Posts Tagged ‘albedo’

A good week for defense

January 19, 2015

I successfully defended my dissertation last Thursday! Defense title: “Measured and modeled albedos of sea-ice surfaces on the oceans of Snowball Earth.”

Here’s how I summed up my talk:

A planet orbits round a yellow sun
Light years away or megayears ago
Its seas are dark, its continent is dun
But brilliant sea ice sets its pole aglow

Its CO2 drops dangerously low
Tendrils of ice reach from the polar caps
That sparks a feedback: oceans turn to snow,
Glaciers push in to close off any gaps

The oceans roiled with countless living cells
Who learned to take their energy from light
Now locked beneath the cold of Dante’s hells
They starve; how long must they endure this night?

Why has this happened, and by what device?
To know, we must investigate the ice.

As sea ice freezes, tiny drops of brine
Are trapped between the quickly forming plates
When cold enough, their molecules align
Into sodium chloride dihydrates

The solid crystals catch and scatter light
Reflect it back, refuse its energy
And I myself have quantified how bright
The surface of the sub-eutectic sea

And in our lab we’ve watched salt ice sublime
With instruments ingeniously designed
The secrets of another space and time
Unfold beneath my models and my mind

This is my work; I hope you will agree
That it is worthy of a Ph.D.

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A Field Scientist’s Work is Never Done

April 7, 2011

Original audio post.

Today’s science fell a bit flat, at least on my end. Ruschle and I went out to re-take some albedo measurements, but were frustrated by rapidly-changing cloud conditions that confused the instrument.



Ruschle covers the Sun so I can photograph the clouds around it, in an effort to compensate for their effects; eventually we just gave up.

Mel and Martin had better luck making snow measurements and taking samples. Right now, Martin is sitting behind me with his makeshift lab, pouring diethyl phthalate into snow samples to preserve them. We’ve been having a little trouble getting the stuff cold enough to set, but that’s another post, I think.

Anyway, I figured I’d talk a little more about the rhythms of life in camp. Life in any camp seems to be defined by its chores, so I’ll start with those. Thinking about chores, actually, I was surprised at how few I could come up with. Perhaps that’s because a lot of chores at normal camps involve cleaning things, and we happen to be living in a place with essentially zero dirt.

Responsibilities begin in the morning, when whoever slept with the satellite phone (it has to be kept warm, so it’ll work when it’s needed) calls McMurdo to let them know we’re still OK. Every field camp does this check-in. It’s a sort of failsafe, in case something disastrous happens and we aren’t able to contact anyone to call for help.

Whoever’s cooking breakfast–or sometimes whoever’s in the kitchen tent first–collects a bucket of snow from outside to melt for water. Melting snow is probably the most constant and time-consuming chore. All the water for cooking, drinking, dishwashing, and other miscellaneous uses must be melted in the big pot on the propane stove. Notably, bathing is not really on the list of uses. Living in unheated tents means that getting wet is more or less courting hypothermia, so we mostly do without. I’m sure there are ways to manage it [bathing], but for three weeks, in a place without dirt, we think we can get away with it.


The water-melting pot, which was in near-constant use when we were in the tent.

We hold an informal meeting after breakfast to decide, mostly based on the weather, what to do with the day. If we’re going out, we need to take the covers off the snow machines and inspect them before heading out to the field to do science. Coming back, we re-fuel and re-cover the snow machines. Without the covers, the engine compartments fill with drifted snow.



Snowmachines, neatly put away.

The cook for the day (tomorrow, that’s me) starts dinner while everyone else puts away the scientific instruments. (I’m thinking I might make some kind of Malaysian-style curry.) More snow is melted–again, usually by the Cook for the Day. Someone might sweep the kitchen, if it’s filled up with drifted snow again. Snow is kind of our equivalent of dirt, but it’s much less bothersome, since it generally evaporates if you leave it long enough. Still, when it accumulates in the kitchen, it becomes somewhat irritating.

[SATPHONE CUTS OUT UNEXPECTEDLY]
Supplementary audio post.

