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Posts Tagged ‘glaciers’

An ice sheet forms when snow falls on land, compacts into ice, and forms a system of interconnected glaciers which gradually flow downhill like play-dough. In Antarctica, it is so cold that the ice flows right into the ocean before it melts, sometimes hundreds of kilometres from the coast. These giant slabs of ice, floating on the ocean while still attached to the continent, are called ice shelves.

For an ice sheet to have constant size, the mass of ice added from snowfall must equal the mass lost due to melting and calving (when icebergs break off). Since this ice loss mainly occurs at the edges, the rate of ice loss will depend on how fast glaciers can flow towards the edges.

Ice shelves slow down this flow. They hold back the glaciers behind them in what is known as the “buttressing effect”. If the ice shelves were smaller, the glaciers would flow much faster towards the ocean, melting and calving more ice than snowfall inland could replace. This situation is called a “negative mass balance”, which leads directly to global sea level rise.

Photo by Tas van Ommen

Respect the ice shelves. They are holding back disaster.

Ice shelves are perhaps the most important part of the Antarctic ice sheet for its overall stability. Unfortunately, they are also the part of the ice sheet most at risk. This is because they are the only bits touching the ocean. And the Antarctic ice sheet is not directly threatened by a warming atmosphere – it is threatened by a warming ocean.

The atmosphere would have to warm outrageously in order to melt the Antarctic ice sheet from the top down. Snowfall tends to be heaviest when temperatures are just below 0°C, but temperatures at the South Pole rarely go above -20°C, even in the summer. So atmospheric warming will likely lead to a slight increase in snowfall over Antarctica, adding to the mass of the ice sheet. Unfortunately, the ocean is warming at the same time. And a slightly warmer ocean will be very good at melting Antarctica from the bottom up.

This is partly because ice melts faster in water than it does in air, even if the air and the water are the same temperature. But the ocean-induced melting will be exacerbated by some unlucky topography: over 40% of the Antarctic ice sheet (by area) rests on bedrock that is below sea level.

bedmap2

Elevation of the bedrock underlying Antarctica. All of the blue regions are below sea level. (Figure 9 of Fretwell et al.)

This means that ocean water can melt its way in and get right under the ice, and gravity won’t stop it. The grounding lines, where the ice sheet detaches from the bedrock and floats on the ocean as an ice shelf, will retreat. Essentially, a warming ocean will turn more of the Antarctic ice sheet into ice shelves, which the ocean will then melt from the bottom up.

This situation is especially risky on a retrograde bed, where bedrock gets deeper below sea level as you go inland – like a giant, gently sloping bowl. Retrograde beds occur because of isostatic loading (the weight of an ice sheet pushes the crust down, making the tectonic plate sit lower in the mantle) as well as glacial erosion (the ice sheet scrapes away the surface bedrock over time). Ice sheets resting on retrograde beds are inherently unstable, because once the grounding lines reach the edge of the “bowl”, they will eventually retreat all the way to the bottom of the “bowl” even if the ocean water intruding beneath the ice doesn’t get any warmer. This instability occurs because the melting point temperature of water decreases as you go deeper in the ocean, where pressures are higher. In other words, the deeper the ice is in the ocean, the easier it is to melt it. Equivalently, the deeper a grounding line is in the ocean, the easier it is to make it retreat. In a retrograde bed, retreating grounding lines get deeper, so they retreat more easily, which makes them even deeper, and they retreat even more easily, and this goes on and on even if the ocean stops warming.

retrograde_bed

Diagram of an ice shelf on a retrograde bed (“Continental shelf”)

Which brings us to Terrifying Paper #1, by Rignot et al. A decent chunk of West Antarctica, called the Amundsen Sea Sector, is melting particularly quickly. The grounding lines of ice shelves in this region have been rapidly retreating (several kilometres per year), as this paper shows using satellite data. Unfortunately, the Amundsen Sea Sector sits on a retrograde bed, and the grounding lines have now gone past the edge of it. This retrograde bed is so huge that the amount of ice sheet it underpins would cause 1.2 metres of global sea level rise. We’re now committed to losing that ice eventually, even if the ocean stopped warming tomorrow. “Upstream of the 2011 grounding line positions,” Rignot et al., write, “we find no major bed obstacle that would prevent the glaciers from further retreat and draw down the entire basin.”

They look at each source glacier in turn, and it’s pretty bleak:

  • Pine Island Glacier: “A region where the bed elevation is smoothly decreasing inland, with no major hill to prevent further retreat.”
  • Smith/Kohler Glaciers: “Favorable to more vigorous ice shelf melt even if the ocean temperature does not change with time.”
  • Thwaites Glacier: “Everywhere along the grounding line, the retreat proceeds along clear pathways of retrograde bed.”

