though, the increases or decreases could be larger. For example, in parts of the the Tarim River catchment, in north-west China, flows are predicted to increase by anywhere from 15-60% before they begin to decrease. And, in around a third of catchments looked at in the 2018 study, there would eventually be a reduction in water flow greater than 10% for at least one month of the year. It might not sound like much, but for areas which are already water-stressed, it may be a real problem.

These kinds of changes will be seen over the next few decades, and may prove a real problem for many countries, including New Zealand. But the immediate concern is not rivers, but lakes. Glacial lakes are formed in a number of different ways.  Lakes Pūkaki and Tākapō are both dammed by rock and rubble left by retreating glaciers at the end of the last ice age – that’s what gives the lakes their blunt-ended shapes. Shorter-lived dams can be formed from mounds of ice and gravel, or from tongues of ice blocking the path of a river.

Since 1990, glacial lakes have been growing in size, and new lakes are forming where there were none before. And the dams holding these lakes are not always stable. The dams can collapse, causing what are known as “outburst” floods – sudden and potentially deadly. Such collapses can be triggered by earthquakes, as happened in Peru in 1970, when 70,000 people were killed, most of them not by the earthquake, but by the catastophic landslide of glacial ice and rock which followed. But rain or high temperatures can also trigger these floods. Last year, Pakistan’s Gilgit-Baltistan region saw 16 outburst floods, one of which destroyed a major bridge.

It’s not yet clear whether climate change is making outburst floods more likely. Rather, what is changing is that with more, and larger, lakes, the area and the number of people at risk has increased. A recent study estimated that 15 million people worldwide live in areas threatened by outburst floods from glacial lakes, half of them in just four countries – India, Pakistan, Peru and China. New Zealand is also on the list of countries with vulnerable populations, but we are among the countries with the smallest number of people at risk (although that may be of no comfort to those who are potentially in the path of a flood).

Work is underway to better predict outburst floods – both to predict which areas are most at risk and to give warnings so that people can get away in time. But some areas are so close to glaciers that there may be very little warning of floods.

There’s one more effect of glacier melting that I came across while researching this story, and it is so remarkable and strange that I had to include it. In Canada, over the course of approximately four days, the flow of water from a retreating glacier stopped flowing into the Slim River and began to flow into the Kaskawulsh River. As a result, the Slim River dried up, leaving nothing but a bed of muddy sediment. I found it hard to visualise what could have happened, but the video I have linked here gives a good explanation. This kind of event is rare indeed, but it’s also a reminder of how unpredictable and startling the effects of climate change can be.

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