Positive feedbacks, processes in which an initial change brings about an additional change in the same direction, are of the most significant climatic factors on earth. Those mechanisms have the potential of turning the earth into a very different planet over a short period of time. It has happened repeatedly in the past and is just as likely to happen in the future, especially when taking into account the unprecedented scale of the environmental changes caused by humans. With the right help in the right places, they can totally alter earth’s living conditions.

Some of these feedbacks effect the earth’s carbon distribution. When found on land or in the ocean, carbon does not have a greenhouse effect. But in the atmosphere (as CO2) carbon is a very significant greenhouse gas, by no doubt the one that draws most of the attention. Some of it for political reasons and some since most scientists agree that largely due to its concentration increase in the atmosphere, the climate is changing.

And still the discussion regarding CO2 refers to the chronic issue (its gradual accumulation in the atmosphere due to humans’ emissions) while we are obviously interested in acute solutions and so are interested in carbon storages which potentially can quickly reach the atmosphere in significant amounts and so significantly change the earth’s climate.

And indeed, the earth contains hundreds of millions of Gigatons of carbon. Only a fraction of it is found in the atmosphere (as CO2) and most is held across the world in what is called carbon reservoirs.
Carbon can be stored in many forms - as mineral compounds in the ground, dissolved in water, as hydrates (like Methane Hydrates), and as organic compounds in live and dead matter.

When in storage and out of the atmosphere, the carbon does not act as a greenhouse gas. Some of the dead organic matter is stored as fossils – oil, gas and coal deposits that after hundreds of millions of years are now releasing their carbon back to the atmosphere by humans who use them as fuel.

There is a constant flow of carbon from one reservoir to the other, this rotation is called the carbon cycle. Reservoirs that release carbon to the atmosphere are called carbon sources and reservoirs that take carbon out of the atmosphere and store it are called carbon sinks.
Land biosphere, including forests and other vegetation, (uptake CO2 through photosynthesis) and oceans (dissolving CO2) are earth’s carbon sinks and so are very significant in determining its climate.

Without the increased absorption of carbon dioxide by the oceans and land sinks since the industrial revolution about 200 years ago, atmospheric concentrations of CO2 would already be above 500 parts per million (ppm), and the world would be much warmer. The ocean sink and land sink have absorbed 56% of human carbon emissions since 1750 (with more or less the same absorption impact), keeping global carbon dioxide concentrations ‘down’. The rest of the emissions, that were not absorbed by the carbon sinks, increase the global average temperature.
CO2 atmospheric levels have increased from a pre-industrial concentration of 280 ppm to 395 ppm in 2013, and they continue to increase with about 10.4 Gigaton carbon, annually released by anthropogenic emissions (fossil fuels, cement and land use change). Of which about a half stays in the atmosphere causing an increase of about 2 ppm each year.

As mentioned, about half is absorbed and stored by those two sinks, but there is a limit to how much more they can hold. According to many scientists’ predictions, when the temperature rises above a certain degree, the sinks will cross a critical tipping point and shift to become carbon sources.

Both geological records and model calculations predict an amplification of the greenhouse effect occurring after carbon sinks would turn into a massive producer of atmospheric carbon as it gets warmer. And of course the warmer it gets, the more abruptly it happens.

Terrestrial Carbon Sinks

The soil and vegetation weren’t always carbon sinks. Obviously CO2 was always absorbed by vegetation through photosynthesis (fixing CO2 absorbed from the atmosphere into carbohydrates like sugar), and released back to the atmosphere by respiration by the vegetation itself and by all the other organisms, however in pre-industrial time the amount of carbon taken by them from the atmosphere and the amount of carbon released by respiration to the atmosphere were more or less the same so soil and vegetation were reservoirs of carbon but not a sink. Since industrial time soil and vegetation became a sink, meaning more CO2 is being taken from the atmosphere to the land vegetation than is being released back to the atmosphere.
The increase in the concentration of atmospheric CO2 since the industrial revolution, changed the carbon balance, turning the land into a sink.

Overall, terrestrial vegetation, as living biomass, contains today about 450-650 Gigaton carbon with tropical forests account for roughly 40% of this carbon reservoir. Some of the carbon within the vegetation ends up in the soil (as the vegetation rots and piles up). Overall the amount of organic carbon stored in soils worldwide, as dead organic matter, is 1500-2400 Gigatons, with boreal forests storing more than third.

Greenhouse gases and climate change affect the terrestrial biosphere carbon reservoir in different ways (the effect is a part of a very complex system which is not yet fully understood).
The increasing concentration of CO2 and Nitrogen deposition (mostly from fossil fuel combustion and high intensity agriculture), on one hand strengthen the carbon sink since they unintentionally act as fertilizers and so increase photosynthesis, allowing vegetation in general and forests specifically to uptake more CO2 from the atmosphere.
On the other hand some studies have shown that in the tropical forests along with greater growth of trees and bushes comes more litterfall and so more organic matter on the forests floor. This increased litterfall can increase carbon release due to "priming effect", as it provides easily decomposable organic matter which stimulates microbial activity and results in increase of CO2 emission from the soil.

