James Weber explains the role of positive feedback loops and how they could lead to runaway environmental disaster
Our atmosphere can be thought of as a single, highly complex system. The complexity arises in part due to the coupling of a vast array of different elements, such as temperature, wind speed, and chemical composition. Should one element be disturbed, others will also change, and this perturbation will propagate throughout the atmosphere, much like plucking a single strand of a spider’s web and witnessing its other strands vibrating across the structure. Feedback loops are a simple extension of this idea, whereby the vibrations of other strands in the web accentuate or dampen the vibration of the strand originally disturbed.
Not only does the phenomenon of feedbacks play a vital role in the atmosphere: it is pervasive throughout all disciplines in the sciences and everyday life. Take the unpleasant high pitched noise that sometimes occurs while using a microphone. This occurs when a microphone gets too close to its amplifier; the microphone relays background noise to the amplifier, whereby it is emitted at a greater volume. This enhanced volume is received by the microphone and again relayed to the amplifier for an even louder result. As the process is repeated, the sound quickly becomes unbearable, until the microphone is rapidly pulled away.
This is a feedback loop, in which one variable (in this case, the volume of sound detected by a microphone) affects a second variable (the volume of sound emitted by the amplifier) which in turn affects the first variable. Since the second variable in this case increases the first variable, the microphone-amplifier scenario is classified as a positive feedback loop.
Positive feedbacks are often described as a ‘vicious cycle’. In contrast, negative feedbacks are those in which the change in the second variable opposes the original change in the first variable. Crucially, while positive feedbacks can lead to rapid changes away from a system’s original state, as in the case of the microphone, negative feedbacks act as a restoring force, working to push systems back towards their initial state. Upsetting this delicate balance can have severe consequences, and in the case of the atmosphere, these occur on a global scale.
A Sea of Troubles
The decline of the Arctic ice pack has been well documented, with some estimates predicting Arctic summer sea ice will be non-existent by mid-century. While the ocean only reflects 6% of energy from the sun, sea ice reflects between 50-70%, greatly reducing the amount of energy absorbed by the planet—and the resultant temperature rise. Unfortunately, this protection is crumbling thanks to human activities, such as the overuse of fossil fuels in industry: increased atmospheric carbon dioxide (CO2) has raised temperatures in the Arctic, causing sea ice to be replaced by open water. As a result, an even greater fraction of solar energy is absorbed by the planet, leading to further warming and ice loss in a positive feedback loop. Even more alarming is the potentially rapid release of large quantities of methane gas trapped in ice crystals, called hydrates, that lie deep in the sea floor. As the planet warms, this methane will escape, bubble through the ocean and enter the atmosphere. Once in the atmosphere, methane acts as a greenhouse gas—akin to CO2—by absorbing energy emitted by the planet and re-radiating a portion back to Earth’s surface, causing further warming in another positive feedback loop. As a warming gas, methane is around 100 times more powerful than CO2, such that a large release of this gas could potentially lead to runaway climatic warming.
Muddying the Waters
Water, like carbon dioxide, can act as a powerful greenhouse gas, increasing global temperatures in a positive feedback loop. The condensation which forms on a bathroom mirror after a shower arises because warm air can hold more water vapour than cold air. When warm air from the shower is cooled by the mirror’s surface, liquid water precipitates. The same principle applies in the atmosphere: as surface temperatures rise, the quantity of water vapour in the air increases. A 2°C rise in temperature would lead to the lower atmosphere holding about 10% more water vapour. This would result in a multitude of effects—though in an apparent paradox, not all would be harmful. Unlike CO2, which is a relatively unreactive greenhouse gas (and therefore difficult to remove from the atmosphere), water can react with high-energy oxygen atoms (which arise from interactions with solar rays) to produce hydroxyl radicals. This chemical destroys the potent greenhouse gas methane, which would otherwise heat the atmosphere. In this negative feedback loop, as the concentration of water vapour increases, the quantity of atmospheric methane should decline. The hydroxyl radical also can react with organic compounds such as isoprene, which is emitted by plants to alleviate heat stress and so is expected to increase with rising surface temperatures. The downstream effect of isoprene emissions is the formation of aerosols, small drops of liquid-like material suspended in the air. These aerosols themselves can reflect solar radiation and can aid cloud formation, increasing cloud coverage. Clouds, aside from facilitating precipitation, reflect a large fraction of energy from the sun, cooling the Earth. Such negative feedback is a topic of active research.
“It is still uncertain how the combination of droughts, heat stress and increased CO2 will affect plant emissions, and this could have important implications for future climate,” said University of Cambridge Lecturer in Atmospheric Chemistry, Dr. Alex Archibald.
An Earth That is Under the Weather
Though the aforementioned feedback loops represent just a fraction of those at play in the atmosphere, they illustrate the complexity of a system that humankind is swiftly changing. Over the duration of history that humans have been able to study, the global climate has varied greatly, but the causes have been predominantly external (e.g. small variations in Earth’s orbit of the sun) and over a vast timescale (approximately 10,000 to 100,000 years). However, due to human activities, the atmosphere is now changing faster than at any previous time for at least the last 800,000 years. The negative feedback loops in the atmosphere might work to cool a warming planet, but it is highly unlikely that they will be powerful or fast enough to counteract significant human-driven perturbations, particularly the 45% rise in atmospheric CO2 in the last 160 years. Indeed, it is suggested by scientists from the Stockholm Resilience Centre that we could soon pass a point of no return, in which the positive feedback loops—such as the release of methane from hydrates—can no longer be contained.
Should Earth reach this so-called ‘hothouse’ phase—marked by the highest global temperatures in the last 1.2 million years—there would be significant disruption to economies, ecosystems and society as a whole.
Whether or not such an outcome is near, the study of feedback loops and their incorporation into climate models is vital as it allows scientists to predict with increasing certainty the effects of humankind’s actions on our planet. Such studies are vital in providing politicians with ever stronger evidence that action must be taken to prevent catastrophic climate changes
James Weber is an atmospheric chemistry PhD student at Pembroke College.