Hey, sorry–I think my last post got cut off when the satellite phone went out. Somewhere around where I was talking about how it’s irritating when the kitchen fills up with drifted snow. I’m actually rather surprised that the satphone hasn’t cut out mid-post before now. Unfortunately the WordPress voice-post function is a bit primitive; the only option is to call, leave a post, and hang up. You can’t really edit it afterwards. So, I won’t try to re-record the whole post this time, since I was mostly finished anyway. After dinner, pretty much all that’s left to be done is to wash the dishes and decide who’s going to sleep with the satphone. So, uh, it’s about time for bed here, so I will bid you all goodnight and talk to you soon. ‘Bye.

Repost: Cracks and Bubbles

February 8, 2011

Original post, called in on January 13, 2011. Thanks once again to Jonathan Beall for the transcription!

“Hi. Today was a quiet day. Well, in terms of activity. It was a quiet day in terms of activity because it was a noisy day in terms of wind. We had a good day of spirited scientific discussions in the tent instead. We were mostly discussing bubbles and their contribution to the amount of light reflected from ice, or its albedo.

Bubbles are awesome.

Ice is very transparent. Light can travel a long way before it gets absorbed. If there are no bubbles in the ice, most light will just travel straight through, and the albedo will be low. If there are bubbles, though, light can hit the bubbles and bounce away in a different direction. A lot of the light that goes into the ice will end up bouncing right back out, which is why bubbly ice has a higher albedo than clear ice. Ice in glaciers and on ice sheets, like where we are, has lots of bubbles because it’s formed from compressed snow. The spaces between snowflakes at the surface will turn into bubbles as the snow is squeezed into ice and a small amount of the air remains behind as bubbles form.



Complex bubble shapes are partly a result of the complex shapes in the snow crystals that formed them.

We also talked about cracks in the ice, which can increase the albedo just like bubbles do. The blue ice here has lots of cracks in it, mostly quite thin. We think they form partly because the ice is cracking as it moves, and partly because, as the top of the ice erodes away from the constant wind, the ice lower down is no longer under as much pressure, and it expands creating more cracks.

Cracks in the ice (picture taken using the Crack Box, an invention I'll explain in a later post)

We actually put on our parkas and went out to look at the ice near camp to see how the cracks behaved and to figure how we might account for them in our measurements. We probably looked a little silly lying face down on the ice and wriggling along the ice like seals, but then science is frequently a little silly; that’s one of the reasons I like it.

Another incident of science being silly.

Cheers!”

Some Science Occurs

September 3, 2009

On Tuesday we managed to get a slightly earlier start heading out to the sea ice. It was still later than we’d planned; starting up a new project always seems to mean discovering several dozen things you forgot to bring.

The ride out was as bumpy as before, except this time we had the Analytic Spectral Device (for measuring albedos) along and thus had to be extra careful about excessive vibration.

We were looking for first-year sea ice that was bare of snow. We ended up near Tent Island, where our Kiwi friends had told us we might be able to find something sufficiently wind-scoured. No bare ice was immediately apparent, but we did find this iceberg trapped in the sea ice:


Click for larger version.



Say 'freeze'!

We investigated it, being careful to stay well clear of any possible falling ice. Icebergs are not to be trifled with; you can see the size of this one, and imagine how even a smallish piece of it might feel if it were to detach while you were underneath it.

Having found no bare ice even in the most wind-scoured of spots, and with the sun gradually sinking towards the horizon, we retraced our steps to an area which at least didn’t have very much snow and broke out our gear.



Rich uses the ASD to measure light levels. An overcast day is best for this, because the light comes in more evenly from all directions.



Me and the ice-coring device. Wrestling one of these down into the ice warms you up rather nicely, even at temperatures in the -30 range.



The snow was not very deep at all, but unfortunately even not-very-deep snow has quite a significant effect on albedo.

We got our albedo measurements, and we got our ice core, and we got very cold indeed.

On the way back we stopped in the sea ice hut the Kiwis have been using all winter, where they graciously fed us tea and cake. They also showed us their conductivity/temperature/depth sensor and the nifty hole in the sea ice–conveniently located within the warm confines of the hut–through which they lower it to get profiles of the ocean water below. Afterwards they kindly directed us to their own road back towards base, which felt orders of magnitude smoother than the route we’d been using and allowed us to get home in one hour instead of three despite being technically longer. An excellent day all round.

