Mosquito experiment concluded

It’s hard to see from the photo but when I checked my bucket after a week away, there were definitely a few mosquito larvae swimming around. There was also an impressive biofilm on the bottom of the bucket. It took less than a month for mosquitoes to breed in a newly formed pool of stagnant water. My son also noticed that a nearby flower pot with water only a few centimeters deep also had larvae. So the claims that mosquitos will breed in tiny amounts of stagnant water is true.

Mosquito update

It’s been about two weeks since I first set out my bucket, although I had to move it to a less obtrusive location. Still no signs of mosquito larvae, although judging from my bite frequency even with mosquito repellant, mosquito activity is still high in my garden. I see the occasional insect trapped (they are not really floating since at their size water is highly viscous) in the surface and there is a nice collection of plant debris at the bottom. The water level seems a little bit higher. It has rained at least once every two days since my first post although it has also been very hot so the input seems mostly balanced by the evaporative loss. I’m starting to believe that mosquitos have their prefered gestation grounds that they perpetually use and only exploit new locales when necessary.

The mosquito experiment

This month, Baltimore, along with much of the mid-Atlantic has been inundated with rain.  It was already the rainiest July on record as of last week. My yard has also been infested with mosquitos this summer. It was a very wet early summer, then it was extremely hot and dry for two weeks before the most recent deluge. Supposedly, mosquitos will breed in any amount of stagnant water. I thus decided to do an experiment to see how long it takes for a mosquito to find a suitable pool of water and go through a life cycle.  I started this on a whim by putting out some buckets a week ago.  Unfortunately, I wasn’t serious at first and didn’t record the exact date but I think it was Friday, July 20, which was right before the epic rains started. Below is a photo of my two buckets.  As you can see, one is filled to the brim and it is a pretty big bucket.  I haven’t measured the height yet but it looks like it is around 30 cm.  You can see a presumably dead mosquito floating in the orange bucket.  On the bottom, along with plant debris, are what I believe to be egg sacks, which look like 1 cm long beads on a string. The eggs are on the bottom and not floating, which is what I thought they were supposed to do. Maybe they are failures but we will see. There are also some other smaller insects and ants floating or trapped in the surface of the water.  I don’t see any larvae yet. The water was pretty clear as of Thursday of last week so it took about a week for the mosquitos to find the water.  That gives you a time window for how long you have before you should empty out any trapped water.  I plan to run this experiment until the larvae pupate and then I’ll end it before they become full adults.

CO2 and the return of the dinosaurs

The dinosaurs lived during the Mesozoic Era, which was divided into the Triassic, Jurassic, and Cretaceous Periods. Many of the iconic dinosaurs that we know and love such as Tyrannosaurus rex and Triceratops lived at the end of the Cretaceous while others such as Stegosaurus and Apatosaurus (formerly known as Brontosaurus) lived 80 or so million years earlier in the Jurassic. I used to picture all the dinosaurs co-existing simultaneously but the time span separating Stegosaurus from T. rex is larger than that between T. rex and the present! Dinosaurs also weren’t the only creatures alive at that time, just the most dominant ones. Technically, the term dinosaur only applies to land-based reptiles with certain features. The avian reptiles, such as Pteranodon, or the marine ones, such as Plesiosaurs, resembled dinosaurs but were not classified as such. Aside from those dinosaur-like animals, there were also invertebrates, fish, sharks, and a class of animals called Synapsids, defined by an opening in the skull behind the eyes, from which all mammals are descended.

Synapsids were small marginal creatures during the Mesozoic but came to dominate the land after the dinosaurs went extinct at the end of the Cretaceous (the KT extinction event). The consensus theory is that a large asteroid or comet strike in the Yucatan set off fire storms, seismic events and a cloud that blocked sunlight for up to a year. This caused plants to die globally, which collapsed the food chain.The only survivors were creatures that could go deep underwater or bury underground and survive long periods with little or no food. Survivors of the KT extinction include some fish, small sharks, small crocodiles and other cold blooded reptiles, small bipedal theropod dinosaurs, of which T-Rex is a member, and small rodent-like synapsids.

