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[ Negative feedback | Positive feedback | Counter-intuitive ]

Growing or decaying systems dominated by positive feedback

The urban development of farmland in California's Central Valley (click image for larger version)

The 400 mile long stretch of central California that is the Central Valley has one of the highest rates of development in the country, with farmland being paved over to construct new residential suburbs and giant retail outlets. In terms of city income, this has both beneficial and detrimental effects.

Increased development attracts more people, thus increasing revenue for the city (whether from property tax, sales tax or utility fees). However, the previously agricultural land requires new infrastructure to be constructed (sewers, roads) and an increase in annual maintenance fees (including emergency services, schools, etc.). The increased income must also be weighed against contributions to the local economy made by the old farmland which required far less infrastructure. A further detrimental effect is that new construction causes the value of nearby property to drop --most people would prefer to move into the new suburbs rather than those which are a few years old -- resulting in a corresponding drop in city revenue from property taxes.

With new income not matching new expenses, cities have been forced to take on bond debt to make up the shortfall, with the cost of this due to be paid by increased taxes and fees on future residents and developers. With expenditure continually increasing and income not keeping pace, the reinforcing loop of bonds and repayment is set to stay unless the contributing factors in the system are changed.

One solution would be to enforce land use restrictions making developers rebuild on more central, previously developed, land. If "Development on undeveloped land" in the diagram were replaced with "Redevelopment" we could remove the downward force on "Amount of farmland" (leaving its tax contribution intact), and the negative effect on neighbouring property values would be reduced. Additionally, we could remove the need for new infrastructure, thus halting the increase in expenditure.

An alternative would be to force developers to pay the infrastructure costs themselves, again relieving pressure on city expenditure. This move is unlikely to be popular with developers, smart growth enthusiasts or the home buyers who would see themselves footing the bill with increased house prices.

(Source: Ecology of Fear, Mike Davis, pp95-112.)

The relationship between hurricane frequency and atmospheric temperature (click image for larger version)

There are two factors relating the frequency of hurricanes and the temperature of the Earth's atmosphere, creating a systems of positive feedback which causes both to rise. Computer modelling implies that as the planet warms up the frequency and intensity of hurricanes will both increase. Global temperature already seems to be rising due to the amount of carbon dioxide human activity is releasing into the air, but it appears that hurricanes themselves can add to the problem: such a storm can be so powerful that carbon dioxide is pulled from the sea's surface and released into the atmosphere. At the moment this contribution is only 1/12 the size of the carbon dioxide emissions released by human activity, but this could rise, depending on the strength of the positive feedback loop. Cutting our own contributions would of course slow the process dramatically.

(Source: 'Vicious Circles', BBC News Online, 1999-09-25)

[ Negative feedback | Positive feedback | Counter-intuitive ]