Earth’s climate has both positive and negative feedback mechanisms.
Negative feedbacks counteract changes and have a stabilizing effect, while positive feedbacks enhance an initial change and have a destabilizing effect.
In the Arctic, positive feedbacks have dominated, playing a major role in Arctic amplification – the phenomenon of enhanced warming in this region compared to the rest of Earth.
Ecosystems in the northern high latitudes are impacted by global warming, and respond with biogeochemical feedbacks related to the earth-atmosphere exchange of greenhouse gases, and biogeophysical feedbacks related to energy absorption.
Biogeochemical feedbacks involve interactions between biological, geological and chemical changes, and biogeophysical feedbacks involve interactions between biological, geological and physical changes.
A recent collaborative study for the Department of Energy by Qianlai Zhuang and C. Adam Schlosser, et. al., compared the relative strengths of biogeochemical and biogeophysical climate feedbacks occurring in the Arctic.
The Permafrost Carbon Feedback
The permafrost carbon feedback is a positive biogeochemical feedback.
The permafrost carbon feedback consists of the permafrost methane feedback, which is strongly positive, and the permafrost carbon dioxide feedback, which is weakly negative.
The briefest explanation of the permafrost carbon feedback loop is this:
Higher temperatures cause permafrost to melt. This melting releases carbon dioxide and methane. Releasing carbon dioxide and methane warm the climate further – which leads, in turn, to more melting.
Permafrost is a layer of permanently frozen soil that exists in cold Arctic regions, including Alaska, northern Canada, and Siberia. Each summer, the top layer of the permafrost temporarily melts, forming small shallow depressions filled with meltwater and rain, known as “thaw ponds.”
The seasonal cycle of freezing and superficial thawing is natural, but it has gone into hyper drive due to increasing temperatures causing excess melting in the past few decades, and the number of thaw ponds has increased dramatically as a result.
This is where the permafrost carbon feedback loop comes into play.
Carbon remains trapped in ground that is frozen, but melting and higher temperatures increase activity of microorganisms in soil. Soil microbes convert carbon to carbon dioxide (CO2) in dryer oxygen-rich environments, and to methane (CH4) in wetter oxygen-constrained environments (such as wetlands).
Since wetlands and ponds are a source of methane (a greenhouse gas 27 times more potent than CO2), increasing their number releases more methane into the atmosphere, leading to further warming, and yet more thaw ponds, in a repeating cycle.
According to the study, rising air temperatures are the dominant factor in determining how much methane is currently emitted and will be emitted in the future. Since the northern permafrost region is warming almost twice as fast as the rest of the planet, methane emissions are already significant, and are projected to be between 2.5 and 2.9 times greater than current levels by the year 2100.
The enhanced decomposition by soil microbes will also release excess CO2 into the atmosphere. However, the permafrost carbon dioxide feedback is actually a negative feedback, since this CO2 release will likely be more than offset by photosynthesis in the extra vegetation that will grow in warmer conditions.
Since the positive permafrost methane effect is greater than the negative permafrost CO2 effect, the combination of the two feedbacks produces a net positive permafrost carbon feedback – or a situation in which warming continues to increase.
It is worth noting that under the greatest warming scenario, the permafrost carbon dioxide feedback diminishes, reducing this negative feedback and enhancing the warming even further.
The Ice-Albedo Feedback
The ice-albedo feedback is a positive biogeophysical feedback, meaning that biology, geology and physics interact in a positive (increasing the effect) feedback relationship. It consists of the snow cover feedback and the vegetation biomass dynamics feedback.
Albedo is the ratio of sunlight reflected to the total sunlight hitting a surface. The higher the albedo, the more reflective the surface. Ice has the highest albedo because it reflects the most incoming sunlight, whereas black asphalt has an extremely low albedo because it absorbs and reemits most of the sunlight that hits its surface. Forests and water have albedos that are higher than asphalt but lower than snow and ice. Therefore, when snow melts, exposing surfaces covered by water or dry land, more sunlight gets absorbed rather than reflected, which has a warming effect.
The ice-albedo feedback makes a warming climate even warmer since warming melts snow and ice, reducing the albedo and leading to further warming. Likewise, this feedback makes a cooling climate even cooler since cooling expands the extent of snow cover, raising the albedo and leading to further cooling.
The study found that the snow cover feedback is expected to be consistently stronger than the effect of vegetation growth (known as “vegetation biomass dynamics”) on albedo.
The Relative Strength of Biogeochemical and Biogeophysical Feedbacks in the Arctic
The study further found that the snow cover and permafrost methane feedbacks will be consistently stronger than the permafrost carbon dioxide and biomass dynamics feedbacks, and that the biogeophysical feedbacks will dominate because the net positive biogeochemical feedback is reduced by the negative permafrost carbon dioxide feedback.
I asked Dr. C. Adam Schlosser, Deputy Director of the MIT Joint Program for Global Change, to compare the effects of the snow cover feedback to the permafrost methane feedback. He responded:
“The way in which each of these two factors’ feedbacks have an effect on climate are quite different and so the extent to which one is “greater” than the other is difficult to judge.
The snow(cover) feedback affects the “albedo” of a surface and the ability to reflect sunlight. Greater amounts of snow(cover) reflect sunlight – less amounts allow more sunlight to be absorbed and surfaces can heat up at faster rates. So we can regard that effect as having a much faster impact on the “local” climate. For example, if you were to put a thermometer on a surface (particularly a darker one) with snow cover during a sunny day and measure its temperature and then remove the snow cover and take it again – you would get a warmer reading almost right away.
The carbon (NCE) and methane (NME) feedbacks don’t have this sort of “local” and “fast” effect – they must mix and interfere with the climate system as a whole (that is to say “globally”) in order for their feedback effect to be measured or sensed. If you knew there was more NME coming out of the ground (and into the air) you would not see the type of thermometer response as in the snow example above. That said – these feedbacks because they operate on the global climate scales, these changes in local or regional NME and NCE – if strong enough – can affect climate globally.”
Are We Underestimating Future Warming?
Currently, the ice-albedo feedback and the permafrost carbon feedback are working in tandem to cause further warming by increasing the amount of energy absorbed at the surface and the concentrations of greenhouse gases, respectively.
According to the study, northern terrestrial ecosystems will lead to a total radiative forcing equivalent to a release of 40~225 Pg CO2 into the atmosphere over the years 2010-2099, depending on the climate warming scenario.
Current global climate models do not take into account this feedback-enhanced source of methane and carbon dioxide emissions, and, therefore, may be underestimating potential future warming.
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