A feedback loop is defined as a system that can control a variable, or some part of the output of a system to increase or decrease the variable and intensify or modulate the effect the variable has on the system.
One example of a feedback loop in climatology/geology is the effect of snow cover on air temperature and formation of snow or ice fields. . During a cooling climatic phase, snowfall accumulates and covers larger areas of the earth’s surface. With these areas covered in white, reflective snow, sun light is more effectively reflected away, and little heat is absorbed ( relatively more solar a radiation is reflected back into space) and the earth surface and atmosphere cool. This cooler atmosphere keeps temperatures low enough so that fallen snow does not melt away in warmer seasons. The cooler air permits more snow to fall and accumulate covering even larger areas intensifying the cooling effect. This may be described as a positive feedback loop.
Our climate fluctuations and “global warming” were to a large extent controlled and modulated by feedback loops.
The many advances and retreats of the Pleistocene Ice sheets were the result of a great feedback loop—involving a “ battle” between the advances of temperate forests and of continental glaciers.
When s the Earth’s orbit, precession and the positions of continental masses on the earth’s surface are appropriate, and when forests grew and expanded over large areas of the Earth surface, these long-lived green plants absorbed and stored CO2. Withdrawl of this gas from the atmosphere cooled the atmosphere by lowering concentrations of a greenhouse gas.
As the earth cooled, winter snow accumulated in the high latitudes and did not melt away in summer. This snow, packed down into ice, forming massive ice sheets thousands of feet in thickness. The weight of the overlying ice generated pressures in its lower levels high enough to cause the ice to flow plastically— generating a “glacial age” in which massive continental ice sheets flowed southward from accumulation centers to eventually cover as much as one third of the existing continental surfaces.
But these ice sheets advanced into forested areas —the very forests which had absorbed carbon dioxide gas and cooled the earth to a temperature where glaciers might form—were now buried and killed as they were overrun by advancing glaciers. The demise of vast temperate forests (and consequent rise in CO2) sowed the seeds of an eventual global warming trend.
With reduced areas of forest cover, CO2 levels in the atmosphere could rise again causing global warming and reversing the process and causing melting and retreat of of the ice sheets . Homeostasis ( or the steady state when all effects were somehow equal) was a fleeting period lasting only briefly in geologic time and occurred when the ice sheets and forests were in some level of balance, at which time ice sheets were advancing as fast as they were melting.
Thus is the history and major causes of the earth’s heating and cooling or of glacial ages and warning stages or interglacials all tied up with the expansion and retreat of global forests. Take a look at Google Earth today to see how humans have actually devastated our forests and encouraged global warming.
For the last several million years ( @6 my) the Earth has cycled through long glacial and short interglacial stages . This glacial interglacial cycle has lasted roughly 100,000 years, with about 90,000 years of ice and glaciers and about 10,000 years of a warm interglacial.
The last glacial advance ended some 20,000 years ago. Since then as the earth warmed and the ice retreated, reforestation followed the retreating ice fronts. This growth in forests withdrew CO2 and initiated a cooling trend…which began some 6000 years ago and which could have initiated a new glacial stage. But it has not. (Humans may have averted that event by deforestation and burning of fossil carbon. )
That expected cooling trend may have been counteracted by a new element— human deforestation and large scale farming!
Humans in their preternatural destructive way first multiplied and as they did began burning down and cutting forests, and prevented natural reforestation. In some place the escape of methane gases from extensive rice paddies may have been a factor as well. Though volcanism, natural decay of vegetative matter, forest fires and other possible anthropogenic processes may have stalled the impending ice age too. Certainly in the last 200 years deforestation and the use of fossil fuels have added huge amount of CO2 to the atmosphere which perhaps has actually averted a new ice age.
Going far back in Earth’s half a billion year history we can decipher from fossils that the evolution of terrestrial green plants were a major part of the great feedback loops in the atmosphere and the rocky earth and oceans (geosphere) had starling effects effect on atmospheric temperatures.
Thus for us moderns, global warming is nothing new. It has been going on for at least the last 17,000 years in most of eastern North America and longer elsewhere. Another factor is the importance of of forests in the equation. They are the regulators of carbon dioxide levels and as such have a great impact on atmospheric temperatures.
Whatever we do as modern fossil burning humans, the earth was (and will remain) in a warming trend for most of the last 20,000 years. We helped it along perhaps avoiding an onset of a new glacial stage with our destruction of forests and using the atmosphere to dump our waste carbon dioxide.
Let’s not get panicky. The Earth systems are essentially conservative…which change only slowly. The earth has been much warmer in ages past and survived, We should address our efforts to modulate the impact of natural warming and anthropogenic warming on this only planet we will ever have by limiting population growth, reestablishing our great expansive forests, and slowly convert from fossil fuels to renewable and other sources of energy which are safe, clean and create no long term pollution.
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