Atmospheric carbon dioxide plays an important role in climate change on the geological time scale. To combate the climatic change and risk of global warming, and to save the planet, it is urgent to reduce and stabilize the concentrations of carbon dioxide (CO₂) and other greenhouse gases (GHGs) in the earth's atmosphere. This review has mainly studied the possible strategies of carbon sequestration by natural and engineering techniques, and off-setting the GHGs emissions, especially focusing on the long-term storage of carbon in oceans, soils, vegetation, geologic formations, industrial materials, and engineered construction works. Carbon can be sequestrated by absorption of carbon dioxide via physicochemical process and biological pump by phytoplankton, by application of iron nutrients in oceans, and engineering techniques for direct injection in oceans by enhanced dilution by CO₂ hydrate particles from the moving ship hydrate discharge. The photosynthesis by terrestrial plants, largest carbon sinks, can augment the carbon sequestration by (1) the increase of forest growth on existing and unmanaged all available lands in the tropical and other regions, (2) the increase of forest or grassland in arid and semiarid regions, desert, semi desert and savanna areas, (3) afforestation of marginal agricultural land, (4) development of forestry or grassland in degraded mining areas, (5) cultivating urban and residential turf grass, (6) preserving forestry in polar region, (7) soil restoration and soil management practices for regenerative agriculture, grassland and pastureland, (8) agroforestry practices and growing energy crops on spare lands. The artificial engineered techniques can be used for carbon sequestration by energy-efficient and energy-capturing building construction, geopolymer cement, capture and storage of industrial carbon emissions, engineered underground geological sequestration along with mineral carbonation of CO₂ (e.g., in deep ocean, geological strata, declining oil field, saline aquifer, and unminable coal seam), and construction of landfills and safe-guard wetlands. Engineering techniques are expensive and have leakage risks. In comparison, natural biotic techniques are cost-effective processes and have numerous ancillary benefits. Although all the possible strategies for the carbon sequestration are implemented, it would be difficult to reduce the concentrations of GHGs from atmosphere and mitigate the effects of climatic change, unless the viable strategies are adopted in anthropogenic activities for (1) the lowering of CO₂ emissions from energy, fossil fuel combustion, process industry, degrading soil cultivation, land-use change, deforestation, biomass burning, draining of wetlands etc, (2) reducing the global energy use, (3) off-setting CO₂ emissions and (4) developing low or no-carbon fuel from bio-diesel and renewable energies of solar, wind and hydraulic power.

Increasing concentration of green house gases in the atmosphere has already resulted in chaotic weather systems that will have major effects on the coastal ecosystem, and drought and salinity will escalate. Of all the green house gases, the major culprits being CO₂ and methane. To bring down the CO₂ levels in the atmosphere, spreading of green cover seems to be one of the biological remedies to mitigate the threatening effects of global warming. Photosynthetically four kinds of plants can be recognized such as C3, C4, CAM and C3-C4 intermediates plants, of which C4 is an efficient fixer of atmospheric CO₂. In addition, attempts are being made to introduce C4 genes into C3 plants (C3 plants harboring photosynthetic genes such as Phospho-enolpyruvate carboxylase (PEPCase) and pyruvate orthophosphate dikinase (PPdK)) to make them more adoptable to drought, salinity and high temperature the major environmental effects of global warming. Exploitation of the coastal regions for cultivation of many types of seaweeds as vegetables for human consumption, Phytoplankton enrichment both in saline waters of oceanic environment and fresh water will lead to further CO₂ depletion in the atmosphere. CAM plants in particular to some extent can take care of respired out CO₂ at night.

In the last decade, the energy sector has suffered several changes related not only with the decrease of conventional hydrocarbons reserves, i.e. oil and associated natural gas, but also and especially with restrictions imposed by mechanisms in the scope of sustainable environment. The energy dependency is one of the major problems that all countries must deal nowadays, and all the international energy bodies agree that it will be impossible, to the most part of the countries, to become energetically independent. Some international entities advised either governmental parties, as well as, other energy players to develop strategies in different fields in order to reduce external dependency. Additionally, the sustainable energy plan developed by the European Commission is closely related to sustainable environment and consequently to all policies involved in reducing the greenhouse gases effect. In this perspective, and knowing that nowadays it is not yet possible to displace fossil fuels from the energy scenario, it is pertinent to apply new technologies, such as CCS (carbon capture and storage) technologies. One of the current main objectives in CCS technologies deals specifically with CO₂ geological storage/sequestration, mostly in depleted oil and gas reservoirs, saline aquifers and in unminable coal seams, the later taking into account the so-called hydrocarbons (CBM) enhanced production.
This paper deals with the study of different coal samples in what concerns their storage and gas circulation capacities. In fact, both processes are highly dependent on physical and chemical properties of coal, which are intimately related to its genetic conditions. As a matter of fact, to understand the mechanisms involved in coal formation process, it is crucial to study in detail the deposition environmental conditions, as well as, the incarbonization process. Moreover, the coal organic components evolution is also directly related to both deposition conditions and the incarbonization process. In terms of petrographic parameters and besides the inorganic components (mineral matter content), the organic compounds of coal correspond to three quite different maceral groups (vitrinite, inertinite and liptinite) and the incarbonization stage, or rank, can be determined by the mean random vitrinite reflectance. All these parameters strongly influence the storage and the gas circulation capacities of a coal, since they can change the pore sizes, as well as, the porous structure organization and therefore the internal surface area.

Storage of CO₂ as hydrates in sub-seabed sediments has been pointed out as an alternative solution for the geological storage of CO₂, particularly suitable for offshore areas where large ocean depths and low temperatures are reached at short distance from the onshore. This is the case for Portugal, where the continental shelf can be as narrow as 10 km. This paper presents the works conducted to identify the CO₂ Hydrates Stability Zone, i.e. the areas where CO₂ hydrates may form and remain stable, in the deep offshore of Portugal. The methodology adopted involved building maps of geothermal gradient, temperature of water, detailed bathymetry and conversion to hydrostatic pressure, and maps of sediment thickness. These maps were integrated in a GIS environment and a Fortran code was implemented to compute the thickness of the Hydrate Stability Zone, based on the pressure and the temperature conditions on the sub-seabed sediments. Preferential areas, where further studies should be conducted, were defined based on thickness and thickness variation of the Stability Zone, depth of the water column and distance from the main harbours.

Since carbonates are at a lower energy state than free CO₂, storage through carbonation of silicate rocks is thermodynamically favoured and proceeds spontaneously by releasing heat. In an in-situ CO₂ injection site, the heat released in these exothermic reactions can be exploited in a geothermal plant, effectively contributing towards the economic viability of the storage process.
Our calculations suggest the possibility of generating up to about 65 TWh of electrical energy while capturing permanently about 1Gton CO₂ per 1 km3 of peridotite or basalt rock. That broadly corresponds to exploring an electric power plant having up to 150 MW gross output during a period of 50 years.
These results show that geothermal energy and CO₂ storage, often portrayed as conflicting uses of the subsurface, can actually work together, enhancing the economic feasibility of each other in case mafic and/or ultramafic rock formations are used as reservoirs.