Climate Feedbacks Are Secondary Influences
There are two main factors that can change the climate of our planet: forcings and feedbacks. In simple terms, climate forcings are primary causes of climate change, while climate feedbacks are secondary influences that simply boost or reduce the impact of a forcing. This article examines climate feedbacks, and explains how they arise and what effect they have on Earth’s temperature and environment. See also our article: What Are Climate Forcings?
- Climate Feedbacks Are Secondary Influences
- What’s The Difference Between Climate Feedbacks And Climate Forcings
- What Is A Tipping Point?
- What Are The Main Climate Feedbacks?
- 1. Water Vapor Feedback
- 2. Ice Albedo Feedback
- 3. Methane Release From Arctic Permafrost
- 4. Methane Release From Hydrates/Clathrates
- 5. Decomposition
- 6. Rainforest Drying And Wildfires
- 7. Cloud Feedback
- 8. Black Carbon Albedo Feedback
- 9. Stefan-Boltzmann Law – Blackbody Radiation
- 10. Chemical Weathering
- 11. Land Albedo Feedback
What’s The Difference Between Climate Feedbacks And Climate Forcings
Forcings are the prime causes of climate change. They are independent, stand alone factors that force the climate system to warm up or cool down. 1 By contrast, feedbacks are secondary influences that simply boost or diminish the warming or cooling caused by the forcing.
An example of a climate forcing is rising levels of greenhouse gases (GHGs), because they lead to a greater greenhouse effect and force the planet to warm up. But as the Earth’s surface warms and the ice in the Arctic or Antarctica starts to melt, it leads to a reduction in the amount of white polar surface that would normally reflect sunlight back into space. The loss of this reflectivity (albedo) is a positive climate feedback, because it leads to more heat from the sun reaching the surface of the planet, which in turn leads to more warming and further loss of ice, and so on. The loss of ice boosts the initial climate warming begun by the GHGs.
A climate feedback that boosts global warming is called a positive feedback; one that diminishes it is called a negative feedback. Rather confusingly, some feedbacks can be both positive and negative.
An interesting example of a something that acts as both a positive and negative feedback, is deforestation caused by tinder-dry trees igniting in the hotter temperatures. This leads to a reduction in the amount of carbon dioxide (CO2) taken out of the atmosphere by growing trees during photosynthesis – a positive feedback, because it boosts warming. At the same time, it leads to less respiration of CO2, a negative feedback because it slows warming, as well as increased reflectivity of sunlight because trees absorb more sunlight than grassland – another negative feedback.
Some feedbacks operate by themselves, while others operate in concert with other elements of the climate system, which makes it difficult to quantify the exact effect of a particular forcing or feedback.
For example, scientists monitoring the effects of greenhouse gas emissions during the 1950s and 1960s were surprised to see a (counter-intuitive) decrease in global temperature. This cooling anomaly is now thought to have been caused by an increase in sulphate aerosols (caused by the post-war upsurge in industrial activity and also by volcanic eruptions) which reflected incoming sunlight back into space. The point is, this cooling masked the increase in global warming and thus delayed the realization that our planet was in danger. For more, see our challenging article: What is the Root Cause of Climate Change?
What Is A Tipping Point?
Climate tipping points are important thresholds which, once crossed, lead to a runaway process of further warming. One of the problems about our climate crisis is lack of transparency caused by climate inertia – the Earth’s slowness to respond to major climate forcings, such as greenhouse gas levels. To put it simply, the planet may be in a much worse condition than we think.
A recent joint study performed by climatologists from Germany, Sweden, Denmark and Australia, concluded that even if global warming was limited to the target of 2 degrees Celsius (3.6 degrees Fahrenheit) enunciated by the Paris Climate Agreement (2015), there is still a risk of the planet entering what the scientists call “Hothouse Earth” conditions. This means temperatures 4-5°C higher than the pre-industrial baseline with sea levels 10-60 meters higher than today, the paper says.
“Our study suggests that human-induced global warming of 2°C may trigger other Earth system processes, often called “feedbacks”, that can drive further warming – even if we stop emitting greenhouse gases,” says lead author Will Steffen from the Australian National University and Stockholm Resilience Centre. 3
What Are The Main Climate Feedbacks?
1. Water Vapor Feedback
As the atmosphere heats up due to the presence of greenhouse gases, the amount of water vapor in the atmosphere increases. And since water vapor is a heat-trapping greenhouse gas, the more of it there is in the atmosphere, the more it heats up, resulting in even more vapor in the atmosphere (a positive feedback). This feedback leads to a much larger greenhouse effect than that due to CO2 alone, equivalent to roughly double the warming that would otherwise occur. 4
Like all climate feedbacks, water vapor only adds to the warming effect caused by the original driver (in this case the greenhouses gases, like CO2). Without the original warming effect, no extra water vapor (and no extra warming) would have appeared.