Salt, Sea Ice and Science

August 27, 2009

Science-wise, our team has been busy getting settled in to the lab and taking all the trainings required for us to be allowed to do things. We won’t be able to get out on the ice until after our Sea Ice Training on Friday, so I thought I’d take this opportunity to talk a little about the actual science we’ll be doing.

You may recall from earlier posts how increased albedo can lead to a Snowball and CO2 can end one. As the planet cooled off and the climate system change, new types of ice could form that are rarely, if ever, found on our modern Earth. That’s why we’re down here in Antarctica in the winter–to find modern analogs of this very, very cold ice.

During our first couple of weeks here we’ll be looking for cold sea ice. “Sea ice” in science terms means ice that forms when seawater freezes–ice that formed from fresh water just happens to be floating on the sea, like icebergs, doesn’t count. When sea water begins to freeze, it forms a large number of small crystals of ice, called frazil. This stage is also called “grease ice” because it looks a bit like an oil slick on the water. Eventually the crystals begin freezing together into a solid sheet. As they freeze they trap small pockets of salt water, and these are what we’re really interested in.

As you may know, salt lowers the melting point of ice so that it becomes liquid at lower temperatures, a property that is useful when melting ice off of driveways or making ice cream by hand. Sea water averages about 3.5% salt (that’s six or seven teaspoons in a quart of salt for you American types, or 35g in a liter for the rest.) This particular salinity level means it freezes at 28F or -2C. As it freezes, the ice pushes out the salt, and the water in the pockets of salt water–the brine pores–gets saltier. If the ice gets colder, this saltier water will also start to freeze, pushing out more salt and making the brine pockets smaller and saltier still.

At -9F/-23C, some of the salt in the water starts to form crystals, called hydrohalite. Like the many small crystals in snow or table salt, these crystals of hydrohalite are good at scattering light. By sending light back out the way it came, the crystals in the brine pockets can increase the amount of light reflected from the sea ice. Our colleague Bonnie Light demonstrated this effect in the lab, and we are hoping to find it out in the field. These albedo measurements will help improve models of Snowball Earth. Because so much area in the sunlit tropics is covered in sea ice on Snowball Earth, small changes in albedo can have large effects, so it’s helpful to have measurements that are as accurate as possible.

Not all of our equipment is here or unpacked yet, but I’ll post photos when it is.

Escaping the Snowball

August 22, 2009

So, how does a planet get itself out of a Snowball state? The short answer, as I mentioned in my last post, is “carbon dioxide.” When the Earth was frozen over, volcanic activity didn’t stop–volcanos kept pumping gases, including greenhouse gases such as CO2, into the atmosphere.

Now we get into the geochemical aspects, which are a bit outside my field, so please take my explanation with a grain of salt. Normally, CO2 that enters the atmosphere reacts with rocks on the Earth’s surface, creating carbonate minerals. This process gradually removes CO2 from the atmosphere. When the amount of CO2 in the atmosphere increases, the Earth warms, which speeds up the weathering process and removes CO2 more quickly. This CO2 feedback helps keep the Earth at a comparatively stable temperature over the long term (CO2 weathering is slow, so it can take thousands of years for CO2 to leave the atmosphere even at warmer temperatures.) Here’s a paper on the CO2 weathering feedback (link goes to PDF file) for those who are interested in a more in-depth treatment.

Much of this CO2 weathering process occurs when CO2 combines with minerals to form carbonates, such as calcium carbonate (the principal component of limestone, among other things.) This process is helped along when CO2 mixes with water in rain, rivers, and the ocean. However, on a Snowball Earth, little rain or snow would fall, and few, if any, rivers would flow. No CO2 could dissolve into the oceans, because they would be cut off from the atmosphere by a thick layer of ice. Instead of weathering out of the atmosphere, the CO2 would simply build up over time. As carbon dioxide levels increased, so would the greenhouse effect. Eventually, the CO2 would reach such high concentrations–hundreds of times modern levels, by some estimates (for example, Hoffman and others, 1998)–that the warming greenhouse effect would overcome the cooling effect of the light-reflecting ice.

