For the past fifteen or so years, there have been concerted global efforts to reduce the concentrations in the atmosphere of the so-called greenhouse gases, since they contribute to the greenhouse effect by absorbing solar radiation that have bounced off the earth’s surface and emitting this back to earth, leading to relative global warming. Without this, radiation incident on the earth surface would escape into space. Examples of greenhouse gases include water vapor, carbon dioxide, methane, nitrous oxide, ozone, chlorofluorocarbons (CFCs), and hydrofluorocarbons (such as HCFCs and HFCs).
The relative amount of the emission of the gases by the US in 2016 is as follows: carbon dioxide (81%), methane (10%), nitrous oxide (6%) and fluorinated gases (3%). (Note that although water vapor is more abundant than these other species, it is not introduced into the atmosphere by humans.) Also note that without greenhouse gases the average temperature of the surface of the earth would be approximately equal to −18°C (0°F), as opposed to the normal average of 15°C (59°F).
Carbon dioxide enters the atmosphere through the burning of fossil fuels such as coal, natural gas, and oil; as well as from solid waste, trees and wood products, and also as a result of certain chemical reactions (e.g., manufacture of cement). Carbon dioxide is removed from the atmosphere when it is consumed by plants as part of photosynthesis. Methane is also emitted during the production of coal, natural gas, and oil. In addition, methane emissions result from livestock and other agricultural practices and by the decay of organic waste in municipal solid waste landfills. Nitrous oxide is emitted during agricultural and industrial activities, and during the combustion of fossil fuels and solid waste. Hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride are synthetic, powerful greenhouse gases that are emitted from a variety of industrial processes.
According to a Wikipedia entry, human activities since the beginning of the Industrial Revolution (around 1750) have produced a 40% increase in the atmospheric concentration of carbon dioxide, from 280 parts per million (ppm) in 1750 to 406 ppm in early 2017. This increase has occurred despite the uptake of more than half of the emissions by various natural “sinks” involved in the carbon cycle. The vast majority of carbon dioxide emissions come from the combustion of fossil fuels, with additional contributions coming from deforestation, changes in land use, soil erosion and agriculture (including livestock).
What is important in this article is to note that the failure to keep the concentration of carbon dioxide in the atmosphere in check will lead to ever increasing global temperatures, which will threaten the existence of life on earth. So, it is imperative for us to control not just the increase of the concentration of carbon dioxide in the atmosphere, but to also remove a significant portion of what is already present.
The Paris Agreement of 2015 – signed by 175 countries in New York in 2016 – aims to strengthen the global response to the threat of climate change by keeping a global temperature rise this century to well below 2 degrees Celsius above pre-industrial levels and to pursue efforts to limit the temperature increase even further to 1.5 degrees Celsius. However, at current carbon dioxide emission rates, experts have recently reported that temperatures could increase beyond 2 °C, which the United Nations’ Intergovernmental Panel on Climate Change (IPCC) designated as the upper limit to avoid dangerous levels by 2036.
Rchard Conniff in the January 2019 issue of the Scientific American magazine goes into some details on the costs and effectiveness of the various strategies that are being investigated for carbon capturing. These techniques include bioenergy carbon capture and storage, biochar, enhanced weathering, direct air capture, ocean fertilization, and soil carbon sequestration. To illustrate two of these techniques, direct air capture uses machines to pull in ambient air, chemically separating out carbon dioxide and pumping it underground for permanent storage. In enhanced weathering, rock is pulverized into dust, which can draw carbon dioxide from the air to fertilize soil when spread on fields. When the dust is sprinkled on the ocean, it reacts with seawater to convert carbon dioxide into carbonates which settle on the ocean floor.
No satisfactory solutions have yet been found among the seven carbon dioxide capturing techniques discussed. In particular, the potential carbon dioxide that can be removed substantially falls short of the cost of removal. For example, it could be that a particular technique requires too much (green) solar or wind power, sufficient to be put on a grid and enabling the turning off of a coal plant! Moreover, numerous negative side effects have been identified, such as air pollution, biodiversity, food security, ground and water pollution, soil quality problems, potential carbon dioxide leakage, and so on. Also, because some of the methods discussed compete for the same resources, you cannot just add up the potential of the methods.
In general, corporations have been reluctant to invest in carbon dioxide removal technologies since they see no clear business case. That is, they see the whole idea as public (social) benefit.