If the KT extinction event was a transient perturbation then it is reasonable to expect that whatever it was that allowed dinosaurs to become dominant would remain and the surviving theropods would come to dominate again. But that is not what happened. Theropods did survive to become modern birds but aside from a few exceptions, most are small and almost all are avian. Instead, the Synapsids came to dominate and the largest creature to ever live, namely the Blue Whale, is a Synapsid. Now this could be purely due to random chance and if we played out the KT event over and over there would be some distribution where either Synapsids or dinosaurs become dominant. However, it could also be that global conditions after the Cretaceous changed to favour Synapsids over dinosaurs.

One possible change is the atmospheric level of carbon dioxide. CO2 levels were higher than they are today for much of the past 500 million years, even with the recent rapid increase. The levels were particularly high in the Triassic and Jurassic but began to decline during the Cretaceous (e.g. see here) and have continued to decrease until the industrial revolution when it turned upwards again. Average global temperatures were also higher in the Mesozoic. The only other time that C02 levels and global temperatures have been as low as they are now was in the Permian before the Great Dying. During the Permian, the ancestor to dinosaurs was a small insectivore that had the ability to run on two legs while the dominant creatures were none other than the Synapsids! So, mammal-like creatures were dominant before and after the dinosaurs when CO2 levels and temperatures were low.

Perhaps this is just a coincidence but there is one more interesting fact to this story and that is the amount of stored carbon (i.e. fossil fuels) has been very high twice over the past 500 million years – the Permian and now. It had been believed that the rise in CO2 at the end of the Permian was due to increased volcanism but a paper from 2014, (see here), speculated that a horizontal gene transfer event allowed an archaea microbe to become efficient in exploiting the buried carbon and this led to an exponential increase in methane and CO2 production. The active volcanos provided the necessary nickel to catalyze the reactions. Maybe it was simply a matter of time before some creature would find a way to exploit all the stored energy conveniently buried underground and release the carbon back into the atmosphere. The accompanying rise in temperatures and increased acidification of the oceans may also spell the end of this current reign of Synapsids and start a new era. While the smart (rich?) money seems to be on some sort of trans-human cyborg being the future, I am betting that some insignificant bird out there will be the progenitor of the next dominant age of dinosaurs.

Cecil and the hunter

Like many others, I was first outraged when I heard about the death of the beloved lion in Zimbabwe at the hands of a hunter from Minnesota. But I then quickly realized that I am in no position to judge the man. Over the past week, I have dined on salmon, chicken, pork, tuna, and beef. Just because I don’t go into the brush to kill an animal I consume doesn’t mean that I am not directly responsible for its demise. The only difference between me and a hunter is that I do not find any sport in the shooting of animals. There are nearly a hundred million cows at any given time in the US waiting to be slaughtered. Is the life of a cow not as valuable as that of a lion? It is no fault of the cow that she is not an iconic symbol like the lion. Fish are wild animals and we are hunting them to extinction. Tuna can live very long lives and are partially warm blooded. Sharks exhibit very complex behavior and have live births. I would suggest that the death of a big fish is no less tragic than the death of a big cat.

The unfortunate hunter paid a lot of money to go on what he thought was a legal hunt. The guides he hired may have misled him and broken the law but hunting lions in Zimbabwe is not a crime. Remember that this is a country that was near economic collapse just a decade ago and could use an infusion of hard currency. I have argued before that hunting may ironically be a way to preserve wildlife and habitat. The interests of hunters and environmentalists could be aligned. Regulated hunting could be an antidote to illegal poaching. If the hunter broke a law then he should be prosecuted. Otherwise, his choice of recreation is protected by the First Amendment of the US Constitution.

One of the most intellectually stimulating radio shows (and podcasts) is Ideas with Paul Kennedy on CBC radio. It basically covers all topics. Many of the shows span several hour-long segments. One inspiring show I highly recommend is devoted to landscape architect Cornelia Hahn Oberlander. She was a pioneer in green and sustainable architecture. She is also still skiing at age 93!

Did microbes cause the Great Dying?