When the greenhouse finally overcame the ice, it melted quickly, suddenly transforming most of the world’s surface from brilliantly reflective ice to dark, absorbent sea. Without the reflective effect of the ice, the greenhouse effect took over and the temperatures soared. At the same time, the massive amount of CO2 in the atmosphere began to dissolve into the ocean, where it precipitated out to form thick layers of carbonates. These “cap carbonates” are still visible today and form one piece of evidence for the Snowball Earth theory.

This is, of course, just one of several ideas about how the Snowball Earth scenario might have played out. It’s possible that, instead of Snowball Earth, there was a “Slushball” Earth, with open water at the equator. A Slushball Earth would have much less trouble supporting life, which would thrive in the areas of open ocean. On the other hand, that open ocean would act as a sink for CO2, and the Slushball might not be able to accumulate enough CO2 to overcome the albedo of the ice on the rest of the globe. Snowball Earth researchers continue to search for a model that will balance these various factors–account for sea-level glaciers at the tropics, provide a refuge for life, and permit enough CO2 buildup to initiate the return to a normal climate.

But you don’t have to take my word for it, as Levar Burton would say. Much more information on all these topics is available at SnowballEarth.org, which gives a good comprehensive overview of the whole business.

<A diagram from SnowballEarth.org, which gives a good graphical overview of the Snowball event.

A diagram from SnowballEarth.org, which gives a good graphical overview of the Snowball event.

Ice, albedo, and the Snowball Earth theory

August 4, 2009

Life on a planet is a fragile thing, particularly if the life is at all complex. Naturally, astrobiologists like to catalog the ways that life can be decimated or snuffed out entirely. Asteroids, comets, and bursts of high-energy gamma radiation are commonly cited catastrophes, but a planet can also run into trouble without any cosmic interference. One of these more self-contained scenarios is “runaway ice-albedo feedback.”

Albedo, if you aren’t familiar with the term, is just the amount of light that’s reflected from a surface relative to the total amount of light hitting it. You can measure it using a device that collects all the light coming down from the sky, then flips over to collect all the light being reflected back from the surface below it. Divide the latter by the former and you have your albedo. Dark-colored things have a low albedo–for instance, the open ocean has an albedo of around 0.06, meaning that it reflects only 6% of the light that hits it and absorbs the rest, which is mostly converted to heat. Light-colored things have a high albedo–for instance, sea ice has an albedo between 0.5 and 0.7, which goes up to 0.9 or higher when the sea ice is covered by snow. The relationship between albedo and absorbed heat will probably be obvious to anyone who has ever worn a black shirt on a hot, sunny summer day. Summer clothes tend to be light-colored because it maximizes your personal albedo and keeps you pleasantly cool.

Ice-albedo feedback occurs because ice and snow are highly reflective. Suppose a planet cools down a little, perhaps because of a reduction in solar output or the amount of CO2 in the air. The ice caps will grow a bit, which will reflect more sunlight back into space. With less sunlight being absorbed, the planet cools a bit more, the ice caps grow a bit more, and so on. (This works in the other direction, as well–the ice at the North Pole is melting faster because it exposes low-albedo ocean as it retreats.) Usually this feedback loop runs into some limit and stops. Theoretically, however, it can go into a “runaway” state. Computer models of this process suggest that once ice reaches 30 degrees latitude, the feedback becomes unstoppable and the entire planet freezes over very rapidly.

A diagram of a planet undergoing runaway ice-albedo feedback.

A diagram of a planet undergoing runaway ice-albedo feedback.

This fully-frozen state is fairly stable; the ice reflects most of the sunlight back into space, keeping the planet cold. In fact, when Mikhail Budyko first modeled this phenomenon in the 1960s, he concluded that it could never have happened on the Earth because there would have been no way to escape from it.

However, in recent decades an increasing amount of geological evidence seems to suggest that the entire Earth did freeze over, around 600 million years ago (and perhaps at other times before that.) How did the Earth transform from its frigid Snowball Earth alter ego back into the warm, pleasant planet we know today? Tune in next time to find out!