Monthly Archives: October 2009

Preposterous extrapolations: climate change and marathon runners

Burning fossil fuels has increased atmospheric levels of carbon dioxide. Yet an inevitable corollary of this fact remains widely unappreciated. Combustion theorists have long noted that fire, whether it occurs in a coal power plant, an internal combustion engine, a gas turbine, both fuel and oxygen. Both are consumed by the fire.

So stoichiometry tells us that oxygen levels in the atmosphere must be going down. Have they? Yes, they have: Andrew Manning and Ralph Keeling of the Scripps Institute of Oceanography have measured the decrease in atmospheric oxygen arising (mostly) from combusion of fossil fuels. From 1990 to 2000, oxygen in the air decreased by about 0.0031% 0.015%.

I find that fact to be amazing in and of itself. “But how will having less oxygen in the air change your life,” you might say. “Where’s the news I can use?” If you’re a runner, well, here we go…the atmosphere is thinner at altitude, and as a result, runners go slower. Conveniently for preposterous extrapolaters, some intrepid physiologists have developed a semi-theoretical (does that sound better or worse than semi-empirical?) estimate for the effect that altitude has on running. At sea level oxygen partial pressure is about 160 mmHg, but air up at an elevation of 520 meters has an oxygen partial pressure of 150 mmHg or so. And the physiologists’ semi-theory says that marathon world record equivalent times at 520 m are about 128 seconds slower.

Combining the atmospheric and phsyiological data, we see that world-record equivalent marathon times in the year 2000 might be 0.067 0.32 seconds slower than in 1990, due to the depletion of atmospheric oxygen by fossil fuel combustion. And, since the decline in running performance at altitude is somewhat offset by decreased wind resistance in the thinner air, decreasing oxygen at a constant pressure might be twice as worse as just thinning out the air.

For men between 18 and 34, the qualifying time for the Boston Marathon is 31:10:59. That’s about 51% slower than world record pace, meaning that slower runners huff, puff, and struggle for oxygen for a longer time. So I think it’s reasonable to assume that fossil-fuel-driven depletion of oxygen from the atmosphere lowers finishing times by a corresponding lower amount.

The end result? If you’re a runner, and you miss qualifying for the Boston Marathon by 0.2 1 second or less, you can use climate change as an excuse.

(The exercise of calculating the most probable number of people who have missed qualifying for Boston due to climate change is left to the reader.)

UPDATE: I was off by five-fold! The change in atmospheric oxygen from 1990 to 2000 was not 0.0031%; it is closer to 0.015% (as should have been clear to had I read the caption to Table 2 in this paper more carefully.) That means that world-record equivalent marathon times may have gone up by between 0.3 and 0.6 seconds due to oxygen depletion in the atmosphere. Times for male would-be Boston qualifiers have gone up from 3:10:59 by a full second. Thanks to Ralph Keeling for the correction, and also be sure to check out his new web site on atmospheric oxygen research.

Absolutist Statement of the Day

Find it here:

“The bottom line is that you can’t meet your nutritional needs in six cookies and one meal a day. It’s not possible,” said Keri Gans, a registered dietitian in New York City.

Where do all those raindrops go?

An earlier Fact of the Day noted that about 15,000 km3 of rain falls on the North American continent every year. Where do all those raindrops go? One approach to measuring and tracking water flows is “water footprint” analysis. This approach to the question would seem on a cursory examination to indicate that more than one in every twenty-five raindrops in all of North America goes to producing beef, chicken, soybeans, wheat, and corn in the United States. Here’s a table of some water footprints for various agricultural products multiplied by annual US production.

Water Footprints of agricultural products and production volume in the U.S. Note: Don't believe these numbers!

Water Footprints of agricultural products and production volume in the U.S.


Does that number seem high? It does to me. In fact I don’t believe them at all! The main problem is in the way the water footprint analysis is done. Some of the problems I have with water footprint analysis I have discussed before, at least in the abstract. I’d like to offer more specific examples of what I find misleading about the water footprint.

