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[ Negative feedback | Positive feedback | Counter-intuitive ]

Unusual systems that display counter-intuitive behaviour

The effect on a deer population of its predators being hunted (click image for larger version)

As settlers spread into the West, they killed vast numbers of animals, from grizzlies to mice, in an effort to make the wild more suitable for habitation. Hunting on this scale had a variety of effects on the rest of the ecosystem, and one of the best examples is that of hunting in the national game preserve of Arizona's Kaibab Plateau.

As the number of humans in the area increased, the need (and the ability) to kill the predatory animals increased. In just over ten years 674 cougars, 3,000 coyotes and 120 bobcats were killed. As the number of predators available to kill the areas deer population decreased, so the number of deer increased. With almost all predators being wiped out, the deer population shot from around 3,000 in 1906 to over 100,000 in 1924. This increase meant the rate at which the local fauna was consumed similarly shot up, past its ability to regenerate itself. With the deer eating vegetation faster than it could grow, soon there was almost no food left, with the result that most of the deer swiftly died, leaving an expanse of largely bare (but predator-free) land.

(Source: Ecology of Fear, Mike Davis)

How global warming could make Europe cooler (click image for larger version)

Most people are familiar with the concept of global warming caused by the increasing emissions of carbon dioxide and other greenhouse gasses. This warming could, however, see temperatures in Europe drop due to the disruption of an otherwise stable system.

In the Atlantic Ocean there is a convection system known as the conveyor belt, illustrating its movements: warm water from the south moves northwards (the North Atlantic Drift) releasing its heat into the atmosphere, before sinking to 2-3km below the surface and travelling south (North Atlantic Deep Water). This rolling system is fuelled by the salinity and temperature of the waters which determine their density. The northern waters have lost much of their heat, have higher salinity and are thus more dense, causing them to sink and keep the conveyor rolling. Freshwater (rain, rivers, melting ice and snow) join the waters in the northern latitudes and threaten to reduce the salinity, and hence density, of the sea. However, the freshwater is continually flushed away to the south and the system keeps rolling.

One of the effects of global warming is to increase the amount of rainfall and melting ice which will increase the flow of freshwater into the sea. This is reducing the salinity of the sea water flowing south and could cause the conveyor belt to slow. With less warm water reaching the North Atlantic on the conveyor, temperatures in North Western Europe (currently higher than many in similar latitudes) could drop. Any significant drop in temperature could cause greater amounts of sea ice to form and this would reflect more sunlight from the surface, reducing air temperatures further.

CO2 use may reach a peak and then slow in the next century, causing temperatures to gradually (over centuries) fall back. The circulation in the Atlantic could either slow and recover or stop entirely. Models have shown that either is possible, depending on whether the freshwater disruption reaches a certain threshold which would cause a collapse of the system.

(Sources: 'Freezing Future', New Scientist, 1999-11-27.
'Ocean Currents and Climate Change', by Stefan Rahmstorf.
'Long-term Global Warming Scenarios Computed With an Efficient Coupled Climate Model', by Stefan Rahmstorf and Andrey Ganopolski.)

[ Negative feedback | Positive feedback | Counter-intuitive ]