Originally published on June 8th, 2014 at generalcirculation.blogspot.com
I have often gone looking for mental clarity in a good children's story.
During finals week in undergrad, I used to camp out in my university's School of Education library, where the collection of "juvenile" literature was kept. Between reworking Geophysics problem sets and rereading notes on Existentialism, I would duck between the tall, metal bookshelves and clear my brain with a few chapters from a childhood favorite--Sideways Stories from Wayside School, From The Mixed-Up Files of Mrs. Basil E. Frankweiler, A Wrinkle in Time. The simple syllogisms and straightforward analogies to be found in the dim, quiet aisles of J/YA Fiction were an excellent way to refocus when a technical theory or philosophical concept had me thinking in circles.
Perhaps this is why, as I prepared to give a talk a few weeks ago, I found myself gravitating towards one of the best known children's parables to frame the context for my work: Aesop's fable of The Tortoise and the Hare.
As an acceleration in my research responsibilities has led to a deceleration in my blog posts, it's high time for me to introduce the focus of my research: black carbon aerosol--what I like to think of as the hare to carbon dioxide's tortoise.
There are many ways in which the relationship between black carbon and carbon dioxide and their interaction with the climate system reminds me of the hare and his reptilian rival. And I must admit, I'm rather fascinated by the hare.
The old fable tells the story of the overconfident hare--fast, flighty, and inconsistent--who challenges the slow-footed tortoise to a race, but loses when he brashly takes a nap halfway and wakes to find that his unhurried competitor has crossed the finish line ahead of him while he slept. The story is the very embodiment of "slow and steady wins the race," but has variously been interpreted as a warning against idleness and a reprimand of capitalism, among other things.
Although the tortoise wins the race, the hare would probably make for a much better psychological study.
So, let me introduce the protagonists of this version of the fable:
Carbon Dioxide in the role of the tortoise is a familiar antihero--the colorless, odorless gas that comes from fossil fuel combustion and warms the planet by trapping thermal radiation and preventing it from escaping to space. Once carbon dioxide is in the atmosphere, it takes from a century to millennia to remove it. Because it lasts for so long, it gets uniformly mixed in the atmosphere. In this way, carbon dioxide is slow, steady, and predictable.
Black Carbon in the role of the hare is a less notorious villain. It refers to the tiny, dark, solid particles that are emitted whenever something (fossil fuels, biomass, etc.) is burned without enough oxygen. It's basically the part of soot that absorbs sunlight, hence its first name, Black. It also warms the climate system (setting aside its complicated interactions with clouds), but by absorbing solar radiation rather than thermal radiation. In fact, it warms the climate by much more than carbon dioxide does per unit mass, but there's thankfully much less of it. These tiny particles last in the atmosphere for only about a week before they either fall out or are washed out by rain. Fast, flighty, and inconsistent.
Carbon dioxide will ultimately win the race. Even if we were to stop emitting carbon dioxide, it's long lifetime means that its concentrations in the atmosphere (what matters for the Earth's energy balance) would only decrease by about 10% over the next century. If we stopped emitting black carbon today, on the other hand, it would be gone from the atmosphere in a matter of weeks. These characteristics have made black carbon the target of United Nations efforts to reduce the short-lived climate pollutants--black carbon, methane, ozone--that contribute to global warming but are easier to tackle than carbon dioxide.
There are also some interesting geopolitical implications, though, of the difference in the character of our two protagonists.
Because Black Carbon doesn't last in the atmosphere for very long, it stays concentrated near where it's emitted--its distribution is spotty and localized, as opposed to carbon dioxide's more-or-less uniform one. And because black carbon interacts with sunlight, which is unevenly distributed from the equator to the poles, where it is concentrated matters for how much sunlight it absorbs and how much it affects the earth's energy balance. Because of this fact alone, the climate system cares about where black carbon is emitted in a way that it doesn't for carbon dioxide (not to mention the fact that black carbon can interact directly with cloud formation and regional circulation on fast time-scales in a way that carbon dioxide doesn't).
This adds an extra dimension to my earlier blogpost on the Ethics of Air, contemplating the ethics of pollution that we outsource to China through trade. Moving emissions of black carbon from one place to another through trade not only raises issues about who is responsible for those emissions (a concern that is also present with carbon dioxide emission), but also creates the added dimension that the climate response to those emissions may be very different in one place than another (a concern that is not present with carbon dioxide).
It is this mischievous, mercurial behavior of black carbon that fascinates me. Although the tortoise ultimately wins the race, the hare would probably make for a much better psychological study. Despite carbon dioxide greater importance to the climate system in the long run, I am still drawn to understanding the climate impact of black carbon, in all its heterogeneous, transitory glory. My research so far has touched on understanding how black carbon affects precipitation, clouds, and the amount of sunlight at the Earth's surface--little puzzle pieces in the intricate jigsaw of how this tiny, fickle particle can affect the climate.
So back I go to looking for the next puzzle piece and figuring out where it fits. Perhaps I'll publish my next article in the Journal of Geophysical Anthropomorphisms.