In one of my very first posts almost a decade ago, I wrote about the end-Permian extinction 250 million years ago, which was the greatest mass extinction thus far. In that post I covered research that had ruled out an asteroid impact and found evidence of global warming, possibly due to volcanos, as a cause. Now, a recent paper in PNAS proposes that a horizontal gene transfer event from bacteria to archaea may have been the main cause for the increase of methane and CO2. This paper is one of the best papers I have read in a long time, combining geological field work, mathematical modeling, biochemistry, metabolism, and evolutionary phylogenetic analysis to make a compelling argument for their hypothesis.

Their case hinges on several pieces of evidence. The first comes from well-dated carbon isotopic records from China.  The data shows a steep plunge in the isotopic ratio (i.e ratio between the less abundant but heavier carbon 13 and the lighter more abundant carbon 12) in the inorganic carbonate reservoir with a moderate increase in the organic reservoir. In the earth’s carbon cycle, the organic reservoir comes from the conversion of atmospheric CO2 into carbohydrates via photosynthesis, which prefers carbon 12 to carbon 13. Organic carbon is returned to inorganic form through oxidation by animals eating photosynthetic organisms or by the burning of stored carbon like trees or coal. A steep drop in the isotopic ratio means that there was an extra surge of carbon 12 into the inorganic reservoir. Using a mathematical model, the authors show that in order to explain the steep drop, the inorganic reservoir must have grown superexponentially (faster than exponential). This requires some runaway positive feedback loop that is difficult to explain by geological processes such as volcanic activity, but is something that life is really good at.

The increased methane would have been oxidized to CO2 by other microbes, which would have lowered the oxygen concentration. This would allow for more efficient fermentation and thus more acetate fuel for the archaea to make more methane. The authors showed in another simple mathematical model how this positive feedback loop could lead to superexponential growth. Methane and CO2 are both greenhouse gases and their increase would have caused significant global warming. Anaerobic methane oxidation could also lead to the release of poisonous hydrogen sulfide.

They then considered what microbe could have been responsible. They realized that during the late Permian, a lot of organic material was being deposited in the sediment. The organic reservoir (i.e. fossil fuels, methane hydrates, soil organic matter, peat, etc) was much larger back then than today, as if someone or something used it up at some point. One of the end products of fermentation of this matter would be acetate and that is something archaea like to eat and convert to methane. There are two types of archaea that can do this and one is much more efficient than the other at high acetate concentrations. This increased efficiency was also shown recently to have arisen by a horizontal gene transfer event from a bacterium. A phylogenetic analysis of all known archaea showed that the progenitor of the efficient methanogenic one likely arose 250 million years ago.

The final piece of evidence is that the archaea need nickel to make methane. The authors then looked at the nickel concentrations in their Chinese geological samples and found a sharp increase in nickel immediately before the steep drop in the isotopic ratio. They postulate that the source of the nickel was the massive Siberian volcano eruptions at that time (and previously proposed as the cause of the increased methane and CO2).

This scenario required the unlikely coincidence of several events –  lots of excess organic fuel, low oxygen (and sulfate), increased nickel, and a horizontal gene transfer event. If any of these were missing, the Great Dying may not have taken place. However, given that there have been only 5 mass extinctions, although we may be currently inducing the 6th, low probability events may be required for such calamitous events. This paper should also give us some pause about introducing genetically modified organisms into the environment. While most will probably be harmless, you never know when one will be the match that lights the fire.

Saving large animals

One  story in the news lately is the dramatic increase in the poaching of African elephants (e.g. New York Times). Elephant numbers have plunged dramatically in the past few years and their outlook is not good. This is basically true of most large animals like whales, pandas, rhinos, bluefin tuna, whooping cranes, manatees, sturgeon, etc. However, one large animal has done extremely well while the others have languished. In the US it had a population of zero 500 years ago and now it’s probably around 100 million.That animal as you have probably guessed is the cow. While wild animals are being hunted to extinction or dying due to habitat loss and climate change, domestic animals are thriving. We have no shortage of cows, pigs, horses, dogs, and cats.