Only one small glacier, Haynes Glacier, is not necessarily doomed, since there are mountains in the way that cut off the retrograde bed.

From satellite data, you can already see the ice sheet speeding up its flow towards the coast, due to the loss of buttressing as the ice shelves thin: “Ice flow changes are detected hundreds of kilometers inland, to the flanks of the topographic divides, demonstrating that coastal perturbations are felt far inland and propagate rapidly.”

It will probably take a few centuries for the Amundsen Sector to fully disintegrate. But that 1.2 metres of global sea level rise is coming eventually, on top of what we’ve already seen from other glaciers and thermal expansion, and there’s nothing we can do to stop it (short of geoengineering). We’re going to lose a lot of coastal cities because of this glacier system alone.

Terrifying Paper #2, by Mengel & Levermann, examines the Wilkes Basin Sector of East Antarctica. This region contains enough ice to raise global sea level by 3 to 4 metres. Unlike the Amundsen Sector, we aren’t yet committed to losing this ice, but it wouldn’t be too hard to reach that point. The Wilkes Basin glaciers rest on a system of deep troughs in the bedrock. The troughs are currently full of ice, but if seawater got in there, it would melt all the way along the troughs without needing any further ocean warming – like a very bad retrograde bed situation. The entire Wilkes Basin would change from ice sheet to ice shelf, bringing along that 3-4 metres of global sea level rise.

It turns out that the only thing stopping seawater getting in the troughs is a very small bit of ice, equivalent to only 8 centimetres of global sea level rise, which Mengel & Levermann nickname the “ice plug”. As long as the ice plug is there, this sector of the ice sheet is stable; but take the ice plug away, and the whole thing will eventually fall apart even if the ocean stops warming. Simulations from an ice sheet model suggest it would take at least 200 years of increased ocean temperature to melt this ice plug, depending on how much warmer the ocean got. 200 years sounds like a long time for us to find a solution to climate change, but it actually takes much longer than that for the ocean to cool back down after it’s been warmed up.

This might sound like all bad news. And you’re right, it is. But it can always get worse. That means we can always stop it from getting worse. That’s not necessarily good news, but at least it’s empowering. The sea level rise we’re already committed to, whether it’s 1 or 2 or 5 metres, will be awful. But it’s much better than 58 metres, which is what we would get if the entire Antarctic ice sheet melted. Climate change is not an all-or-nothing situation; it falls on a spectrum. We will have to deal with some amount of climate change no matter what. The question of “how much” is for us to decide.

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If you haven’t yet watched the television series Game of Thrones or read George R. R. Martin’s A Song of Ice and Fire books on which the show is based, I would urge you to get started (unless you are a small child, in which case I would urge you to wait a few years). The show and the books are both absolute masterpieces (although, as I alluded, definitely not for kids). I’m not usually a big fan of high fantasy, but the character and plot development of this series really pulled me in.

One of the most interesting parts of the series – maybe just for me – is the way the seasons work in Westeros and Essos, the continents explored in Game of Thrones. Winter and summer occur randomly, and can last anywhere from a couple of years to more than a decade. (Here a “year” is presumably defined by a complete rotation of the planet around the Sun, which can be discerned by the stars, rather than by one full cycle of the seasons.)

So what causes these random, multiyear seasons? Many people, George R. R. Martin included, brush off the causes as magical rather than scientific. To those people I say: you have no sense of fun.

After several lunchtime conversations with my friends from UNSW and U of T (few things are more fun than letting a group of climate scientists loose on a question like this), I think I’ve found a mechanism to explain the seasons. My hypothesis is simple, has been known to work on Earth, and satisfies all the criteria I can remember (I only read the books once and I didn’t take notes). I think that “winters” in Westeros are actually miniature ice ages, caused by the same orbital mechanisms which govern ice ages on Earth.

Glacial Cycles on Earth

First let’s look at how ice ages – the cold phases of glacial cycles – work on Earth. At their most basic level, glacial cycles are caused by gravity: the gravity of other planets in the solar system, which influence Earth’s orbit around the Sun. Three main orbital cycles, known as Milankovitch cycles, result:

  1. A 100,000 year cycle in eccentricity: how elliptical (as opposed to circular) Earth’s path around the Sun is.
  2. A 41,000 year cycle in obliquity: the degree of Earth’s axial tilt.
  3. A 26,000 year cycle in precession: what time of year the North Pole is pointing towards the Sun.

