In a positive feedback system, the result of a process encourages the continuation and acceleration of that process. Positive feedback systems are common in nature, and their understanding is central to many scientific theories. Let’s look at two examples relating to climate change.
Positive feedback in natural science
Light-colored objects reflect more of the Sun’s energy than dark objects. Most people understand this, as people in warm climates buy white cars and wear light clothing. Likewise, snow-covered ground reflects more sunlight than dark-colored ground. If more of the Earth’s surface is covered with snow, more solar energy reflects back away from the Earth, cooling the atmosphere. A cooler atmosphere encourages more snow and ice, which then reflects more sunlight, and so on. The reverse is also true. If a glaciated region loses snow and ice cover, it absorbs more solar energy, heating the atmosphere, and encouraging more snow and ice loss.
The second example relates to evaporation. Evaporation from the oceans is in part a function of the ocean temperature, and the ocean surface area. Warmer water and larger surface area produce greater evaporation rates. Water vapor is a greenhouse gas, and more water vapor in the atmosphere helps the atmosphere retain heat. This concept is easily illustrated by noting how nighttime temperatures are cooler when the weather is dry versus when the air is humid. A warmer atmosphere causes sea level rise because of ice melt and thermal expansion of the liquid water. Higher sea level means greater ocean surface area, as oceans replace coastal land areas. The increased surface area along with warmer water increases the evaporation rate, which increases the water vapor concentration in the atmosphere, which causes warming that further causes higher sea level.
Positive feedback elsewhere
I derive great enjoyment in applying the same principles I use as a scientist to everyday life. Positive feedback systems are easily applied to many phenomena.
The other night, I was drinking a beer at my local bar, when around 9pm, I was the only customer. At one point, two individuals walked in the door, but quickly left without coming inside. The bartender made a remark about how the bar may not have been lively enough for their taste. Perhaps this is another example of positive feedback. Bars are boring without interesting patrons. Having interesting patrons in a bar encourages more patrons, which encourages even more business as a busy bar becomes known as a “place to be”. People don’t just go to bars to drink, they like to meet new people, and see their friends. The more people in a bar, the greater the attraction of the social atmosphere. The same could be applied to most restaurants. Sometimes people judge a restaurant as “good” by the number of cars in the parking lot. A busy restaurant must be good, and that attracts customers, which attracts even more business. A restaurant could theoretically serve lousy food, but attract many customers simply because they see other people eating there.
And of course, another well known positive feedback example is best summed up with the phrase:
“it takes money to make money”.