Global warming is one of the most critical issues the world is facing in the twenty-first century, one that will affect every living creature on the planet. It is also an extraordinarily complex problem, which everyone needs to understand as clearly and comprehensively as possible. The climate of the earth is constantly changing due to a variety of factors. These factors include, among others, changes in the earth's orbit, changes in the sun's intensity, changes in the ocean currents, volcanic emissions and changes in greenhouse gas concentrations (IPCC, 2007; Rahman, 2013). The temperature of the earth is controlled by the balance between the input from the energy of the sun and the loss of this back into space. Certain atmospheric gases are critical to this temperature balance and are known as greenhouse gases. The two most important greenhouse gases are carbon dioxide and water vapor. Currently, carbon dioxide accounts for 0.03–0.04% of the atmosphere, while water vapor varies from 0 to 2% (Seinfeld & Pandis, 2016). Over the past 150 years, the amount of carbon in the atmosphere has increased by 30%. Most scientists believe there is a direct relationship between increased levels of carbon dioxide in the atmosphere and rising global temperatures. One proposed method to reduce atmospheric carbon dioxide is to increase the global storage of carbon in terrestrial systems (Luo et al., 2017).
An environmental component that absorbs and stores carbon for an indefinite period of time is known as a carbon sink, for example, forests and soils; and one that is discharging or emitting carbon is known as a carbon source, for example, volcanic eruptions and industrial emissions. The flow of carbon from one stock to another (carbon fluxes) involves various physical and biological processes e.g. fossil fuel combustion and biological growth (Le, et. al., 2009).
Terrestrial carbon sequestration is the process through which CO2 from the atmosphere is absorbed by trees, plants and crops through photosynthesis, and stored as carbon in biomass (tree trunks, branches, foliage and roots) and soils (USEPA 2017). In the terrestrial system, carbon is sequestered in rocks and sediments, in swamps, wetlands and forests, and in the soils of forests, grasslands and agriculture. About two-thirds of the globe’s terrestrial carbon, exclusive of that sequestered in rocks and sediments, is sequestered in the standing forests, forest understory plants, leaf and forest debris, and in forest soils. In addition, there are some non-natural stocks. For example, long-lived wood products and waste dumps constitute a separate human-created carbon stock. Given increased global timber harvests and manufactured wood products over the past several decades, these carbon stocks are likely increasing as the carbon sequestered in long-lived wood products and waste dumps is probably expanding (Le, et. al., 2009).
Forest Ecosystems As Carbon Sinks
In contrast to many plants and agricultural crops, which have short lifespans or release much of their carbon seasonally, forest biomass stores carbon over decades and centuries. Moreover, carbon accumulation potential in forests is large enough that forests offer the likelihood of sequestering significant amounts of additional carbon in relatively shorter time periods. On the other hand, forest carbon can also be released rapidly, as in forest burning (Le, et. al., 2009).
Unexpectedly, forests managed for timber, flora and fauna or for ecological restoration stocks carbon through sequestration as a by-product phenomenon. Forests may also be managed specifically for sequestration purpose. But this could reduce the expanse of other forest ecosystem services such as biodiversity. Yet, if forests managed for carbon sequestration are allowed to reach maturity, one of the long-term impacts may be enhanced biodiversity (Osmani 2012).
Components and Mechanism of Carbon Storage
There are four main elements of carbon storage in a woodland ecological unit, namely trees (the main forest canopy), plants growing on the forest floor (understory plants), detritus (and other decomposing matter), and forest soils (Osmani, 2012).
During the growth process, a plant captures carbon within its cells. As the number of leaves, branches and other plant materials increases, the carbon stock in the form of a biomass of the plant (and in turn the whole forest) increases. Eventually, when these materials fall off, they accumulate carbon during the decaying process (Greenpeace International 2017). Forests do not only act as a reserve for carbon stocks, they are also known to be the carbon sources. The forest transitions from one ecological state to another produces significant carbon fluxes. It is essential to cautiously assess the net carbon ratios within a forest to determine the forest’s sink/source contribution (Le, et. al., 2009).
The reduction in biomass due to anthropogenic activities e.g. fire, deforestation for timber and fuelwood etc. can lead to increased carbon release, designating the forest as a carbon source. Conversely, the forest may again turn into a carbon sink as it is restored through reforestation (Osmani, 2012).
The Great Northern Forest – Earth’s Largest Terrestrial Carbon Stock
Winter landscape in Kotavaara, northern Finland
The boreal forest landscape, famously recognized as the Great Northern Forest, represents approximately one-third of the forested areas left on Earth. It is the second largest forest ecosystem in the world and possesses a rich biodiversity of native mammals. The Great Northern Forest covers the regions of Alaska, Canada, Scandinavia and Finland, Russia and Siberia. It is expanded over 16 million km2 which is more than twofold the size of Amazon rainforest. Its extreme meteorological conditions make it distinctive in terms of biodiversity. The tree cover is dominated by slow-growing conifers (Qie, et. al., 2017)
The Great Northern Forest is earth’s largest terrestrial carbon sink because of its vast regions of peat soils and permafrost. But, extensive deforestation, mainly as a consequence of wildfires, worsened by industrial logging, pests and disease, is converting this great carbon sink into a carbon source by rapidly changing the net carbon fluxes (Osmani, 2012). The boreal region is one of the rapidly warming areas of the planet, triggering heat and drought-related pressures on local ecosystems, which eventually leads to severe pest epidemics. These dynamics, in turn, lead to the reduction in tree number, which along with the drier surroundings make the forest more vulnerable to fire. Logging and other industrial activities have also increased the fire occurrences by degrading and fragmenting the forests, particularly in Siberia. The effect of logging is particularly noteworthy. The slow-growing boreal forest takes several years to restore after clearcutting; furthermore, its structural and biological diversity are declined, along with its resilience to climate change (Greenpeace International. 2017).
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