Climate change is caused by global warming as a result of trapping of the heat in the earth’s atmosphere caused by the absorption of infrared radiation by greenhouse gas concentration present in the atmosphere. As per the Kyoto protocol, the greenhouse gases are carbon dioxide, methane, nitrous oxide, and the fluorinated gases such as hydrofluorocarbons (UNFCC 2008).
The first conference on world climate was held in Geneva in 1979. It acknowledged climate change realities. In 1988, the Intergovernmental Panel on Climate Change (IPCC) was established to advise governments and official bodies on the scientific aspects of climate change necessary to bring forth climate policies (Hakala 2021). The adoption of the United Nations Framework Convention on Climate Change (UNFCCC) in 1992 was an important step. Climate modelling established that to confine global warming to within 1.5 °C temperature rise, a 45% reduction in anthropogenic greenhouse gas emissions was mandatory by 2030 as compared to 2010 levels. Added to this, emissions should be brought down to zero levels by 2050.
Achieving net zero emission demand negative emissions technologies (NET) for capturing and sequestering CO2 from the atmosphere. Carbon capture and storage (CCS) captures CO2 emitted by fossil fuel usage and stores it in geological reservoirs like dried up oil and gas fields, coal beds and sub-surface aquifers for long periods. Technologies for CCS and utilization are yet to be proven.
Bioenergy carbon capture and storage is another NET. Growing biomass absorbs atmospheric CO2 through photosynthesis. Its combustion is a process for energy production. The sources of biomass used for this can be waste from either agriculture or forests or even energy crops. The biomass feedstocks required would be in direct competition with food and feed crops (UNFCC2008). This is an issue.
Growing trees build up their bulk by capturing CO2 from the atmosphere. Forestation is thus a biogenic NET. Forestation can mean new forests, or re-establishing degraded forest areas. New forests promote CO2 uptake for 20–100 years until trees reach maturity. Demand for land will compete with alternate land use.
Biomass, when pyrolysed, produce Biochar. The carbon present in biomass is processed into a char that can be applied to soils. The carbon in this form resist decomposition and hence is very stable. Excessive biochar in the soil leads to decreasing reflectivity and increasing soil temperature, thus countering the benefit of carbon storage realised this way.
In direct air carbon capture and storage (DACCS), CO2 is captured from the air by flowing ambient air through chemicals which have an affinity for CO2. Heating the sorbents release CO2, which can be converted chemicals or mineral carbonates. A key issue is the significant energy required by DACCS plants. However, the plants are flexible and can be located at sites where low-carbon energy and adequate infrastructure is available. Many processes are currently being developed at laboratory-scale or pilot-scale phases. The global potential for carbon dioxide removal has been estimated to be in the range of 0.5–5 GtCO2/year by 2050 (Fawzy 2020).
Adding minerals and iron to the sea water makes phytoplankton to grow which can stimulate CO2 absorption by oceans. Phytoplankton is a microscopic organism, found at the surface layer of oceans. Ocean fertilization can potentially sequester up to 3.7 GtCO2/year by 2100 with a total global storage capacity of 70–300 GtCO2 (Royal Soc 2018).
The weathering process which disintegrates Silicate rocks consumes atmospheric CO2 and releases metal ions as well as carbonate and/or bicarbonate ions. The dissolved ions, transported through groundwater streams, eventually end up in the ocean where they are stored as alkalinity, or they precipitate in the land system as carbonate minerals. Enhanced weathering is an approach that can accelerate this process to enhance CO2 uptake. This is achieved through milling silicate rocks to increase its reactive surface and enhance its mineral dissolution rate. The ground material is then applied to croplands providing a multitude of co-benefits.
The oceans already absorb a significant amount of atmospheric CO2, which, reacting with water forms carbonic acid, which further dissociates into bicarbonate and carbonate ions. As alkalinity increases, more carbonic acid is converted to bicarbonate and carbonate ions and greater amounts of carbon are stored in inorganic form.
Wetlands are high carbon density ecosystems that facilitate atmospheric carbon sequestration through photosynthesis and subsequent storage in biomass as well as in soil. While peatlands and coastal wetlands are estimated to store between 44 and 71% of the world’s terrestrial biological carbon, they are vulnerable to deterioration due to habitat degradation. A major drawback is the substantial emissions of greenhouse gases such as CH4 and N2O associated with wetland habitats.
The utilization of biomass materials in construction aids carbon to be stored for decades in the built environment. Replacing conventional building materials such as steel and cement help emission reduction since these are made by carbon-intensive processes. Estimates of 14–31% reduction in global CO2 emissions and 12–19% reduction in global fossil fuel consumption can be realized through this approach.
Radiative forcing geo-engineering techniques are a set of technologies that aim to alter the earth’s radiative energy budget to reduce global temperatures. This is achieved by either increasing the earth’s reflectivity by increasing shortwave solar radiation that is reflected to space, or by enhancing long wave radiation that is emitted by the earth’s surfaces to space. Stratospheric aerosol injection aims to mimic the cooling effect caused by the volcanic eruption by artificially injecting reflecting aerosol particles in the stratosphere.
Marine sky brightening, also known as marine cloud bright- ening aims to reduce global temperatures by enhancing cloud reflectivity. This is achieved through cloud seeding with seawater particles or with chemicals. The potential cooling effect has been estimated between 0.8 and 5.4 W/m2.
Space-based mirrors that aims to reflect part of the incoming solar radiation can reduce global temperatures. Space mirrors or reflectors need to be transported into orbit around the earth or placed at the Lagrangian L1 location between the earth and the sun, where the gravitational fields are in balance. While this approach can have a considerable cooling effect based on model simulations, development of such technology is still at a very infant stage.
References
Fawzy (2020): Fawzy, S., Osman, A.I., Doran, J. et al. Strategies for mitigation of climate change: a review. Environ Chem Lett 18, 2069–2094.
Hakala (2021): Kaija Hakala, Climate change induced challenges for research — a case study of crop production research in Finland Agricultural and Food Science, p. 344, ISSN : 1795–1895
RoyalSociety (2018) Greenhouse gas removal. https://royalsociety.org/-/media/policy/projects/greenhouse-gas-removal/royal-society-greenhouse-gas-removal-report-2018.pdf.
UNFCCC (2008) Kyoto protocol reference manual on accounting of emissions and assigned amount. https://unfccc.int/resource/docs/publications/08_unfccc_kp_ref_manual.pdf. Accessed 22 Dec 2019