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. Positive climate feedbacks are often discussed in the context of climate change and is one sub-type of positive feedback.
Similar to how keeping money in a savings account earns interest and compounds to earn more money, positive climate feedbacks increase some initial change in the climate. Some initial warming is accompanied by additional warming from some other process, which in turn results in even more warming. For further explanation on what a positive feedback is in general, click here.
Although there are also negative climate feedbacks, positive feedback cycles are what are so disconcerting. Positive feedback cycles accelerate climate change as they increase warming, leading to large non-linear responses. With positive feedback, some minor change in the state of the climate can result in a large change overall. 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.
There are disconcerting positive feedback cycles being observed and predicted. It is important to note that these different types of positive feedback result in a different amount of warming overall. The following are a few of the climate endangering cycles.
One example of a positive feedback is the melting of ice - particularly sea ice - and corresponding decrease in albedo (see Figure 1). Ice is white and highly reflective - corresponding to a high albedo. This reflectivity prevents some incident sunlight from being absorbed. However, when ice melts the vegetation, soil, or water beneath it is exposed. These surfaces are darker and heat faster and thus have a lower albedo. As these surfaces with a lower albedo are exposed to the Sun, they absorb more solar radiation than the ice would have. This absorption further raises temperatures and causes even more ice to melt, continuously heating and pushing the cycle. 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.
The human-caused release of greenhouse gases into the atmosphere as a result of fossil fuel combustion is the initial cause of climate change. These human activities increase global temperature as these gases trap heat in the atmosphere as a result of their basic properties. The feedback comes when the warmer atmosphere results in more evaporation of water on the surface of the Earth. As the water from the Earth evaporates, it turns into water vapour in the atmosphere. This water vapour thus causes even more warming of the atmosphere, creating a self-reinforcing cycle of heating.
As the temperature on the Earth increases, permafrost begins to thaw. This permafrost contains large amounts of carbon that was contained in frozen soil. Thus the melting of the permafrost releases methane into the atmosphere. As these greenhouse gases are released they cause more warming, which in turn causes more thawing and more carbon release.