Given that current conservation efforts are struggling to save the animals we love, we may need to try a new strategy. A complete ban on ivory has not stopped the ivory trade just as a ban on illicit drugs has not stopped drug use. Prohibition does not seem to be a sure way to curb demand. It may just be that starting some type of elephant farming may be the only way to save the elephants. It could raise revenue to help protect wild elephants and could drop the price in ivory sufficiently to make poaching less profitable. It could also backfire and increase the demand for ivory.

Another counter intuitive strategy may be to sanction limited hunting of some animals. The introduction of wolves into Yellowstone park has been a resounding ecological success but it has also angered some people like ranchers and deer hunters. The backlash against the wolf has already begun. One ironic way to save wolves could be to legalize the hunting of them. This would give hunters an incentive to save and conserve wolves. Given that the set of hunters and ranchers often have a significant intersection, this could dampen the backlash. There is a big difference in attitudes towards conservation when people hunt to live versus hunting for sport. When it’s a job, we tend to hunt to extinction like  buffalo, cod, elephants, and bluefin tuna. However, when it’s for sport, people want to ensure the species thrives. While I realize that this is controversial and many people have a great disdain for hunting, I would suggest that hunting is no less humane and perhaps more than factory abattoirs.

Weather prediction

I think it was pretty impressive how accurate the predictions for Superstorm Sandy were up to a week ahead.  The hurricane made the left hand turn from the Atlantic into New Jersey just as predicted.  I don’t think the storm could have been hyped any more than it was.  The east coast was completely devastated but at least we did have time to prepare.  The weather models  have gotten much better from even ten years ago. The storm also shows just how vulnerable the east coast is to a 14 foot storm surge.  I can’t imagine what a 20 foot surge would do to New York.

I listened to two Long Now Foundation talks on my way to Newark, Delaware and back yesterday for my colloquium talk.  These podcasts tend to be quite long, so they were perfect for the drive.  The first was by environmental activist and journalist Mark Lynas and the second by National Geographic photographer Jim Anderson.  Both were much more interesting than I expected.  Lynas, who originated the anti-genetically modified organism (GMO) food movement in Europe in the 1990s, has since changed  his mind and become more pragmatic.  He now advocates for a more rational environmental movement that embraces technological solutions such as GMO foods and nuclear energy.  He argues that many more people are killed by particulate matter from coal-fired generating plants in a year than over the entire history of nuclear use.  I have always felt that nuclear power is the only viable technology to reduce carbon emissions.  I have also argued previously that  I’m more worried about the acidification of the ocean due to CO2 than an increase temperature.  I think we should start building CANDU reactors now and head towards fast breeder reactors.

Jim Anderson talked about the loss of diversity of domesticated plants and animals and how they are essential for the survival of humans.  For the first 9,900 years of agriculture, we increased the diversity of our food stuff.  For the last hundred, we have gone in the other direction. We used to have hundreds to thousands of varieties of fruits and vegetables and now we’re down to a handful.  There are at most 5 varieties of apples I can buy at my local supermarket, yet a hundred years ago, each orchard would produce its own variety.  This leaves us extremely vulnerable to diseases.  The world’s banana supply is dominated by one variety (the Cavendish) and it is under siege by a fungus that threatens to wipe it out.  The Irish potato famine was so severe because they relied on only two varieties that were both susceptible to the same blight. Our fire wall against future blights are seed banks, where we try to preserve as many varieties as we can.  However, not all seeds can remain viable forever.  Many have to be planted every few years from which new seeds are harvested.  This replanting is often done by amateur horticulturists.  The podcast made me think that with the cost of genome sequencing dropping so rapidly, what we need now is for someone, like Google, to start sequencing every living being and making it publicly available, like Google Books.  In fact, if sequencers become cheap enough, this could be done by amateurs.  You would find some plant or animal, document it as well as you can, and upload the sequence to the virtual seed bank.  This can be a record of both wild and domesticated species.  We can then always resurrect one if we need to.  There could also be potential for mischief with highly dangerous species like small pox or anthrax, so we would need to have a public discussion over what should be available.