These three cycles combine to impact the timing and severity of the seasons in each hemisphere. The way they combine is not simple: the superposition of three sinusoidal functions with different periods is generally a mess, and often one cycle will cancel out the effects of another. However, sometimes the three cycles combine to make the Northern Hemisphere winter relatively warm, and the Northern Hemisphere summer relatively cool.

These conditions are ideal for glacier growth in the Northern Hemisphere. A warmer winter, as long as it’s still below freezing, will often actually cause more snow to fall. A cool summer will prevent that snow from entirely melting. And as soon as you’ve got snow that sticks around for the entire year, a glacier can begin to form.

Then the ice-albedo feedback kicks in. Snow and ice reflect more sunlight than bare ground, meaning less solar radiation is absorbed by the surface. This makes the Earth’s average temperature go down, so even less of the glacier will melt each summer. Now the glacier is larger and can reflect even more sunlight. This positive feedback loop, or “vicious cycle”, is incredibly powerful. Combined with carbon cycle feedbacks, it caused glaciers several kilometres thick to spread over most of North America and Eurasia during the last ice age.

The conditions are reversed in the Southern Hemisphere: relatively cold winters and hot summers, which cause glaciers to recede. However, at this stage in Earth’s history, most of the continents are concentrated in the Northern Hemisphere. The south is mostly ocean, where there are no glaciers to recede. For this reason, the Northern Hemisphere is the one which controls Earth’s glacial cycles.

These ice ages don’t last forever, because sooner or later the Milankovitch cycles will combine in the opposite way: the Northern Hemisphere will have cold winters and hot summers, and its glaciers will start to recede. The ice-albedo feedback will be reversed: less snow and ice means more sunlight is absorbed, which makes the planet warmer, which means there is less snow and ice, and so on.

Glacial Cycles in Westeros?

I propose that Westeros (or rather, the unnamed planet which contains Westeros and Essos and any other undiscovered continents in Game of Thrones; let’s call it Westeros-world) experiences glacial cycles just like Earth, but the periods of the underlying Milankovitch cycles are much shorter – on the order of years to decades. This might imply the presence of very large planets close by, or a high number of planets in the solar system, or even multiple other solar systems which are close enough to exert significant gravitational attraction. As far as I know, all of these ideas are plausible, but I encourage any astronomers in the audience to chime in.

Given the climates of various regions in Game of Thrones, it’s clear that they all exist in the Northern Hemisphere: the further north you go, the colder it gets. The southernmost boundary of the known world is probably somewhere around the equator, because it never starts getting cold again as you travel south. Beyond that, the planet is unexplored, and it’s plausible that the Southern Hemisphere is mainly ocean. The concentration of continents in one hemisphere would allow Milankovitch cycles to induce glacial cycles in Westeros-world.

The glacial periods (“winter”) and interglacials (“summer”) would vary in length – again, on the scale of years to decades – and would appear random: the superposition of three different sine functions has an erratic pattern of peaks and troughs when you zoom in. Of course, the pattern of season lengths would eventually repeat itself, with a period equal to the least common multiple of the three Milankovitch cycle periods. But this least common multiple could be so large – centuries or even millennia – that the seasons would appear random on a human timescale. It’s not hard to believe that the people of Westeros, even the highly educated maesters, would fail to recognize a pattern which took hundreds or thousands of years to repeat.

Of course, within each glacial cycle there would be multiple smaller seasons as the planet revolved around the Sun – the way that regular seasons work on Earth. However, if the axial tilt of Westeros-world was sufficiently small, these regular seasons could be overwhelmed by the glacial cycles to the point where nobody would notice them.

There could be other hypotheses involving fluctuations in solar intensity, frequent volcanoes shooting sulfate aerosols into the stratosphere, or rapid carbon cycle feedbacks. But I think this one is the most plausible, because it’s known to happen on Earth (albeit on a much longer timescale). Can you find any holes? Please go nuts in the comments.

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Recently I was lucky enough to pay a visit to the South Island of New Zealand. I am actually a Kiwi by birth (that’s why it’s so easy for me to work in Australia) but grew up in Canada. This was my first visit back since I left as a baby – in fact, we were in my hometown exactly 20 years to the day after I left. We didn’t plan this, but it was a neat coincidence to discover.

Among the many places we visited was Aoraki / Mt. Cook National Park in the Southern Alps. It was my first experience of an alpine environment and I absolutely loved it. It was also my first glacier sighting – a momentous day in the life of any climate scientist.

There are 72 named glaciers in the park, of which we saw two: the Hooker Glacier and the Mueller Glacier. The latter is pictured below as seen from the valley floor – the thick, blue-tinged ice near the bottom of the visible portion of the mountain. As it flows downward it becomes coated in dirt and is much less pretty.

