These human activities increase global temperature as these gases trap heat in the atmosphere as a result of their basic properties. The human-caused release of greenhouse gases into the atmosphere as a result of fossil fuel combustion is the initial cause of climate change. This feedback can act more quickly over the ocean than over land as sea ice is able to melt more quickly than large, continental ice sheets. This absorption further raises temperatures and causes even more ice to melt, continuously heating and pushing the cycle. As these surfaces with a lower albedo are exposed to the Sun, they absorb more solar radiation than the ice would have. These surfaces are darker and heat faster and thus have a lower albedo. However, when ice melts the vegetation, soil, or water beneath it is exposed. This reflectivity prevents some incident sunlight from being absorbed. Ice is white and highly reflective - corresponding to a high albedo. One example of a positive feedback is the melting of ice - particularly sea ice - and corresponding decrease in albedo (see Figure 1). The following are a few of the climate endangering cycles. It is important to note that these different types of positive feedback result in a different amount of warming overall. There are disconcerting positive feedback cycles being observed and predicted. This is in stark contrast with negative feedback, which reduces the impact of some initial climate change by acting in the opposite direction, bringing it back to its initial state. With positive feedback, some minor change in the state of the climate can result in a large change overall. Positive feedback cycles accelerate climate change as they increase warming, leading to large non-linear responses. For further explanation on what a positive feedback is in general, click here.Īlthough there are also negative climate feedbacks, positive feedback cycles are what are so disconcerting. Some initial warming is accompanied by additional warming from some other process, which in turn results in even more warming. Similar to how keeping money in a savings account earns interest and compounds to earn more money, positive climate feedback increases some initial change in the climate. Positive climate feedbacks are often discussed in the context of climate change and is one sub-type of positive feedback. Positive climate feedback is a process that is one type of climate feedback wherein some initial change in the climate causes some secondary change that in turn increases the effects of the initial change, essentially magnifying the initial effect. The melting of ice and corresponding change in albedo is one major positive feedback. A map showing the decrease in arctic sea ice over the years. Logistic (S-shaped) growth would be a better choice for modeling world population for this 100 year time interval shown.Figure 1. Also shown is an estimate of future world population which is close to the mid-range United Nation Environmental Program (UNEP) best guess for future population to stress that exponential growth is not realistic for world population although it works fairly well for the time between 19. In the graph below we show the world population predicted for a fixed 2% growth rate from 1950 to 2050. Not all positive feedbacks give exponential growth but all, left unchecked, will result in unlimited (or unstable) growth. The solution to this is P(t)=Po(exp) or exponential growth. This idea can be modeled nicely with the differential equation dP/dt=+rP, where P is population and r is the percent birth rate. In the figure below connecting population to births, large populations cause large numbers of births and large numbers of births result in larger population. Positive feedbacks will result in unlimited growth (until checked) and are sometimes referred to as vicious cycles. This same trick of multiplying the signs of the connections around a loop together to find out whether it is a positive or negative feedback loop works for more complicated loop structures with many more connections.Īn example of positive feedback is world population with a fixed percentage birth rate. Going around the loop the positive connection times the negative connection gives a negative loop feedback effect. When these two connections are combined we get a negative feedback loop as shown at left in which the coffee temperature approaches the stable equilibrium of the room temperature.
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