1. Inconsistent time frame of analysis. The water footprint for beef, as calculated, stems mainly from the water inputs into feed production over the entire lifespan of the animal, from birth to slaughter. Waterfootprint.org assumes cattle take three years to reach maturity, which sounds reasonable enough. Certainly it takes water to grow feed for the cattle in all three years of its life. But the failure to base the water footprint of a product on some consistent unit of time leads to some strange consquences. Take a calf named Betsy. Some of the same water molecules that grows the grass that feeds Betsy in the year after her birth get evapotranspirated back into the atmosphere; others get diverted to streams and rivers and flow to the ocean, from where the evaporate again. Eventually they fall again as rain. The turnover time for this process is much less than a year. For example, water draining to the Mississippi River basin reaches the Gulf of Mexico in less than one month. The mean residence time of moisture in soil before evaporation might be ~90 days. (These numbers are coming from Figures 5.5 and 9.1 of this book.) Thus, those water molecules that we counted as part of the year I water footprint for production of our hypothetical calf might be the same molecules that are counted again in year 2!

COUNTERPOINT AND RESPONSE: Prof. A. Y. Hoekstra, perhaps the chief proponent of water footprint analysis, points to FAQ Question 4 at the waterfootprint.org site:

…But in a certain period one cannot use more water than is available. A river can be emptied and in the long term one cannot take more water from lakes and groundwater reservoirs than the rate with which they are recharged. The water footprint measures the amount of water available in a certain period that is consumed (i.e. evaporated) or polluted. In this way, it provides a measure of the amount of available water appropriated by humans. The remainder is left for nature.

To me, this “answer” acknowledges the problem but provides no indication that water footprint analysis solves it.  Water footprint analysis does not reveal the “amount of available water” that humans could appropriate “in a certain period”, it reveals the amount of water that humans do appropriate in a certain period.  And it appears that in many cases, the period used for water footprint analysis is longer than the time it takes the water cycle to recharge itself.

2. Failure to consider substitution effects. At waterfootprint.org, note the following text:

… to produce one kilogram of boneless beef, we use about 6.5 kg of grain, 36 kg of roughages, and 155 litres of water (only for drinking and servicing). Producing the volume of feed requires about 15300 litres of water in average.

In the US, the grain is probably mostly corn. Even if it were the ostensibly more water-intensive wheat, the water footprint per kg of beef from grain would be only 8450 L (from 6.5 kg grain per kg of beef × 1300 L of water per kg of wheat). That’s only about half of the feed water. The remainder, 7100 L, must come from the roughage. At waterfootprint.org, roughage is described as “pasture, dry hay, silage and other roughages”. Fair enough. But what happens if the beef industry decides it doesn’t need to produce Betsy, and as a result, Betsy is never born? Do we save 7100 L of water for every kg that Betsy would have weighed in year 3 of her life? Possibly, but only if none of the pasture, dry hay, and silage that would have gone to feed Betsy is produced in her absence. If Betsy’s portion of hay is grown anyway, it will still evapotranspirate water that could have been directed to other uses, even if the hay is left on the field instead of harvested. In short, it seems far easier to change the water footprint attributed to a product than it does to change the perturbations to the water cycle caused by the production of that product.

COUNTERPOINT AND RESPONSE: Prof. Hoekstra says: “If not appropriated for human consumption (e,g, hay as input of cows that provide meat), then the water is available to sustain natural vegetation (or in the case of river water: sustain aquatic life)”.  True enough…but why design a method of water use accounting that stacks the deck in favor of “natural” vegetation so highly?  From a water use perspective, what is the difference between a prairie grazed by a “natural” bison population which people do not eat, and a pasture grazed by cattle, which people do eat, other than in the former case people have less food?