Along with most of the world’s glaciers, the Mueller is retreating (see these satellite images by NASA). At the base of the mountain on which it flows, there is a large terminal lake, coloured bright blue and green from the presence of “glacial flour” (rock ground up by the ice). According to the signs at the park, this lake has only existed since 1974.

In the photo above, you can see a large black “sill” behind the lake, which is the glacial moraine showing the previous extent of the ice. Here’s a photo of the moraines on the other side, looking down the valley:

It’s hard to capture the scale of the melt, even in photos. But when you stand beside it, the now-empty glacial valley is unbelievably huge. The fact that it was full of ice just 50 years ago boggles my mind. Changes like that don’t happen for no reason.

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Part 5 in a series of 5 for NextGen Journal
Read Part 1, Part 2, Part 3, and Part 4

Scientists can never say that something is 100% certain, but they can come pretty close. After a while, a theory becomes so strong that the academic community accepts it and moves on to more interesting problems. Replicating an experiment for the thousandth time just isn’t a good use of scientific resources. For example, conducting a medical trial to confirm that smoking increases one’s risk of cancer is no longer very useful; we covered that decades ago. Instead, a medical trial to test the effectiveness of different strategies to help people quit smoking will lead to much greater scientific and societal benefit.

In the same manner, scientists have known since the 1970s that human emissions of greenhouse gases are exerting a warming force on the climate. More recently, the warming started to show up, in certain patterns that confirm it is caused by our activities. These facts are no longer controversial in the scientific community (the opinion pages of newspapers are another story, though). While they will always have a tiny bit of uncertainty, it’s time to move on to more interesting problems. So where are the real uncertainties? What are the new frontiers of climate science?

First of all, projections of climate change depend on what the world decides to do about climate change – a metric that is more uncertain than any of the physics underlying our understanding of the problem. If we collectively step up and reduce our emissions, both quickly and significantly, the world won’t warm too much. If we ignore the problem and do nothing, it will warm a great deal. At this point, our actions could go either way.

Additionally, even though we know the world is going to warm, we don’t know exactly how much, even given a particular emission scenario. We don’t know exactly how sensitive the climate system is, because it’s quite a complex beast. However, using climate models and historical data, we can get an idea. Here is a probability density function for climate sensitivity: the greater the area under the curve at a specific point on the x-axis, the greater the probability that the climate sensitivity is equal to that value of x (IPCC, 2007):

This curve shows us that climate sensitivity is most likely around 3 degrees Celsius for every doubling of atmospheric carbon dixoide, since that’s where the area peaks. There’s a small chance that it’s less than that, so the world might warm a little less. But there’s a greater chance that climate sensitivity is greater than 3 degrees so the world will warm more. So this graph tells us something kind of scary: if we’re wrong about climate sensitivity being about 3 degrees, we’re probably wrong in the direction we don’t want – that is, the problem being worse than we expect. This metric has a lot to do with positive feedbacks (“vicious cycles” of warming) in the climate system.

Another area of uncertainty is precipitation. Temperature is a lot easier to forecast than precipitation, both regionally and globally. With global warming, the extra thermal energy in the climate system will lead to more water in the air, so there will be more precipitation overall – but the extra energy also drives evaporation of surface water to increase. Some areas will experience flooding, and some will experience drought; many areas will experience some of each, depending on the time of year. In summary, we will have more of each extreme when it comes to precipitation, but the when and where is highly uncertain.

Scientists are also unsure about the rate and extent of future sea level rise. Warming causes the sea to rise for two different reasons:

  1. Water expands as it warms, which is easy to model;
  2. Glaciers and ice sheets melt and fall into the ocean, which is very difficult to model.

If we cause the Earth to warm indefinitely, all the ice in the world will turn into water, but we won’t get that far (hopefully). So how much ice will melt, and how fast will it go? This depends on feedbacks in the climate system, glacial dynamics, and many other phenomena that are quantitatively poorly understood.

These examples of uncertainty in climate science, just a few of many, don’t give us an excuse to do nothing about the problem. As Brian, a Master’s student from Canada, wrote, “You don’t have to have the seventh decimal place filled in to see that the number isn’t looking good.”. We know that there is a problem, and it might be somewhat better or somewhat worse than scientists are currently predicting, but it won’t go away. As we noted above, in many cases it’s more likely to be worse than it is to be better. Even a shallow understanding of the implications of “worse” should be enough for anyone to see the necessity of action.

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James Balog has an amazing, but terrifying, presentation of how different glaciers are retreating, shown on time-lapse cameras that take pictures every hour. Watch it here.

(It looks like I should be able to embed the video, but it’s not working. I hardly know html so I’m probably missing something – I am using the “code” and “/code” buttons, though. Any suggestions?)

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