According to some climate models, in a few drier areas, where a change in rainfall patterns has increased water availability, the warming impact of water vapor is partially offset by an increase in plant growth, which removes more carbon dioxide from the atmosphere, via increased levels of photosynthesis, causing a cooling effect.
2. Ice Albedo Feedback
In climate science, the term “albedo effect” describes the capability of a surface to reflect sunlight. Generally speaking, the darker the surface the more sunlight it absorbs, and the brighter it is, the more light it reflects. For example, ice and snow (being white) reflect sunlight back into space, thus helping to keep the planet cooler.
Ice albedo is a positive feedback and works like this. When temperatures rise in the summer (climate forcing), polar ice melts. As it does so, the colour of the polar surface goes from white (highly reflective) to dark blue (the non-reflective colour of the ocean). As a result, more heat is absorbed at the poles, causing more ice to melt, and so on. Because the ice-melt boosted the effect of the forcing, it is classified as a positive feedback.
Similarly, when temperatures fall in the winter (climate forcing), the ocean freezes and turns to ice, while the land becomes covered in snow. The polar surface turns brilliant white and reflects back into space most of the sunlight it receives. As a result, less heat is absorbed, causing more ocean surface to turn to ice, which reflects more sunlight, and so on. Again, a perfect example of a positive feedback that boosts the initial change, in this case warming.
Because polar ice melt is a potential trigger for major sea level rise (SLR) around the world, accurate prediction of its progress is critical. Recent studies tend to paint a gloomier picture of the stability of polar ice sheets and glaciers than the more conservative projections offered by the IPCC. 5 6 7 8
3. Methane Release From Arctic Permafrost
One of the most potentially damaging climate feedbacks concerns the huge carbon reservoir located in the permafrost of the far north.
There are an estimated 1,200 billion tons of carbon – frozen carbon dioxide (CO2) or methane (CH4) stored beneath the permafrost in Arctic regions, mostly in Canada, Russia and Alaska. 9 The rapid increase in the rate of global warming has led to widespread thawing of the permafrost, putting buildings and other infrastructure at risk of subsidence or collapse.
The IPCC have calculated that the permafrost has been thawing at a rate of 4mm per year since the early 1990s, but later studies show that permafrost is thawing much quicker than predicted, creating a worrying positive feedback. 10 11
4. Methane Release From Hydrates/Clathrates
Methane hydrate (also known as methane clathrate) is a solid compound containing a significant amount of methane within its ice-like structure. Extremely large deposits of this compound have been located beneath sediments on the ocean floor. Were these deposits to be released into the atmosphere, it would likely lead to irreversible global warming – similar to the Permian–Triassic extinction event – the most extreme mass extinction event in history. A recent scientific expedition detected levels of methane in the atmosphere of the Siberian Arctic – likely being released by methane clathrates in the area between the Laptev Sea and East Siberian Sea – that were 100 times above normal. 13 14 See also: Why are Methane Levels Rising?
Global warming may cause significant emission of carbon from peat bogs, due to decomposition of its mossy humus. 15 This may be released as methane, which can create a huge feedback effect due to its powerful heat-trapping properties (it is 25 times more potent than CO2).
The world’s largest permafrost peat bog, over 1 million square kilometers in area, is located in Western Siberia. The thawing of its frozen soil is estimated to result in the release (over several decades) of up to 70,000 million tons of methane, constituting a massive additional source of greenhouse gas emissions. 16 Similar observations have been made in the east of the region. 17
Freshwater bodies account for more than 15 percent of the Earth’s natural emissions of methane, chiefly derived from aquatic vegetation. In fact, as much as 77 percent of a lake’s methane emissions come from the decomposition of aquatic plants. Microorganisms break down the plant matter generating methane that bubbles up to the surface.
Warming temperatures stimulate the growth of these plants, resulting in yet more heat-trapping methane being released into the atmosphere. Overall, researchers calculate that the lakes of the northern hemisphere might almost double their methane emissions by 2070. 18
6. Rainforest Drying And Wildfires
The Amazon rainforest makes half its own rainfall. Evaporation from the complex surfaces of the forest greenery combined with heavy transpiration of the trees themselves, releases moisture into the air creating clouds which are then blown westwards by prevailing easterly winds from the Atlantic. 19 Furthermore, the clouds reflect sunlight, thus helping to cool the region and reducing global warming.
The moisture recycles half a dozen times via rainfall and evaporation until it meets the Andes mountain range. Here, the uplift triggers major rainfall, filling up the Amazon river system. The falling rain is washed downstream to the ocean where it evaporates into the air, repeating the cycle. All of which explains the term “rainforest”.
