Category: Aside

Flow down, stock up

A simple example of bathtub dynamics:

Source: NYT

The flow of plastic bags into landfills is dramatically down from the 2005 rate. But the accumulation is up. This should be no surprise, because the structure of this system is:

The accumulation of bags in the landfill can only go up, because it has no outflow (though in reality there’s presumably some very slow rate of degradation). The integration in the stock renders intuitive pattern matching (flow down->stock down) incorrect.

Placing the flow and the stock on the same vertical scale, is also a bit misleading, because they’re apples and oranges – the flow of disposal has units of tons/year, while the accumulation has units of tons.

Also, initializing the stock to its 2005 value is a bit weird. If you integrate the disposal flow from 1980 (interpolating as needed), the accumulation is much more dramatic: about 36 million tons, by my eyeball.

Blood pressure regulation

The Tech Review Arxiv blog has a neat summary of new research on high blood pressure. It turns out that the culprit may be a feedback mechanism that can’t adequately respond to stiffening of the arteries with age:

The human body has a well understood mechanism for monitoring blood pressure changes, consisting of sensors embedded in the major arterial walls that monitor changes in pressure and then trigger other changes in the body to increase or reduce the pressure as necessary, such as the regulation of the volume of fluid in the blood vessels. This is known as the baroreceptor reflex.

So an interesting question is why this system does not respond appropriately as the body ages. Why, for example, does this system not reduce the volume of fluid in the blood to decrease the pressure when it senses a high systolic pressure in an elderly person?

The theory that Pettersen and co have tested is that the sensors in the arterial walls do not directly measure pressure but instead measure strain, that is the deformation of the arterial walls.

As these walls stiffen due to the natural ageing process, the sensors become less able to monitors changes in pressure and therefore less able to compensate.

Braveheart & Rogaine

The Reinhart & Rogoff debt/growth paper continues to make a stir for it’s basic Excel errors. Colbert has the latest & funniest take on it.

Two things about this surprise me.

Confronted with obvious and irrefutable errors, the authors double down and admit nothing. They also downplay the significance of the results, ‘… we are very careful in all our papers to speak of “association” and not “causality” …’

But of course the (amplified) message, Debt/GDP>90%=doom, was taken causally in the policy world; see the multiple clips in the intro to the Colbert video. Politicians are nuts to accord one paper in a sea of macroeconomic thought so much weight, but I guess this was the one they liked.

EU ETS on the ropes

The EU declined backloading, a deferral of permit auctions that would have supported prices in the Emissions Trading System (ETS).

This is described imminent collapse to the system, threatening the achievement of emissions targets. Perhaps a political collapse is imminent – not my department – but the idea that low emissions prices threaten the system is a bit odd. The ETS price is a feedback mechanism. Low prices are a symptom, indicating that the marginal cost of meeting targets is extremely low. That should be a cause for celebration (except for traders).

For the umpteenth time, this shows the difficulty of running a system that invites wrangling over allocation and propagates noise from the economy into a market.

Meanwhile, carbon taxes grind away at their job.

Finding SD conference papers

Some change at google or the SD society site has broken the old approach for filtering searches to SD conference papers, which was

site:www.systemdynamics.org/conferences/* bathtub dynamics

Now it appears that

site:www.systemdynamics.org bathtub dynamics

still works, though with a bit of noise from general SD society pages.

For some reason, the old way still works in a google custom search:

Update: the new preferred syntax appears to be:

site:systemdynamics.org inurl:conferences bathtub dynamics

On the usefulness of big models

Steven Wright’s “life size map” joke is a lot older than I thought:

On Exactitude in Science
Jorge Luis Borges, Collected Fictions, translated by Andrew Hurley.
…In that Empire, the Art of Cartography attained such Perfection that the map of a single Province occupied the entirety of a City, and the map of the Empire, the entirety of a Province. In time, those Unconscionable Maps no longer satisfied, and the Cartographers Guilds struck a Map of the Empire whose size was that of the Empire, and which coincided point for point with it. The following Generations, who were not so fond of the Study of Cartography as their Forebears had been, saw that that vast Map was Useless, and not without some Pitilessness was it, that they delivered it up to the Inclemencies of Sun and Winters. In the Deserts of the West, still today, there are Tattered Ruins of that Map, inhabited by Animals and Beggars; in all the Land there is no other Relic of the Disciplines of Geography.
—Suarez Miranda,Viajes de varones prudentes, Libro IV,Cap. XLV, Lerida, 1658

It’s no less relevant to big models, though.

h/t Benjamin Blonder

Dynamic simulation the hard way

When Alan Turing was born 100 years ago, on June 23, 1912, a computer was not a thing—it was a person. Computers, most of whom were women, were hired to perform repetitive calculations for hours on end. The practice dated back to the 1750s, when Alexis-Claude ­Clairaut recruited two fellow astronomers to help him plot the orbit of Halley’s comet. ­Clairaut’s approach was to slice time into segments and, using Newton’s laws, calculate the changes to the comet’s position as it passed Jupiter and Saturn. The team worked for five months, repeating the process again and again as they slowly plotted the course of the celestial bodies.

Today we call this process dynamic simulation; Clairaut’s contemporaries called it an abomination. They desired a science of fundamental laws and beautiful equations, not tables and tables of numbers. Still, his team made a close prediction of the perihelion of Halley’s comet. Over the following century and a half, computational methods came to dominate astronomy and engineering.

From Turing’s Enduring Importance in Technology Review.