Friday, January 16, 2009

Global Warming, The Carbon Cycle, and Fish Poop

Posted on: January 16, 2009 11:25 AM, by Mike Dunford

When we talk about the role of fossil fuels in climate chance, what we're really talking about is the carbon cycle. That's the term that scientists use to describe the different forms that carbon is stored in on the earth, and the different ways that it can move from form to form. Understanding the carbon cycle is one of the keys to understanding both the effect of burning carbon-based fuels and the issues involved in trying to take carbon dioxide out of the atmosphere. According to a paper in the latest edition of Science, there may still be some pretty significant gaps in our knowledge of the carbon cycle. In particular, it looks like our understanding of the way carbon moves through the oceans may have been suffering because we didn't know poop about fish poop.

Before we get down to the gritty details and talk about what poop has to do with anything, it might be good to start with a quick review of the carbon cycle. Actually, it might be even better to start with a quick review of one of those concepts that we all learn in third-grade physics, but don't think about much in our day to day world: the law of conservation of mass/matter.

Matter is not created or destroyed. Therefore the amount of mass in a closed system will remain constant no matter what happens.

Like most things in science, that might be a bit of a simplification, but when we're looking at something the size of the Earth, it's good enough. Relativity, quantum mechanics, and space dust might all complicate things a bit, but not enough to matter. For our purposes, we can reasonably assume that all the carbon that we're putting into the atmosphere in the form of carbon dioxide has been here since the earth was formed, and that if we want to take the carbon dioxide back out of the atmosphere, we're going to have to find somewhere on this planet to store the carbon.

With that in mind, let's look at the some of the more important ways that carbon can move through the crust, oceans, and biosphere, and atmosphere.

Carbon Cycle-Cute Diagram.Jpeg

Source: NASA via Wikimedia.

Carbon can be found in the atmosphere in a few forms, the most important of which is carbon dioxide. It can be found on land and in the crust in any number of forms, both as organic compounds in living organisms and their remains (including the range of fossil fuels) and in rocks and soils as inorganic minerals like calcium carbonate. It can be found in the oceans as dissolved carbon dioxide, dissolved minerals, and in organisms that live in the oceans.

Carbon can be released into the atmosphere in a number of different ways. Carbonate rocks, for example, can produce carbon dioxide through natural weathering processes, and volcanoes can release carbon dioxide from magmas. At this point in the history of the earth, though, we know what the biggest cause of carbon dioxide entering the atmosphere is - and he is us.

It's possible that our entire success as a species has been the result of our learning how to use a particular chemical reaction:

Organic carbon + Oxygen = Carbon Dioxide + Water + Energy

For most of human history, we were mostly burning plant matter in relatively small quantities, so this wasn't a huge deal. The organic carbon in question had mostly come from photosynthesis, and most of it would have been released as carbon dioxide anyway - we use, along with lots of other living things, use that same chemical reaction when we produce our own energy.

The Industrial Revolution changed that. We started to burn fossil fuels - coal, petroleum products, and natural gas. The carbon that's in the fossil fuels comes from the remains of plants that pulled it out of the atmosphere and fixed it as organic compounds. Most plants, when they die, decompose and their carbon is released back into the atmosphere. The same thing happens to most animals. But not all. Some living things get fossilized after they die.

The thing is, we don't have lots of fossil fuel because lots of things get turned into fossils all the time. We have lots of fossil fuel because there's been a lot of time for things to get turned into fossil fuels. These are deposits of carbon that were formed very slowly, over tens and hundreds of millions of years. Left to themselves, these deposits would have been released back into the atmosphere through weathering and other natural processes over similarly long periods of time. We're taking these deposits that were formed over intervals of tens and hundreds of millions of years, and we're burning them over periods of tens to hundreds of years.

If you think that we're not pumping fossil-fuel derived carbon dioxide into the atmosphere millions of times faster than it would get there on its own, I'd suggest that you go back and look at the two bits of boldfaced text earlier in this article.

The increase in carbon dioxide in the atmosphere is leading to an increase in the amount of heat that we're retaining from the sun because carbon dioxide is a greenhouse gas - it traps energy that would otherwise have radiated out into space.