Unfortunately, rising temperatures near the equator mean less rainfall, and even drought, which leads to fewer trees to absorb and transpire water, which in turns makes conditions still drier. Ultimately the rainforest begins mutating into a drier type of biome through a process known as savannization. This leads to a positive feedback loop of increased warming due to greatly reduced carbon sequestration. 20
An extreme version of this drying out process occurs after forest fires. A number of studies have revealed that forest fires have a huge adverse effect on carbon sequestration while at the same time releasing huge amounts of CO2 into the atmosphere. See: What is the Effect of Wood Burning on Climate Change? This double whammy positive feedback provides a massive boost to global warming.
Wildfires are also a major source of air pollution, characterized by significant emissions of black carbon and other damaging pollutants.
7. Cloud Feedback
Clouds have a significant impact on Earth’s climate. On the one hand they reflect back into space about one-third of the sunlight that reaches Earth’s atmosphere (the albedo effect of clouds), creating a negative feedback. On the other hand, they trap infrared radiation trying to escape from the surface of the planet, creating a positive feedback. Indeed, there is still no clear conclusion as to whether clouds amplify or diminish global warming. For more on this, see: How Do Clouds Affect Climate?
For example, in its Fourth Assessment Report (2007) the IPCC states: 21 “By reflecting sunlight back to space and by trapping heat emitted by the surface and the lower troposphere, clouds exert two competing effects on the Earth’s radiation budget… In the current climate, clouds exert a cooling effect.”
Six years later the cloud cooling effect had changed to a cloud-warming effect. In its Fifth Assessment Report (2013), the IPCC estimated cloud feedback as being “between near-zero and moderately positive.” 22
Although the precise effects of clouds on the climate system are not well simulated by climate models, it seems that the IPCC view – that clouds exert a neutral or slightly positive climate feedback – has replaced the earlier view that, if there were no clouds to reflect sunshine or trap escaping heat, the world would be 5°C warmer. 23
8. Black Carbon Albedo Feedback
Black carbon consists of aerosols (particulate matter) formed by the incomplete combustion of fossil fuels, wood and other fuels. Black carbon emissions are increasing rapidly in many developing countries as a result of open biomass burning and residential solid fuel combustion. Almost 90 percent of emissions are generated in Asia, Africa and Latin America.
Aside from its serious effects on air quality and respiratory health, black carbon is a positive climate feedback, due to its effect on the albedo of the polar ice caps. By blackening the reflective white surface of the ice, it allows more heat to reach the polar surface, thus boosting global warming. According to one study, the melting effect of black carbon on Arctic snow and ice is more than three times greater than that of carbon dioxide. 24 Note, however, a recent study of satellite data from 1982 to 2014, which indicated that the loss of surface albedo in the Arctic (roughly 1.4 percent per decade) was due mostly to melting ice, rather than soot absorption. 25
9. Stefan-Boltzmann Law – Blackbody Radiation
One of the main negative climate feedbacks (incorporated into most climate models produced for the IPCC) is derived from the Stefan–Boltzmann law, which (in very simple terms) says that the warmer the Earth gets, the more heat it releases back into space. It is also known as the Planck feedback. 26
10. Chemical Weathering
When atmospheric carbon dioxide is dissolved by rainfall and lands on exposed rock (particularly silicates, or carbonate rocks like limestone), it results in microscopic chemical weathering of the rock surface – a process which produces calcium and bicarbonate – before being transported to the ocean by river run-off. In the ocean, the calcium and bicarbonate are used by marine calcifying organisms to build shells and skeletons.
When the marine organisms die, their shells and skeletal remains sink into deep water where they end up as sediments on the ocean floor. Over millions of years, chemical and physical processes turn these sediments into sedimentary rock. The carbon remains trapped in the rocky lithosphere until, after millions more years, geological forces expel it into the atmosphere through volcanic activity. (For more. see: The Carbon Cycle: How Does it Work?)
Chemical weathering occurs more rapidly in warmer conditions, because warmer atmospheres contain higher levels of carbon dioxide. This makes the process a negative feedback because it seeks to retard or diminish the initial warming by removing heat-trapping CO2 from the atmosphere.
11. Land Albedo Feedback
Forests generally have a low albedo effect (that is, they have a reduced ability to reflect sunlight). As a result, some of the adverse effects of deforestation can be offset by reduced heat absorption. Indeed, in a situation of evergreen forests with seasonal snow cover, the albedo reduction caused by deforestation may even lead to a net cooling effect. Winter albedos of snow-covered treeless areas are typically 10-50 percent higher than nearby forested areas because snow does not cover the trees as completely as it does pasture or arable land. 27
Reduced albedo diminishes rather than amplifies warming, which means this is a negative climate feedback, although on a relatively small scale compared to the warming effect of deforestation generally, due to its increase in atmospheric CO2.
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