An earlier post has discussed the role of Arctic snow/ice feedback on climate. This post will extend the subject of feedback effects to other parts of the climate system.
The feedback effect we are most familiar with is perhaps the acoustic feedback. The electric circuit connecting a microphone to a speaker passes through an electronic amplifier. The amplifier magnifies the signal coming from the microphone and feeds this magnified signal to the speaker, as shown in the figure below. This closed loop can go on and on, and within a fraction of a second, the original small input signal can magnify into a BOOM. Climate feedbacks work in similar fashion.
Increase in greenhouse gases (GHG, e.g., CO2 and CH4) from burning of fossil fuels, livestock agriculture, and deforestation has led to global warming. About 25% of the CO2 emission is absorbed by the oceans and 25% absorbed by and stored in plants, mainly forests. The rest stays in the atmosphere. The oceans, plants, and atmosphere all play important roles in climate feedbacks.
Besides human activities, something else is at work in causing warming. There are dozens of feedback loops in the climate system. Our warming activities are triggering the earth’s own warming mechanisms—feedback loops that feed on themselves to cause more warming. Once triggered, these feedbacks may become irreversible, self-perpetuating effects that are beyond human control.
Richard Houghton and George Woodwell wrote in a 1989 article, “The warming, rapid now, may become even more rapid as a result of the warming itself and it will continue into the indefinite future unless we take deliberate steps to slow or stop it.” They are referring to climate feedback effects. This post will discuss several dominant feedback effects. If we do not stop them now, they will push the climate to a tipping point, resulting in devastation to the human race.
One of the major feedback effects is the sea ice feedback. Ice surface reflects above 50-60% of sunlight, while the dark ocean surface reflects only 10-20%. So smaller sea ice coverage will result in more absorption of sunlight and warming of the ocean and atmosphere. In the Arctic Ocean, the warmer the ocean and atmosphere, the more sea ice melts. This positive feedback (figure below), once started, can go on by itself even if no more GHG is added to the atmosphere.
Permafrost is found at high altitude mountains, in Antarctica and, especially, in the Arctic. In permafrost, ice binds together soil, rocks, sand, and organic matter. Some of that organic matter includes the remains of plants and animals that have been frozen since the last Ice Age, more than 11,000 years ago. There are an estimated 1,400 gigatons of carbon frozen in Arctic permafrost. That’s about four times more than the carbon humans have emitted since the Industrial Revolution. As global temperatures rise, the permafrost may thaw, exposing the organic matter below the surface. Microbial decomposition of the exposed biomass produces CO2 and CH4. These additional GHG will cause further warming. Thus the endless cycle—warming causes permafrost to thaw and the GHG emitted from thawing causes further warming–-forms a positive feedback (figure below).
The warmer and drier climate trends in many parts of the word have caused more frequent and more extensive droughts, fires, and plant diseases; they are part of a feedback loop (figure below) that perpetuates itself even if no more GHG is added to the atmosphere.
Water vapor is brought into the atmosphere via evaporation and plant transpiration. The atmosphere’s water-holding capacity (the amount of water vapor that the atmosphere can hold before it condenses and falls as rain or snow) increases with warming. Water is a GHG and higher atmospheric-water content increases warming, resulting in a strong feedback in the climate system (figure below). Currently, water vapor accounts for about 60% of warming.