Apart from the direct effect of increased concentrations of greenhouse gases, the increasing concentrations also affect forests indirectly by climate change. Here are a few ways in which climate change affects forests and as a consequence their role as a carbon sink:

The last decades temperatures rise and the more frequent and severer droughts, fires and insects attacks, increased tree mortality in forests in different regions (high and low latitudes). Carbon that was uptaken and held by a tree for a very long time, some for more than a hundred of years, is released when the tree dies and decomposes in a much quicker rate than the tree accumulated the carbon. This is a small scale example of how a sink becomes a source.
These trends in climate change and tree mortality are expected to continue in the future especially as CO2 concentrations increase.

Higher temperatures cause an increase in respiration rates so more carbohydrate is used for breathing. Some scientists believe that this will limit the energy trees would use for growth and hence forests will tend to become smaller as temperatures rise. In forests which normally have cold winters such as the boreal forests, increased respiration during the warmer winters due to climate change, would cause an increased release of CO2 to the point of leaving the trees with carbon deficiency.

Higher temperatures also allow higher microbial activity and therefore faster decomposition of dead organic matter in the soil which releases higher amounts of CO2 to the atmosphere. Also as temperatures increase, insects can expand to new territories and attack trees more efficiently, especially if these trees are already more vulnerable due to other physiological stress such as water stress, heat stress and carbon starvation (explained in the following paragraph).

Higher temperatures, even with no precipitation decrease, increase forests’ water stress. As temperatures rise in many forest regions, the relative humidity decreases, so more water is evaporating from the soil, and the trees are more prone to lose water in transpiration (the process by which water evaporate from the leaves and are replaced by water drawn up from the ground through the roots and stem) so they must close their pores. Closing the pores, which are in charge of absorption of CO2 from the air, leads to carbon starvation (since as the pores are kept closed, so the tree won’t lose more water, they also can’t uptake CO2 from the atmosphere for the photosynthesis process) and if the pores are left open it leads to water stress, both lead to more tree mortality.

Higher temperatures with precipitation decrease obviously increase forests’ water stress. Models predict that most tropical forests will experience less frequent and lower rainfall in the future as a result of climate change. And since much of the rain in the rainforests originates from water evaporated within them, forest clearing (result of human greed) and the reduced transpiration (result of higher temperatures which result of human greed), result in less rainfall downwind from the cleared forests creating even drier conditions.

In boreal forests, climate change is also drying out and warming up and this trend is expected to accelerate in the future. Both tropical and boreal forests experience more frequent and severer droughts and heat waves, water stress, heat stress and carbon starvation, all significantly affect forests as carbon sinks.

These drier and warmer conditions are, as earlier mentioned, beneficial conditions for insects attack. Different species of Bark Beetles, which in these new conditions and with trees being more vulnerable are thriving, can expand to new niches attacking forests more efficiently. In recent decades, billions of trees across millions of hectares of forests, ranging from Mexico to Alaska, have been killed by bark beetles. Several of the current outbreaks are among the largest and most severe in recorded history, and the conditions are only getting better for the beetles.

Another indirect effect of drier and warmer conditions is that they aggravate effects of fires by increasing their frequency, intensity and size. While fires have positive effects on forests growth, such as cleaning up a forest of dead and decaying matter, maintaining ecosystem balance by removing diseased plants and harmful insects and regeneration of seeds through increased sunlight, the negative growth effects outdo them and they will become even more dominant as fires become more intense and frequent.

An obvious effect of fires on the carbon cycle is that they burn organic matter quickly, releasing large amounts of CO2 to the atmosphere. The more intense the fire, the more organic matter it can burn.
After a fire, the loss of forest canopy (the forest’s uppermost layer) allows more sun radiation to reach and warm the surface, which may speed decomposition of organic matter in the soil. In boreal forests the soil holds large amounts of organic matter so increased decomposition there is highly important, and if the soil warms enough to melt the underlying permafrost, even more stored carbon may be unleashed. A study done on the subject suggests that over recent decades, boreal forests have become a smaller sink and may actually be shifting toward becoming a carbon source.

As temperatures rise and it becomes drier, boreal forests are expected to shift northwards (to colder regions) and to shrink, releasing stored carbon into the atmosphere. Besides the shrinking of the forests area, which its effect on the forest carbon sink is obvious, the shifting northwards is also highly significant in decreasing the boreal forests’ sink. This shift northwards means that trees will die in the southern end of the forest and new trees will grow in the northern end. Even if there was no shrinking and for each dead tree a new one would grow, when trees die they quickly release carbon that was accumulated for hundreds of years and more importantly the soil which holds large amounts of organic matter also releases carbon that was accumulated for thousands of years, carbon that the new forest growing in the north end would take thousands of years to accumulate. So the expected shrinking and shifting northwards of boreal forests would create an important carbon source.

The Oceanic Sink

The oceans are the largest active carbon reservoir on earth, containing about 38,000 Gigatons of carbon. They currently absorb more than a third of the CO2 emitted by humans.

