Since the beginning of the Industrial Revolution, we have pumped nearly 2,000 billion tonnes of carbon dioxide into the atmosphere. Moreover, in April of this year, the average concentration of carbon dioxide reached a dubious milestone—410 parts per million—according to data recorded at the Mauna Loa Observatory in Hawaii. Carbon dioxide hasn't been this high in millions of years.
Arguably, such a great accumulation of carbon dioxide and the threat it poses to humanity evoke feelings of helplessness and despair. And the worrisome part is that the rising trend of accumulation is not expected to change anytime soon because our dependence on fossil fuels is not showing any signs of easing off, let alone going down. Besides, the carbon dioxide that is emitted today will remain in the atmosphere for thousands of years trapping additional climate-changing heat. Consequently, leading climate scientists believe that the average global temperature will increase by more than three degrees Celsius in the next 100 years or so. If indeed that's the scenario, it would rip apart the fabric of our civilisation.
As discussed in an op-ed piece published in this newspaper on November 29, 2018, a few ground-based geo-engineering technologies have been developed and deployed on an experimental basis to combat global warming by removing carbon dioxide from the atmosphere, as well as preventing it from entering the atmosphere. Three such technologies are Direct Air Capture (DAC), Carbon Capture and Storage (CCS) and Bioenergy with Carbon Capture and Sequestration (BECCS).
Before a large-scale deployment of these technologies, the big question that needs to be addressed is whether they can keep the global temperature rise under two degrees Celsius by the end of this century, as agreed upon by the stakeholders at the non-binding 2015 Paris Climate Accord. Also, what are their pros and cons?
Carbon dioxide is a relatively small part of the atmosphere—about 0.04 percent. Hence, DAC will work effectively only in the vicinity of power plants and factories where carbon dioxide is emitted in copious amounts. To capture carbon dioxide away from the sources of emission, DAC facilities would have to be rolled out on a vast scale.
An area of major concern with large-scale DAC installations is energy efficiency. It takes far more energy and resources to remove carbon dioxide that is so sparsely distributed throughout the atmosphere. In a report prepared by the American Physical Society in 2011, it is estimated that to extract a billion tonne of carbon dioxide, a figure viewed by many experts as climatically significant, maximally efficient DAC systems would require about 10,000 MW of power. To put this in perspective, this is more than the net capacity of the largest nuclear power plant in the world.
In order for DAC to be feasible as a negative emission technology, it is crucial that the amount of carbon dioxide removed from the atmosphere should be appreciably greater than the amount emitted. Otherwise, it would get snared in a continuous game of catch-up with the voluminous output of carbon dioxide.
Since the extracted carbon dioxide is stored underground, it raises concerns over possible leaks which may bring about greater atmosphere warming undoing any of the gains from capturing carbon dioxide in the first place. If leaked into groundwater aquifers, it could make the latter unsafe for drinking. Some geologists worry that a leak could even trigger minor earthquakes. The storage sites would, therefore, require long-term monitoring to ensure their stability.
Nevertheless, DAC is a promising technology because rather than being attached to a single source of carbon dioxide, it can be built and deployed anywhere in the world, including areas where the environment is otherwise unsuitable for agriculture, human habitation or other natural carbon dioxide mitigation efforts such as reforestation and afforestation.
Unlike DAC, CCS and BECCS are not negative emission technologies. Since they capture and store carbon dioxide at the source before it enters the atmosphere, they are considered to be zero emission technologies. However, in industries where CCS technology is used, it cannot capture 100 percent of the carbon dioxide. As of today, CCS reduces emissions by about 90 percent.
As for BECCS, there are sustainability issues for it involves biomass together with carbon capture and storage, all of which involve many sub-systems. They span from biomass availability and storage capacities to water use. Another major issue with BECCS is that there are carbon dioxide emissions associated with the growing, harvesting and transporting of biomass. Additionally, it may involve cutting down forests, a big carbon sink, in favour of other forms of biomass. Both CCS and BECCS face the same storage issues as DAC.
Compared to DAC which is believed to have a high mitigation cost, the pathway of BECCS has reasonable costs. But the competition for land to grow biomass may limit the large-scale deployment of this technology.
Although it seems to be a valid strategy to use the aforementioned technologies to reduce the concentration of carbon dioxide in the atmosphere, they cannot be considered a panacea to the problems arising from the emission of greenhouse gases. In some ways, they come across as a band-aid solution. Furthermore, they would be slow to reduce climate risks, requiring decades to make an appreciable impact on the atmospheric concentration of carbon dioxide, particularly if we continue to move in the wrong direction, which is burn fossil fuel at a prodigious rate. Analysis of climate data indicates that in 2018 we are on pace to release a record-breaking 37 billion tonnes of carbon dioxide, with China, US, EU and India being the main culprits. If such huge annual emissions continue unabated, it would be a no-win situation for DAC. That being the case, negative and zero emission technologies won't work at the levels required to compensate for mitigation of global warming.
Finally, while the guardians of our planet are delivering recycled speeches at various rambunctious conferences, the latest one at COP24 in Katowice, reminding us about what we already know but failing to understand and act to prevent the calamitous effects of climate change, scientists are working tirelessly behind the scenes on a number of bold new ideas, both ground-based and space-based. As an example of ground-based technology, physicists at Columbia University in New York are working on synthetic trees with materials called sorbents that would absorb carbon dioxide from the air. Most of the space-based technologies are concerned with solar radiation management, with the aim of preventing some of the sun's rays from ever reaching the earth's biosphere.
Quamrul Haider is a Professor of Physics at Fordham University, New York.