Infinite growth on finite resources

At this past summer’s Lindau meeting of Nobel Laureates, Christian Rene de Duve, who is over 90 years old, gave a talk on population growth and its potential dire effects on the world.  Part of his talk was broadcast on the Science Show.  His talk prompted me to think more about growth.  The problem is not that the population is growing per se.  Even if the population were stable, we would still eventually run out of fossil fuels if we consume energy at the same rate.  The crucial thing is that we must progressively get more efficient.  For example, consider a steady population where we consume some finite resource at the rate of $t^\alpha$.  Then so long as $\alpha < -1$, we can make that resource last forever since $\int_1^\infty t^\alpha$ is finite.  Now, if the population is growing exponentially then we would have to become exponentially more efficient with time to make the resource last.  However, making the world more efficient will take good ideas and skilled people to execute them and that will scale with the population.  So there might be some optimal growth rate where we ensure the idea generation rate is sufficient to increase efficiency so that we can sustain forever.

What’s in your sunscreen?

Here’s something to think about from Scientific American:

…And just what are the risks? According to the non-profit Environmental Working Group (EWG), there are two major types of sunscreens available in the U.S. “Chemical” sunscreens, the more common kind, penetrate the skin and may disrupt the body’s endocrine system, as their active ingredients (e.g., octylmethylcinnamate, oxybenzone, avobenzone, benzophone, mexoryl, PABA or PARSOL 1789) mimic the body’s natural hormones and as such can essentially confuse the body’s systems. Quite a risk to take, considering that the chemical varieties don’t even work for very long once applied.

Meanwhile, “mineral” sunscreens are considered somewhat safer, as their active ingredients are natural elements such as zinc or titanium. But “micronized” or “nano-scale” particles of these minerals can get below the skin surface and cause allergic reactions and other problems for some people. EWG recommends sticking with “mineral” sunscreens whenever possible but, more important, taking other precautions to avoid prolonged sun exposure altogether. “At EWG we use sunscreens, but we look for shade, wear protective clothing, and avoid the noontime sun before we smear on the cream,” the group reports.

As for spray varieties, EWG recommends avoiding them entirely: “These ingredients are not meant to be inhaled into the lungs.” With so little known about the effects of sunscreen chemicals on the body when rubbed into the skin, we may never know how much worse the effects may be when they are inhaled. But suffice it to say: When your neighbor at the beach is spraying down Junior, it’s in your best interest to turn away and cover your nose and mouth…

Miraculous technologies

This month’s Scientific American magazine has a story on 7 Radical Energy Solutions.  The link is here although you need a subscription to access the full article.  The 7 solutions are 1) Fusion-triggered fission – using lasers to trigger  fusion in small pellets to produce neutrons to ignite fission; advantage being that a chain reaction is not necessary so nuclear waste can be used as fuel. 2) Solar gasoline – converting solar energy directly into a carbon-based liquid fuel. 3) Quantum photovoltaics – use quantum dots to increase efficiency of solar cells by trapping hot electrons that are lost with existing technology. 4) Heat engines – generate power by capturing waste heat using shape-memory alloys. 5) Shock-wave auto engine – a new internal combustion engine that uses shock waves to propel a turbine. 6) Magnetic air conditioners – make a fridge with no moving parts by using special magnets to replace the refrigerant and  pumps. 7)  Clean coal – use an ionic liquid to pull CO2 out of coal plant exhaust;  the CO2 would then have to be sequestered underground. See here for descriptions of  projects funded by the US Department of Energy.

The article made me think of technology we use today that seems miraculous.  The first thing that comes to mind is the airplane.  People had dreamed of flight for centuries if not millenia but it wasn’t until technology matured enough that the dream was realized in 1903 by the Wright brothers.  The mobile phone was just a science fiction dream to me when I was child.  The refrigerator has always seemed miraculous to me.  Even after thermodynamics and the heat cycle was understood, it is still amazing that actual substances that could act as refrigerants were discovered.  I find  bullet proof glass kind of astounding.  All of our electronic technology is based on silicon, which is made from sand.   Water itself is kind of magical.  The fact that it is so abundant and takes on three phases in a human accessible range of temperatures is astonishing (or maybe not – cf. Anthropic Principle).    I could go on and on.  As Arthur C. Clarke once wrote: “Any sufficiently advanced technology is indistinguishable from magic.”  At any moment, there could be a technological breakthrough that changes history.

