Feedback is essential for normal functioning. An example is if you lurch to the left while walking, your body senses this and compensates by tensing muscles to keep you upright. Another example is when it gets hot you sweat, which cools you off.
Most of these control loops that maintain normalcy are negative; if there is an error, the control mechanism acts to counteract that error. They are essential to every living beingâ€”and to our planet.
Although rare in nature, there are also positive feedback loops, which usually have bad consequences. A common example is a sound amplifying system that squeals. Hereâ€™s how that works.
The singer at a concert is quiet, so someone turns up the volume. The amplifier makes her voice louder, but the microphone also picks up sound from the speaker and amplifies it. At some critical point the noise from the speaker gets amplified more than the singerâ€™s voice and there is a terrible high-pitched squeal. People slap their hands over their ears until someone turns down the volume.
Note that negative feedback is good, because it keeps things under control. Positive feedback is bad because it quickly gets out of hand, and can be thought of as a â€œvicious circleâ€.
The future may hold several destructive positive feedback loops leading to increased global warming. Their end result is that the warmer it gets, the warmer it will get.Â Combined, these may cause such a profound effect on Earthâ€™s climate that the planet becomes uninhabitable except for a relatively small portion of its surface.
Many loops involve increasing greenhouse gases. Like an actual greenhouse, these gases hold in the sunâ€™s heat and keep it from escaping from the atmosphere. Carbon dioxide (CO2) has the highest concentration and is the best known. Although it sounds as though the greenhouse effect might not do very much to the climate, it can cause huge consequences. Other planets provide an example. Although further from the sun, Venus is a lot hotter than Mercury because of runaway greenhouse effect. Almost all scientists agree Earthâ€™s climate is getting hotter. They also agree that the increase is due to human actionsâ€”primarily CO2 from using fossil fuels.
One positive feedback loop affecting the climate takes place in the arctic. Ice and snow reflect much of the sunâ€™s heat back into space. As the climate heats up, the Greenland icecap, and other glaciers, melt. This exposes darker ground that absorbs more of the sunâ€™s heat, causing more glacial melting, and so on.
One advantage (one might think) of less ice in the north is that more trees and plants could growâ€”and grow they do. The growth helps to absorb CO2, thus keeping down greenhouse gas concentrations. But it turns out that these boreal forests are darker than the bare ground, so they absorb more heat. Calculations suggest that the benefit on CO2 is offset by the effect of the darkness, unfortunately.
In addition to trees and plants keeping CO2 out of the atmosphere, the ocean absorbs a huge amount of this gas. Some is dissolved in water, and some is sucked up by the growth of algae. As the oceans heat up, both of these processes will diminish. Open a can of hot soda and it explodes; the CO2 wonâ€™t stay in solution. Algae donâ€™t grow well in warmer water, either, so thereâ€™s more CO2 in the air.
During the sweltering summer of 2003 in Europe, scientists found that trees stopped growing. In fact, their metabolism released CO2 instead of fixing it. The heat also killed plants and animals, releasing more CO2 from the corpses. Tropical rainforests are called â€œthe lungs of the planetâ€ because they breathe in CO2 and exhale oxygen. They will suffer from increased heatâ€”another cause of increased CO2.
Another greenhouse gas, methane, is twenty-four times more powerful than CO2 at trapping heat. In addition to the methane burped out by cows and generated by decay, there are billions of tons of it frozen in the arctic. Methane liberated by global warming will intensify the greenhouse effect.
It seems as though our use of fossil fuels has set into motion vicious cycles that will change the nature of our climateâ€”and of our planet. A relatively small rise in temperature will make many areas of the planet barren and uninhabitable. Hurricane Katrina is an example of a cataclysmic weather event resulting from global warming. Next month Iâ€™ll write more about the effects of heating up our planet.
Â© Richard Grossman MD, 2006