3. Double-counting. Even if Betsy does get produced, and even if we ignore the incommensurate time scales of Betsy’s life and the terrestrial water cycle, there’s yet another problem in adding up the water footprints of various agricultural commodities, as I did above. In the US, about 55% of the US corn crop is used as feed for meat production. That means that 55% of the water we’ve attributed to corn production gets counted again when we tabulate up numbers for beef, chicken, or other meats.

COUNTERPOINT AND RESPONSE: Prof. Hoekstra agrees with me that water footprints cannot be added in the way I attempted.  Opinions may differ, I suppose, on whether this property is a feature or a bug. But in my view, since mixing a gallon of water with a gallon of water results in two gallons of water it would be nice if water footprints had the same property.

I was very curious to hear a proponent of water footrprint analysis point out flaws in my reasoning or defend waterfootprint.org’s use of these numbers. To that end, I contacted Professor A. Y. Hoekstra, whom I believe to be the primary exponent of water footprint analysis, and shared with him my three concerns.  Graciously, he has responded, and I have included his rebuttals (and my response to them) inline with my arguments above.

Ivory tower, home sweet home

This article is the strongest evidence that grad students and post-docs don’t need raises that I have seen.

I mean, if I made more money, would I actually be able to relate to that list? Blegh.

In Defense of Food and creationism

On a recent flight I was finally able to digest (ha!) Michael Pollan’s In Defense of Food.  The first part of the book is a historical narrative of the science, policy, and politics of nutritionism in the US.  In Part I, Pollan mounts a devastating critique of US nutritional science and policy. The climax is Chapter 5, “The Melting of the Lipid Hypothesis”, which is perhaps the most important bit of science writing I’ve read all year.    I was left wondering why anyone ever listens to nutritionists.  The only (and minor) weak point in Part I is the tendency to make the nutritionists’ blunders and the grain industry’s lobbying seem more like a nefarious, well-coordinated conspiracy.

But in the Part III of the book, now that Pollan has knocked the scientific wisdom of the day off its altar, it’s time for him to offer an alternative.  And what he proposes isn’t too convincing.  One problem is that Pollan spends so little effort convincing us that there is any sound scientific basis to his recommendations.  In Part I of the book, Pollan adeptly compares case-control studies, cohort studies, and intervention trials…but the entire scientific basis for Pollan’s recommendations on how to eat in Part III appears to be this passage (note the classic correlation-is-not-causation mistake):

People eating a Western diet are prone to a complex of chronic diseases that seldom strike people eating more traditional diets.  Scientists can argue all they want about the biological mechanisms behind this phenomenon, but whichever it is, the solution to the problem would appear to remain very much the same: Stop eating a Western diet.

Instead of science, Pollan’s recommendations rest on cultural traditions.  “Don’t eat anything your grandmother wouldn’t recognize as food,” he says on pg. 148.  “Cultures have had a great deal to say about what and how and why and when and how much we should eat,” says pg. 133.

This train of thought sounds like some creationists’: Both Pollan and creationists poke holes in the scientific orthodoxy and would have us insert tradition in its place. Similarly, creationists think that atheists plot to discredit their work; Pollan warns us to resist the pernicious myths propagated by the “Nutritionalist Industrial Complex”.

The analogy between creationists and In Defense of Food only goes so far, of course.  Most importantly, creationists are not nearly as persuasive in their efforts to poke holes in the scientific orthodoxy. And, even if creationists are wrong, Pollan might still be right. Personally, as weak as I find In Defense of Food’s scientific basis, I think Pollan might be on to something – and I can’t say the same for creationists.

A billion years of sulfidic oceans

In last week’s PNAS, A.H. Knoll and colleagues lay out the controlling feedbacks that they think buffered the Earth’s atmosphere at a low-oxygen state for about a billion years. Very interesting reading for anyone into paleoclimatology, paleooceanography, and/or biogeochemistry in general.

Fact of the day

Over 15,000 cubic kilometers of rain fall on North America each year. (That’s about 0.5 Sverdrups if you like obscure units.)

That flux is equivalent to dumping out a cube of water 700 m on a side every second onto the surface of North America.