The long view

Physicist and Nobel Laureate Robert Laughlin was on Econtalk this past summer. The link to the podcast is here.  Laughlin, who likes to take on contrarian positions and is always entertaining, talks about the future of carbon and his forthcoming book “When coal is gone”.  Chapter two of his book originally entitled “Geological Time” was excerpted in The American Scholar with the title “What the earth knows” and can be obtained here.  In that chapter and on the podcast, Laughlin argues that the human age of fossil fuels and its effect on climate is but a blink of an eye in geological time.   The earth has endured much larger perturbations then humans will ever inflict.  He claims that we’ll run out of oil in about 60 years but we will still use carbon-based liquid fuels because their energy densities are without peer.  (You can’t fly an airplane without it.)  However, instead of getting it out of the ground we will manufacture it using coal or natural gas as feed stock.  In about 200 years we’ll run out of coal but we’ll still want to make fuels.  At that point, we’ll have to extract carbon out of the air or ocean, mostly likely using plants.  Laughlin tries to avoid taking political positions and does acknowledge that climate change could be bad for this and the next generation of humans even if it won’t matter much in the long term.  He’s confident the earth and humans will survive this crisis.  The one thing he does worry about is biodiversity loss, which is permanent.  There is a switch in topic to Laughlin’s previous book The Crime of Reason 50 minutes into the podcast.  In that book, Laughlin argues that  the US switch from a manufacturing economy to an information economy will stifle  learning and the dissemination of knowledge because if information becomes a commodity, its value depends on its scarcity.  Thus, the rate of innovation will decrease not increase and we will become more secretive in general.

National Elk Refuge

Right outside of Jackson, Wyoming is the National Elk Refuge, which was established in 1912.  It is the wintering ground for a herd of ten thousand elk as well as eight hundred bison.  During winter, the elk come down from the mountains to the Jackson Hole valley where the snow is thinner so they can access grass more easily.  You can take a horse drawn sleigh right out to the herd with the Grand Tetons as the the backdrop. Here are some pictures.

The power of small

Here’s a nice story for Christmas in the New York Times. African villagers can now get electrical power using cheap solar cell units. It can make a significant difference for their lives.

The cost of commuting

It is about 45 miles (70 km) from Baltimore to the NIH campus in Bethesda, MD.  If I were to travel the entire distance using public transit it would cost over 20 dollars for a return trip (one way bus fare in Baltimore is $1.60, commuter rail (Marc train) fare is$7.00, and Metro fare in DC is $3.65 ($3.85 during peak hours)).  That amounts to over $100 per week and$5000 per year.  If I bought a  monthly rail pass, then I could cut the cost down by 75% or so.  Now if instead I were to drive everyday,  ninety miles per day is equivalent to 22,500 miles per year.  A car that could travel 30 miles per gallon of gasoline would use 750 gallons a year.  At the current price of $3 per gallon, this would be$2250 per year.  If I drive my car for ten years and it cost twenty thousand dollars then that is an additional $2000 per year. Insurance, fees, maintenance and repairs probably costs another$2000 per year so driving would cost about $6000 per year. If I drove a cheaper and more efficient car then I could bring this cost down to$5000 per year.  Thus, driving is economically competitive with public transit.  Add in the fact that I would own a car anyway even if I didn’t use it to commute to work and driving is the less expensive choice.

How is this possible?  Well one cost that I didn’t account for is parking.  The NIH happens to have a large campus where parking is nominally free.  Although if I chose not to drive, I could receive a public transit subsidy of  up to $110 per month or$1320 per year.  If the NIH were located in downtown Washington DC, parking could cost over $400 per month or$5000 per year.  So the real reason driving is competitive with public transit is because parking is subsidized.  If I  worked in an urban center  where parking is expensive then driving would be much more expensive than public transit.  Driving is further subsidized because roads and highways are funded by tax dollars while the cost of maintaining transit stations and tracks are only partially funded by taxes.  If transportation infrastructure were publically funded or if subsidies for roads and parking did not exist then public transit would be the prohibitive cost effective option.