The Solubility Pump

CO2 from the atmosphere dissolves in the ocean water. Basically the colder and more turbulent the water are the better CO2 is absorbed. Therefore most of the carbon intake takes place around the poles, in the north Atlantic ocean (which accounts for about 60% of this mechanism alone) and around Antarctica in the southern ocean, where cold surface water are sinking to the deep ocean as part of the Thermohaline Circulation (THC) also called the Ocean Conveyor Belt or the Meridional Overturning Circulation (MOC) – a system of surface and deep currents encompassing all ocean basins.
The fact that warmer water surface dissolves less CO2 than cold water surface, is especially significant at the polar regions where warming occurs faster and as mentioned it is where most of carbon absorption happens.

Another important factor is acidity. The chemical ability of the oceans to absorb CO2 decreases as its concentration rises. CO2 dissolves less in acidic water, and since the CO2 itself makes the water more acidic, after a certain amount of CO2 is dissolved in the water (naturally it takes place at the surface), the acidity rises and the absorption rate decreases.

The ocean currents keep its surface (that absorbs CO2) from acidifying. By dragging down the surface water with the CO2 that was absorbed, to deeper parts of the ocean, the currents "make room" for more CO2 to dissolve at the surface. This down flow of water is part of the mentioned thermohaline circulation, and water temperature is an important part of what moves this circulation. In the north Atlantic, hot water from the Gulf Stream flow north towards the pole, they release heat to the atmosphere and become colder and so tend to sink (as colder water is denser).

Salinity also affects water density (the more saline the water the denser they are). Due to global warming, water temperature is on the rise and water salinity near the North Pole decreases as more fresh water enter from the melting sea ice and ice sheets, decreasing the water density and as a consequence slows down the circulation. A weaker down flow also slows down the CO2 down flow, leaving warmer and more acidic water at the surface, which are less capable of absorbing CO2, what eventually causes a weaker CO2 sink. Some studies show that the Atlantic Meridional Overturning Circulation (part of the thermohaline circulation) has slowed by 30% between 1957 and 2004. Studies have also shown the north Atlantic CO2 sink has reduced since the mid-1990’s due to increased water stratification and slowing overturning circulation which decrease rates of ventilation and tend to slow CO2 uptake.

The Biological Pump

Another sink mechanism in the oceans is driven by phytoplankton (microscopic aquatic plants) and is similar to the mechanism of terrestrial vegetation. Through photosynthesis phytoplankton absorb CO2 to be fixed into organic compounds. Like plants, phytoplankton also release CO2 by breathing.
Phytoplankton activity is found mostly within the first 50-100 meters of the surface and varies widely according to season and location. Phytoplanktons are sensitive to changes in environmental conditions and require specific nutrients. They depend on ocean currents, therefore are found mainly in the areas where deep water are rising, bringing nutrients from the deep to the surface - a process known as upwelling (such as in the North East Atlantic and North East Pacific or along the Southern Ocean).

The higher concentrations of CO2 in the atmosphere can affect this oceanic biological sink to make it stronger or weaker. On one hand the warming can in some locations create higher rates of photosynthesis, meaning more growth and therefore a stronger sink. But on the other hand warming also increases respiration rates and from a certain threshold respiration would become more dominant which would weaken the sink.

In addition warming creates more stratified water, meaning less upwelling and therefore less nutrient supply, which would limit phytoplankton growth, and it also means more acidification of the surface water which makes it much more difficult for certain phytoplankton to survive and build there shell, both would weaken the sink. In the past, nutrient depletion and ocean acidification caused mass extinction of phytoplankton as billions upon billions of organisms no longer conducted photosynthesis, what halted an important climatical mechanism that drives out vast quantities of carbon away from the atmosphere.

From Earth to Venus

Not until long ago the earth's climate was considered as something that will change gradually, as CO2 and global temperatures continue to rise. But nowadays there is a growing consensus among climate scientists that positive feedback mechanisms are likely to abruptly amplify the rate of warming as the earth's climate reaches a "tipping point".

If it still feels a little intangible, maybe looking at another planet in the solar system can help…
Venus is similar to earth in size, mass and chemical composition (sometimes called earth’s sister planet). Its orbit is also similar, with Venus orbiting only 20% closer to the sun than the earth. They both started out several billion years ago, approximately 4.5. Despite the similarity, Venus’ surface temperatures are over 400°C (752°F).
It is hotter than Mercury's surface, even though Venus is nearly twice as distant from the Sun.
The 20% difference in distance from the sun between Venus and Earth, might explain a temperature difference of a few tens of degrees, but not about 380 degrees gap.

The total amount of carbon in planet Venus is much like the Earth's however the atmosphere of Venus is 96.5% CO2 by volume, while the Earth's atmosphere is composed merely of 0.04% CO2.
Venus was under what's known as a runaway greenhouse effect, resulted from a series of positive feedbacks. Somewhere along its geological past, Venus climate went through a number of critical thresholds and positive feedback mechanisms that drove it to its current state.

Before the runaway greenhouse effect hit Venus, it looked much like earth…