Anthropogenic CO2: How Bad Is It?
By Thi Viet Tran
Several decades ago, the concept of global warming was unfamiliar to the general public but today, many recognize it as an urgent environmental problem. A 2017 public survey on what causes global warming revealed that most respondents believed it is due to excessive emission of CO2,. Although that is true, CO2 is not the only greenhouse gas contributor to global warming. Many other gases are also implicated, such as methane (CH4), nitrous oxide (N2O), fluorinated gases (e.g., C2H2F4, CF4, SF6, and NF3), and even water vapor. In terms of their heat absorption capacity per molecule, fluorinated gases trap 23,000 times more heat than CO2, whereas nitrous oxide and methane are 300 and 30 times greater, respectively. This misconception has given CO2 a very bad reputation. However, it's important to remember that CO2 is a vital atmospheric constituent in many natural cycles. For example, it is a carbon source for plants, a climate buffer, and a contributor to the weathering of rocks.
So what exactly is the connection between CO2 and global warming? Although CO2 is a naturally occurring gas, it’s over emission from human activities has exceeded nature’s ability to absorb it. When the sun shines on our planet, most of the energy is absorbed by the surface while some of the heat is radiated back into space. As heat escapes from the surface, it is absorbed by the greenhouse gases in the atmosphere. The greenhouse gases behave like a giant blanket that keeps the earth’s surface and atmosphere temperature stable so that life could thrive below. Unfortunately, when the CO2 concentration in the atmosphere increases beyond normal levels, excessive heat will be trapped inside our planet. A rapid change in temperature over the course of 100 years will be catastrophic to many living species since it typically takes thousands of years for organisms to adapt to their surroundings.
In nature, CO2 is generated from activities such as aerobic respiration, decomposition of organic matter, or environmental disasters (e.g., forest fires and volcanic eruptions) and is absorbed by processes such as photosynthesis in plants, carbonation in the ocean and carbonate formation in weathered rocks. The generation and consumption of CO2 must be well-regulated (~280 parts per million, ppm) to maintain a healthy ecosystem. However, since the beginning of the industrial revolution in the 1830s, human activities have produced massive amounts of CO2 that nature can no longer control. Thus, atmospheric CO2 will continue to increase dramatically (~415 ppm in 2020) unless we take major actions to both curtail emission and absorb some of this excess.
How can we reduce the amount of anthropogenic CO2 in the atmosphere?
Many strategies are being explored to reduce anthropogenic CO2, a few of the most popular ones are listed below.
1) Plant more trees. This option is probably the most attractive to the public for a variety of reasons. Trees not only consume CO2 efficiently but benefit human health, support vital ecosystems and are aesthetically pleasing. Therefore, it is not surprising that many tree-planting campaigns have started around the world. Some of the major ones include the Bonn Challenge, the Trillion Tree Campaign, the One Trillion Tree Initiative, Trees for the Future, Trees Forever, Plant a Billion Trees, and 8 Billion Trees.
2) Reduce CO2 emissions caused by human activities. One of the most impactful ways to minimize our CO2 emission is to reduce reliance on fossil fuels in favor of renewable energy sources such as wind, solar, and hydropower (also known as “clean” or “green” energy). However, it is critical that renewable energy be produced with a net decrease in CO2 emission in comparison to that obtained from hydrocarbons. For example, there can be hidden environmental costs that are not always considered, such as the carbon footprint of transporting, storing, and distributing that particular form of energy.
Many other sectors of society, such as agriculture, transportation, or manufacturing, can take actions to reduce CO2 emissions. For example, Microsoft announced its plan to be carbon negative by 2030. The corporation expanded its internal carbon fee to fund an aggressive program that will reduce the carbon footprint from not only their direct emissions but within their entire supply and value chain as well. In addition, they will support the acceleration and development of carbon reduction and removal technologies by adding $1 billion to the cause. To provide transparency on their progress, they will publish an annual environmental sustainability report. These types of changes are driven by increasing public demand for the companies they purchase from to pursue greener agendas.
3) Develop technologies to convert CO2 into value-added products. Chemists are well equipped to advance this goal. Over the past several decades, numerous synthetic methods have been developed to convert CO2 into useful chemicals.,  A survey of the recent scientific literature suggests that this field has been gaining tremendous popularity. Although CO2 is considered a “green” reagent, the question for chemists is: are the other reagents and solvents used in the synthesis also green?
