A positive feedback is sometimes called a vicious cycle. We all have experienced positive feedback effects. The most common one involves a microphone and a speaker. The electric circuit that connects a microphone to a speaker passes through an electronic amplifier. The amplifier magnifies the signal coming from the speaker and feeds this magnified signal to the speaker. If the microphone and speaker are in proximity of each together, the microphone picks up the signal coming out of the speaker and the amplifier magnifies this signal. This closed loop goes on and on, and within a fraction of a second, the original small signal can magnify into a BOOM.
The climate system involves a number of positive feedback effects. One of the major ones is the ice albedo feedback. Polar ice (both sea ice and land ice) reflects above 50-60% of sunlight; this is much higher than the global average. So smaller polar ice caps will result in more absorption of sunlight and warming of the atmosphere. The warmer the atmosphere, the more polar ice will melt. This positive feedback, once it has started, can go on by itself even if no more greenhouse gas is added to the atmosphere.
In the Arctic Ocean, the area covered by sea ice grows and shrinks over the seasons of the year. Each fall, as less sunlight reaches the Arctic and air temperature begins to drop, additional sea ice forms. The total area covered by ice increases through the winter, usually reaching its maximum extent in early March. Once spring arrives with more sunlight and higher temperatures, the ice begins to melt, shrinking to its minimum extent each September. As shown in the figure below (from the National Snow and Ice Data Center, NSIDC), summer ice declines have been especially rapid since 2001.
According to observations from NASA’s GRACE satellites, the mass of the Greenland ice sheet has rapidly declined in the last two decades due to surface melting and iceberg calving. The satellites observe that between 2002 and 2016, Greenland lost approximately 280 gigatons of ice per year, causing global sea level to rise by 0.03 inches (0.8 millimeters) per year (figure below).
The GRACE satellites also observed that, between 2002 and 2016, the Antarctica lost about 125 gigatons of ice per year (figure below), causing global sea level to rise by 0.35 millimeters per year. In the figure below, orange and red shades indicate areas that lost ice mass, while light blue shades indicate areas that gained ice mass. White indicates areas where there has been very little or no change in ice mass since 2002.
