Positive feedback is a real bitch of a problem
Most homes have a thermostat hanging on the wall in a convenient, central location. This little device connects to heating and cooling systems and regulates indoor temperatures. A thermostat constantly monitors the home’s temperature. If it detects the temperature has dropped below a programmed level, it turns on the heating system to warm the house back up to the desired temperature. This system forms a negative feedback loop where the thermostat responds to home environmental conditions, maintaining a warm cozy abode — if temperatures drops too low, the thermostat orders up more heat.
All in all, the thermostat provides a nifty little feedback system to maintain a constant home temperature. But suppose the system goes awry and works as a positive feedback loop. Then a temperature rise would be a signal for even more heat. Once this loop starts, the home would continue warming until the furnace ran out of heating oil.
Unfortunately, positive feedback loops are far too common in the natural world. The soft, earthy soils blanketing much of the land on our planet are the anchor for most terrestrial ecosystems. Forest and farm crops both thrive on healthy soil. Rich soil is alive with microbes and nutrients, and soil quality is directly related to its organic content.
Soils serve as a valuable carbon sink, but they are a two-edged sword, and they can also release carbon back into the atmosphere, exacerbating global warming.
Carbon sequestration in soils
Current estimates are that 2,500 gigatons of carbon reside in the earth’s soils. This number compares with 800 gigatons of carbon in the atmosphere and 560 gigatons in the biosphere (plant and animal life). So soils hold almost twice as much carbon as the atmosphere and biosphere combined.
The pathway for carbon sequestration in soils starts with plant photosynthesis. Plants remove carbon (primarily CO2) from the atmosphere and then use sunlight to transform it into the carbon compounds they need for growth. Excess carbon from the plant’s photosynthesis exudes from the roots, where it feeds micro-organisms in the soil. Through this process and the decay of dead plants, the carbon generated by plants transforms into organic-rich components of the soil. We refer to these organic components as humus.
The dark, rich texture of high-quality soils is a reflection of the amount of humus available. Humus is also valuable from an agricultural standpoint since it provides the soil with fertility, structure, and water retention capacity. Soil is what farmers desire for growing crops. If you remove the humus and nutrients from the soil, then you just have dirt.
Soil-based carbon-storage can be maintained or destroyed under the influence of agriculture. Farming methods that revitalize the carbon in the soil, also build its sequestration capacity. However, not all farming has historically cared for the land in this way. Since the original agricultural revolution, the earth’s cultivated soils have lost an estimated 50 to 70 percent of their stored carbon.
Most of this soil-carbon loss occurred after the industrial revolution when large-scale farming became possible and many of the practices adopted, as agriculture became an industry, were carbon-reducing in nature. Not only did these practices eliminate carbon-storage capacity, but they released excess carbon into the atmosphere since exposed humus oxidizes into carbon dioxide. Between 50 and 100 gigatons of CO2 are estimated to have been released into the atmosphere during this process.
Heat, metabolic pathways, and positive feedback
Poor farming practices are not the only way carbon is released from the soil into the atmosphere. Think of the soil as a thermostat set to a positive feedback loop. The trigger for this thermostat is a dependency between organic decomposition and heat. Good soil is loaded with partially decayed organic material. It is all part of the “soil carbon turnover” process where plants remove carbon from the atmosphere and sequester it in plant biomass and the soil. Eventually, the plants die and decompose into the soil. Then bacterial microbes in the soil feast on this organic matter, digest it, and expel carbon dioxide back into the atmosphere. Effective sequestration takes place when more carbon is being absorbed than expelled.
But bacteria are cellular creatures with metabolic engines. Heat increases their metabolic rate and causes them to digest more organic material as ambient temperatures rise. This process, of course, results in increased CO2 emissions into the atmosphere. In an era of global warming, soil carbon turnover creates a positive feedback loop where warming from fossil fuel emissions raises global temperatures, resulting in the release of even more CO2 from the planet’s soils..
Researchers at the University of Exeter calculate that a 2 degree Celsius rise in global temperature will release 230 gigatons of carbon from the earth’s soils. For comparison, this number represents over twice the USA’s total carbon emissions during the past 100 years.
These types of annoying complications are completely ignored in the world of political speak, where combating climate change is simply a matter of planting more trees. Mind you, there is nothing wrong with more trees, but they do eventually die and decompose, releasing their sequestered carbon back into the atmosphere — the warmer the planet, the more rapid the carbon release.
There are no simple solutions to global warming. Soils are only one of many positive feedback loops, operating quietly in the environmental background; thermostats gone awry, greeting each incremental increase in global temperatures with the release of carbon to further stoke the engine of climate change. Even if the world reached net-zero emissions today, feedback loops would ensure another rise of at least 1 degree Celsius. Mitigating climate change is possible, but not simple, and certainly not accomplishable without sacrifice. Feedback loops are a troubling reality, particularly since the climate-change train has left the station with the throttle wide open. Stopping all that momentum takes science, planning, and collaboration. So why did we leave the Paris Agreement on November 4th, 2020?
Read more on Medium publications:
Environmental Articles on EarthSphere
Stories, Life Observations and more on Dropstone
See my recent Rand Soler book
Soil as Carbon Storehouse: New Weapon in Climate Fight? (By JUDITH D. SCHWARTZ; Yale Environment 360)
Global Carbon (USDA)
Soil Carbon Storage (By Ontl, T.A. & Schulte, L.A.; Nature Education Knowledge)
Warming of 2°C would release billions of tonnes of soil carbon (Source: Science Daily)