How do the different strategies compare?
|Plant more trees||Trees use solar energy to convert CO2 and H2O into biomass and O2.||Only basic skills needed, aesthetically pleasing, and has many environmental benefits.||Require genetic engineering to improve photosynthetic yield of trees. Planting more trees requires more land.|
|Reduce CO2 emission||Human activity generates a massive amount of atmospheric CO2 each year.||Many industries can benefit from green technology and energy conservation.||May require special skills and knowledge. Could have high cost of investment and operation.|
|Develop technologies to convert CO2||Incorporate CO2 into inexpensive starting materials to create useful products.||Synthetic yields in large-scale processes can be much greater than that of natural processes.||Require extensive research investment. The reagents and solvents used may not be green.|
Evaluating Our Options
Having more trees is definitely the most popular option since many people will agree that living or working in a green environment can improve both their mental and physical health. In fact, green architecture is appearing all over the world. For example, the Apple corporation has designed its headquarters to look more like a nature refuge rather than an office, with 80% of the campus consisting of green space. However, planting more trees alone will not be enough to address the enormous scale of the CO2 problem we currently face. These types of solutions are also capital and land intensive which makes greening projects more accessible to open suburbs as opposed to the dense, urban centers that most of the population live and work in.
The industrial revolution in the past few centuries has significantly boosted the production of goods, increased personal wealth, and improved standards of living. Unfortunately, industrialization has also had a devastating impact on the environment. During the early years of industrialization, keeping productivity high and production costs low were the top priorities in manufacturing, so, little consideration was given to what damage such activities might do to the planet. Increased population and high demand for material goods led to the emergence of mass production, which generated a massive amount of air, soil, and water pollutants that exceeded nature’s capacity to detoxify. Fortunately, in recent decades, efforts have been made to minimize greenhouse gas emissions (including CO2) in production processes. For example, more than a hundred nations have agreed to the Kyoto Protocol and Paris Agreement, which aim to reduce greenhouse gas emissions by a target date. This commitment will help to reduce future anthropogenic CO2 emissions but not the amount of CO2 already present in the atmosphere. There is also concern due to the large number of nations not on track to actually meet their CO2 reduction goals.
Developing industrial-scale methods to directly convert CO2 into useful materials will be a game-changer. The efficiency of an industrial process has the potential to substantially exceed that obtainable by nature. For example, according to Myers and Goreau, one hectare of a plantation of pine (~1000 trees) can reduce about 73,326 pounds of CO2 per year. If we can develop a chemical process that could consume 10,000 pounds of CO2 per day, it will take only 7 days to do what 1000 trees could do in a year and also save one hectare of land. To make this possible, however, it will require a major investment of research dollars to overcome issues like concentrating atmospheric CO2 and purifying industrial CO2 to meet the needs of CO2 harvesting catalysts.
CO2 in the Chemical Industry
The chemical industry has long suffered a bad public image due to its contribution to environmental pollution. Unfortunately, the use of toxic chemicals is unavoidable in some processes due to limitations in current synthetic methodologies. Chemists have been trying to minimize the creation of hazardous waste by using non-toxic reagents or developing environmentally-friendly processes. The concept of “green chemistry” was developed to encourage chemical producers to design products and processes that eliminate the use or generation of hazardous substances. Although green chemistry has grown rapidly in the last 20 years, there are still many challenging scientific and technological issues to overcome, such as product cost competitiveness.
In terms of CO2 utilization, this chemistry is difficult to develop because CO2 is a very stable molecule. To solve this problem, CO2 coupling methods usually employ additional “activating” reagents or harsh conditions to drive the process. Amazingly, nature can use plants to do what chemists are not able to easily accomplish yet. For example, plants harness sunlight to convert CO2 and H2O into O2 and glucose. This reaction occurs at room temperature using non-toxic substances and only require low CO2 concentrations (300-400 ppm). Although photosynthesis may appear to be a simple solution to the CO2 challenge, it is actually quite complicated. The conversion of CO2 to glucose requires at least five enzymes working together in sequence.
Taking inspiration from nature, our NSF Center for Integrated Catalysis (CIC) aims to produce plastics from CO2 and other readily available starting materials using multi-step chemical catalysis. Although this goal is quite ambitious, we believe that the impact of this work in creating new uses for CO2 is well worth the research effort and investment.
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—Thi V. Tran is a postdoctoral researcher at the University of Houston.