Biomass

Since the rise of human civilization,  life forms larger than 10 centimeters to a metre have been systematically culled or eliminated from the ecosystem.  Almost all land megafauna that used to roam wildly a few thousand or even hundred years ago are either extinct or reside in small numbers in protected parks and reserves. Macroscopic sized sea creatures that were reasonably plentiful just two or three decades ago may all disappear shortly.  In that mean time the population of  humans and domesticated plants and animals have exploded.

So, has there been a net gain or loss of total biomass?   I think the conventional wisdom would be that we have replaced large tracts of forest with pavement, lawns and farmland, which would seem like a huge net loss of biomass.  However, we have added extra nutrients (i.e. fertilizer) and carbon (i.e. fossil fuels) into the system. The energy flux from the sun has also not changed significantly in the last millennium.  Hence, the capacity to support life has probably not changed or maybe has even increased. Removing, all of the large wild animals may also create more opportunities for small animals.  Perhaps there are more small and microscopic creatures then there would have been had humans not existed.  I have no idea what the answer is.

Phytoplankton

I have always felt that a rise in global temperatures was the least of our worries about increasing CO2 in the atmosphere.  I’m much more concerned about how it could perturb the delicate balance that allows mammals to live, i.e. us.  One of the things that could be trouble is that CO2 dissolved in water can make the oceans more acidic by forming more carbonic acid, which could make it harder for marine creatures to make shells through calcification, which  in turn could have a large impact on the coral reefs and the ocean food chain.

Another thing I worry about is that our oxygen supply could decrease.  Although the direct effect of converting oxygen to water and CO2 through increased combustion of fossil fuels is small, the effect on photosynthetic organisms that make our oxygen is largely unknown.  I’ve actually been somewhat optimistic on this account thinking that since we are introducing more nutrients into the oceans and CO2 is increasing then perhaps phytoplankton, which make much of our oxygen and is a blanket term for photosynthetic microscopic sea organisms like cyanobacteria and dynoflagellates, might increase.  However, a paper in Nature this week, says otherwise.

Metabolism of Mice and Men

In the 1930’s, Swiss-American animal metabolism pioneer Max Kleiber noticed that the metabolic rate of animals scales as the body mass to the three quarters power.  There is still some controversy over whether the exponent is really three quarters or something else.  Many theories have been proposed for why the exponent  should be three quarters (or two thirds) but I won’t go into that here.  The crucial thing is that it is less than one and that implies that a large animal is more efficient than a small one.  This efficiency with size is not restricted to biological examples.  As Steve Strogatz pointed out in a New York Times column last year, the number of gas stations doesn’t grow linearly with the population of a city but rather grows in proportion to the 0.77 power of the population.  This sublinear scaling also goes for other city infrastructure like the total length of roads and electrical cables. Large cities may in fact be more efficient than small ones.

Now a mouse weighs about 20 to 30 grams so it is about a factor of 3500 times less massive than an average human.  Metabolic rate scales as mass to the three quarters so power density ( e.g. Watts/gram) scales as mass to the minus one quarter.  Hence, a mouse is $3500^{(1/4)}$ or 7 to 8 times less metabolically efficient than a human. A colony of mice weighing as much as a human would have to eat 7 to 8 times as much food.

However, in terms of total energy utilized, first world humans are much less efficient than mice and perhaps all other organisms.  The metabolic rate of an average person is about 10 megajoules per day or 115 watts but according to Wikipedia, the United States uses about 10,000 watts of power per capita.  This is a factor of 90 over the metabolic rate implying that an average American is a factor of ten less efficient than a mouse.  However, a very low energy use nation like Bangladesh only consumes about twice as much energy per capita as the human metabolic rate and thus an average Bangladeshi is more efficient than a mouse.