Posts Tagged ‘SnowballEarth’

Whys and Wherefores

January 4, 2011

This one’s a bit late–sorry about that! I’ve gone for a longer post to make up for the tardiness.

Antarctica, as you may have noticed, is a long way away from almost everything. And even aside from being distant, it’s logistically tricky to get to. The Southern Ocean is stormy and mercurial at the best of times, every iota of fuel and supplies must be shipped in, and the preponderance of ice and snow presents its own special problems. So why have we made the trek back once again?

You may recall that last year we came down seeking types of ice that can’t be found anywhere else on Earth: sea ice so cold that the salts trapped within it can crystallize, and ice whose surface has sublimated away to leave behind a crust of the salt mirabilite. This year we’re once again seeking ice that exists nowhere else. In this case, we’re looking for ice that has formed from the accumulation of snow and eventually become re-exposed to the sun by sublimation without ever experiencing temperatures above freezing.

There are three main layers to the Antarctic ice sheet. At the surface is snow. Over time, as surface snow is buried by the accumulation of later years, it compacts and grows dense and hard. This deep-down, compacted snow is called firn. In the rest of the world “firn” simply means snow that has survived through the summer melt season, but in Antarctica, where there is no summer melt season except at a few places near the coast, firn is sometimes defined as snow that has reached a certain density (550 kilograms per cubic meter, which is the density at which simple rearrangement of ice particles gives way to more complicated densification processes.) The firn continues to grow denser under the weight of the snow and firn above it until the spaces between snow grains close off to become bubbles, and the firn becomes ice.

Blue ice stratigraphy

The ice flows under its own weight. In some places, the ice surface may flow into an area of the ice sheet where sun and wind vaporize (or sublimate, which means to turn directly from solid to vapor) the snow faster than it can accumulate. In this case, the snow on the surface will sublimate away, exposing the firn below and eventually the ice. And that firn and ice—exposed to the sun, yet never melted—is unique to Antarctica. These “blue ice zones” are of interest to meteorite hunters, because meteorites that may have been buried in the snow become exposed, and are easy to spot on the surface. My colleagues already found and collected some meteorites during their first trip out.

We’re interested for a different reason, though–we want to know how much light reflects off of these blue ice zones, because there would have been a lot of them on Snowball Earth. (Here’s a great article in the Antarctic Sun that talks about our research.) Many areas of Snowball Earth, particularly near the equator, would have been so dry that snow sublimated away faster than it could accumulate.

These areas of exposed blue ice, being much darker than snow, would have absorbed a lot of sunlight and had a significant effect on the planet’s balance of energy. So knowing exactly how much sunlight they absorb is important to people trying to model Snowball Earth.

So that’s why we’re here. Mostly, anyway–some of us, including myself, also have a keen interest in snow and want to do some side projects involving the microstructure of snow and bubbly ice. But more on that later!

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!


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