Now that we've figured out that this is a real problem - the odd conservative ideologue notwithstanding - we've started to try and find ways to fix the problem, while still producing enough energy to drive our modern world. A wide variety of solutions have been proposed, many of which involve continuing to burn fossil fuels, but trapping and storing the carbon dioxide that's produced. In fact, there's a coal plant in Germany that's started doing just that.

The German plant is storing (or at least planing to store) the carbon dioxide by injecting it into a depleted oil field, but another storage site that's frequently proposed is the deep ocean. At depth, the ocean is undersaturated with respect to carbon dioxide, which is a fancy way of saying that it can hold more than it currently does. That means, it's been suggested, that we can take the carbon dioxide from fossil fuel combustion and pump it down into the deep ocean through really long pipes.

And this, finally, brings us to the gut of the matter. And to fish poop.

The I haven't talked about it much yet, but the ocean is involved in the carbon cycle. Things like the depth at which the ocean is no longer carbon-dioxide saturated are determined by the chemistry of the ocean, and by the way carbon moves through the seas. An article that was just published in the journal Science by Wilson et al. suggests that we may not have known as much about the oceanic carbon cycle as we thought we did.

In particular, we might not have considered the impact that billions of fish can have, just by living, drinking, and pooping.

You see, the internal environment of a fish contains a higher concentration of water (and lower concentration of salt) than the ocean does. As we all learned at some point in school, this means that water is going to tend to move from the fish back into the ocean. This is the same process that's in play when salt is poured on a slug, and even in the ocean the effects would be similar if the fish weren't able to somehow counteract it.

Fish counteract the tendency of water to leave their body by actively pulling more water in. Fish drink like fish. But, because they're drinking saltwater, they need to do something to pull out the salts. In the case of the calcium and magnesium, they precipitate it out in their guts by forming inorganic calcium and magnesium carbonate crystals. What happens to these "piscine carbonates" is entirely predictable, as Wilson et al. point out:

Carbonate precipitates formed in the gut are excreted either within discrete mucus-coated tubes or pellets, or incorporated with feces when fish are feeding. The organic mucus-matrix is rapidly degraded in natural seawater, leaving only inorganic crystals of CaCO3 with high magnesium content (Mg:Ca ratio ranging from 10 to 33 mol %)
[endnotes omitted]

The importance of this effect is a bit less obvious. There's a lot less fish than plankton, after all, so how important is it really likely to be? The answer is a bit surprising. The authors did a range of calculations based on a number of different estimates of both fish biomass and the rate of production of "piscine carbonates":

To calculate the teleostean contribution to oceanic carbonate budgets requires knowledge of global marine fish biomass. We used two entirely independent models to describe the size composition and abundance of marine fish across the global oceans, one by using a size-based macro-ecological approach and the other by using Ecopath software. The fish biomass estimates generated for each size-class and the relevant average local sea temperatures were then combined with individual fish carbonate excretion rates to predict global fish CaCO3 production ranging from 3.2 x 1012 to 8.9 x 1012 mol year-1 (0.04 to 0.11 Pg of CaCO3-C year-1). This range accounts for 2.7 to 15.4% of estimates for total global new CaCO3 production in the surface oceans.

[endnotes omitted]

In simple terms, the authors of the paper have just informed us that we may not have noticed a process that's responsible for a substantial amount of the carbon movement in the oceans. That's kind of a big deal (which would explain the Science article).

The effect that climate change is going to have on this particular form of carbon movement is not entirely clear (at least to me), nor is the effect that this might have on climate change. "Not none" is probably a reasonable guess, and I'm sure that we'll learn more in the future.

What is clear is that this shows us something that we would be advised to remember when we start to talk about things like pumping a gajillion tons of CO2 into the oceans:

Do we really want to take chances messing around with things that we might not know piscine carbonates about?


R. W. Wilson, F. J. Millero, J. R. Taylor, P. J. Walsh, V. Christensen, S. Jennings, M. Grosell (2009). Contribution of Fish to the Marine Inorganic Carbon Cycle Science, 323 (5912), 359-362 DOI: 10.1126/science.1157972