Carbon Sequestration and its Ways | Environment

In this article we will discuss about the carbon sequestration and its three major ways.

Most analyses of options for mitigating the risk of global climate change have focused on the reducing emissions of carbon dioxide and other greenhouse gases. Much less attention has been given to the potential for storing significant amounts of carbon in forests and other ecosystems as an alternative means of offsetting the effect of future carbon emissions in the atmosphere.

Since the late 1980s, much of the interest in carbon sequestration has been prompted by suggestions that sufficient lands are available to use sequestration for mitigating significant shares of annual CO₂ emissions and this approach provides a relatively inexpensive means of addressing climate change.

Carbon sequestration involves removing carbon from the atmosphere and depositing it in a reservoir. The term carbon sequestration is used to describe both natural and deliberate processes by which CO₂ is either removed from the atmosphere or diverted from emission sources and stored in the ocean, terrestrial environments (vegetation, soils and sediments) and geologic formations.

Before human-caused CO₂ emissions began, the natural processes that make up the global carbon cycle maintained a near balance between the uptake of CO₂ and its release back to the atmosphere. However, existing CO₂ uptake mechanisms or carbon sinks are insufficient to offset the accelerating pace of emissions related to human activities.

Carbon can be sequestered in three major ways:

i. oceanic,

ii. geological and

iii. terrestrial/biological.

1. Oceanic Carbon Sequestration:

The world’s oceans are the primary long term sink for human-caused CO₂ emissions, currently accounting for a global net uptake of about two gigaton of carbon annually. This uptake is not a result of deliberate sequestration, but occurs naturally through chemical reactions between seawater and CO₂ in the atmosphere. While absorbing atmospheric CO₂, these reactions cause the oceans to become more acidic.

Many marine organisms and ecosystems depend on the formation of carbonate skeletons and sediments that are vulnerable to dissolution in acidic waters. Laboratory and field measurements indicate that CO₂ induced acidification may eventually cause the rate of dissolution of carbonate to exceed its rate of formation in these ecosystems. The impacts of ocean acidification and deliberate ocean fertilization on coastal and marine food webs and other resources are poorly understood. Scientists are studying the effects of carbon sequestration on these important environments.

2. Geologic Carbon Sequestration:

Geologic sequestration begins with capturing CO₂ from the exhaust of fossil fuel power plants and other major sources. The captured CO₂ is piped one to four kilometers below the land surface and injected into porous rock formations. Compared to the rates of terrestrial or biological carbon uptake, geologic sequestration is currently used to store only small amounts of carbon per year. Much larger rates of sequestration are envisioned to take advantage of the potential permanence and capacity of geologic storage.

The permanence of geologic sequestration depends on the effectiveness of several CO₂ trapping mechanisms. After CO₂ is injected underground, it will rise buoyantly until it is trapped beneath impermeable barrier or seal. In principle, this physical trapping mechanism, which is identical to the natural geologic trapping of oil and gas, can retain CO₂ for thousands to millions of years.

Some of the injected CO₂ will eventually dissolve in ground water and some may be trapped in the form of carbonate minerals formed by chemical reactions with the surrounding rock. All of these processes are susceptible to change over time following CO₂ injection.

Scientists are studying the permanence of these trapping mechanisms, economic costs and environmental risks and the chance of geologically sequestered CO₂ to leak back to the atmosphere. Geological sequestration, which has a fairly large technological potential, has not yet been demonstrated on a scale anticipated to mitigate CO₂ emissions.

3. Terrestrial or Biological Carbon Sequestration:

Terrestrial carbon sequestration, also termed biological sequestration, is typically accomplished through forest and soil conservation practices that enhance the storage of carbon (such as restoring and establishing new forests, wetlands and grasslands) or reduce CO₂ emissions (such as reducing agricultural tillage and suppressing forest fires). These practices are implemented to meet a variety of land management objectives.

Existing terrestrial carbon storage is susceptible to disturbances such as fire, diseases and changes in climate and land use. Terrestrial carbon sequestration is a form of indirect sequestration whereby ecosystems are maintained, enhanced, or manipulated to increase their ability to store carbon.

Biological carbon sequestration transfers carbon (from CO₂) in the atmosphere to biomass through photosynthesis and ultimately stores it in plants (foliage, wood and roots) and soils. Biological sequestration encompasses various ways of using agricultural and forest land to enhance the natural storage of atmospheric CO₂.

Examples include planting or preserving trees, altering crop production practices, planting vegetation in areas prone to soil erosion and changing the way grazing lands are managed. When forests, croplands and grazing lands sequester carbon, they are referred to as “carbon sinks”.

Biological carbon sequestration increases the net fixation of atmospheric CO₂ by terrestrial vegetation with emphasis on enhancing physiology and rate of photosynthesis of vascular plants. It retains carbon in plant materials and enhancing the transformation of carbon to soil organic matter.

It reduces the emission of CO₂ from soils caused by heterotrophic oxidation of soil organic carbon and also increases the capacity of deserts and degraded lands to sequester carbon. There are two main reservoirs available for biological carbon sequestration- soil and forest.

i. Soil Carbon Sequestration:

Soil carbon sequestration is the storage of carbon in soils. Although oceans store most of the Earth’s carbon, soils contain approximately 75 per cent of the carbon pool on land, which is three times more than the amount stored in living plants and animals. It is the process of transferring carbon dioxide from the atmosphere into the soil through crop residues, leaf fall and other organic solids and in a form that is not immediately reemitted.

This transfer or sequestration of carbon helps to offset emissions from fossil fuel combustion and other carbon-emitting activities while enhancing soil quality and long term productivity. Soil carbon sequestration can be accomplished by management systems that add high amounts of biomass to the soil, cause minimal soil disturbance, conserve soil and water, improve soil structure and enhance soil fauna activity. Continuous no-till crop production is a prime example.

Carbon is added to the soil as litter when plants die and decompose. The primary way that carbon is stored in the soil is as Soil Organic Matter (SOM). The amount of carbon found in SOM ranges from 40 to 60 per cent by mass. SOM is a complex mixture of carbon compounds, consisting of decomposing plant and animal tissue, soil biota (protozoa, nematodes, earthworms, fungi and bacteria), and carbon associated with soil minerals.

Carbon can remain stored in soils for millennia or be quickly released back into the atmosphere. Climatic conditions, natural vegetation, soil texture and drainage all affect the amount and length of time carbon is stored. Globally, the top soil stores approximately 1,500 Gt as organic carbon and an additional 900-1,700 Gt as inorganic carbon and exchanges 60 Gt of carbon per year with the atmosphere.

ii. Forest Carbon Sequestration:

In 1992, over 180 countries joined in signing the United Nations Framework Convention on Climate Change, agreeing in principle to stabilize GHG levels in the atmosphere. Since that time, attention has been given to ways to decrease or at least decelerate the flow of carbon from fossil fuels to the atmosphere. Additionally, it may be possible to increase the rate at which ecosystems remove CO₂ from the atmosphere and store the carbon in plant material, decomposing detritus and organic soil.

In essence, forests and other highly productive ecosystems can become biological scrubbers by removing (sequestering) CO₂ from the atmosphere. Forest growth naturally sequesters carbon and the carbon remains in the wood after it is processed into a product. Forest carbon sequestration is a practice to provide sink zone for atmospheric carbon dioxide in term of biological accumulation by increase in forest cover.

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