Are negative carbon emissions possible?

It is remarkable how public awareness of climate change has grown over the past decade and a half. Today, there is a strong international scientific consensus that greenhouse gas emissions from human activities are upsetting the natural atmospheric heat balance, and causing climatic changes that are far faster than those that have occurred in the Earth’s past, especially when measured in terms of geological time scales. The bad news is, despite this growing recognition of what may well be the single biggest environmental crisis facing humanity, very little progress has actually been made in curbing emissions of key greenhouse gases — particularly carbon dioxide (CO2), which accounts for an overwhelming proportion of global warming impacts. Earlier this year, atmospheric CO2 concentration reached 400 ppm, well above the sustainable limit of 350 ppm proposed in a 2009 article in Nature by Rockström and colleagues. Burning of fossil fuels now adds about 30 billion tons per year of CO2 (or about 8 billion tons per year of carbon) to the atmosphere; to make matters worse, rising standards of living throughout the world means that the rate of release is increasing, and shows no signs of slowing down despite well-intentioned efforts to reach a global climate accord. At this rate, it will take only a few decades for the amount of carbon in the atmosphere to reach a trillion tons (from the current level of about 500 billion tons) — at which point climatic effects may prove to be catastrophic and practically irreversible.

A large part of the problem is the strong correlation among economic development, energy use and carbon emissions. The failure of the Kyoto Protocol, in fact, resulted from an impasse among key greenhouse gas emitting countries (most notably China and the US) over issues of equity. Clearly, part of the solution must lie either in decoupling growth from energy use, or, alternatively, decoupling energy use from emissions. These can be achieved through various engineering interventions, including increasing energy efficiency, switching from high- to low-carbon fossil fuels (e.g., from coal to natural gas) or displacing fossil fuels with renewables (e.g., wind, solar, hydroelectric power). In many cases, the technologies needed are already available, and their adoption hinges on economics. In other cases, the solution is still unproven and potentially controversial. A case in point is CO2 capture and storage (CCS), which involves removing CO2 from industrial exhaust gases and injecting it underground for permanent long-term storage. CCS is based on the simple premise that CO2 will only cause climate change if it enters the atmosphere. Thus, a large part of the solution may lie “simply” in developing the infrastructure to dispose of CO2 in the ground. The next decade may prove whether or not this is feasible on a large scale. All these technologies make it possible to reduce CO2 emissions per unit of useful energy produced. Nevertheless, even large-scale use of such low-carbon technologies will only reduce the rate of increase of atmospheric CO2 levels; the actual carbon stock will still be increasing. For as long as fossil fuels are burned, carbon will continue to be transferred from the ground into the air, as a matter of simple mass conservation laws. In fact, even if CO2 emissions were to somehow cut to zero overnight, the half a trillion tons of carbon already in the atmosphere would still be problematic.

A better solution may lie in combining low-carbon technologies into a fundamentally negative-carbon system. While this may sound like wishful thinking, consider an energy system where biomass (e.g., wood) is grown in sustainable plantations where trees are planted, cut down for fuel, and then replanted to ensure an indefinite energy supply (since the wood regrows to make up for the harvest). The wood, in turn, is burned as fuel to run steam turbines, which run generators to make electricity. In effect, such a biomass-based system acts like a giant solar cell, harnessing photosynthesis to turn sunlight into chemical energy (in the wood), and then that into useful energy (electricity). In addition, it is easy to figure out that the CO2 emissions from burning the wood are reabsorbed by the plantation as the trees regrow at just the right rate to supply the fuel requirements. Carbon fixation balances out the carbon emissions, and the system is said to be “carbon neutral” because the carbon just cycles back and forth between photosynthesis and combustion. Now, consider what happens if the wood-fired power plant is equipped with CO2 capture equipment. The CO2 from the burned wood, instead of re-entering the atmosphere, is now pumped away into some naturally occurring underground geological reservoir. This combination, known as bioenergy with CCS (BECCS), appears to do the impossible: it produces useful energy, while effectively transferring carbon from the atmosphere (via photosynthesis) into the ground (via CCS). Note that this direction of flow is the reverse of what happens in normal systems; it thus achieves negative carbon emissions!

The concept of negative emissions may seem like science fiction, but many of the various pieces of the technological puzzle are surprisingly well-developed. The CCS part may be the least mature at present. Even then, many low-tech alternatives are also potentially capable of achieving the crucial reverse, air-to-ground flow of carbon. Consider, for example, that when powdered charcoal is mixed into soil as a conditioner, the same effect is achieved (I leave it to the reader to work out how this happens, by following the same chain of logical deduction described in the previous paragraph); such a process is known as biochar-based carbon sequestration, and a couple of my graduate students are now doing research on how such schemes can be made integral to complex energy systems. Thus, negative carbon emissions are indeed possible to achieve for some man-made systems, at least in principle. The catch is that such reverse flows need to take place at a massive scale in order to make any significant impact on global emissions; and that, as many people now recognize, time is fast running out.

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Raymond R. Tan is a professor of chemical engineering, university fellow and current vice chancellor for research and innovation at De La Salle University. His main areas of research are process systems engineering and process integration. He received his BS and MS in chemical engineering and Ph.D. in mechanical engineering from De La Salle University. He is the author of more than 80 published and forthcoming articles in ISI-indexed journals in the fields of chemical, environmental and energy engineering. He currently has over 100 publications listed in Scopus with an h-index of 23, and is a member of the editorial board of the journal “Clean Technologies and Environmental Policy” (Springer) and is editor of the book “Recent Advances in Sustainable Process Design and Optimization” (World Scientific). He is also the recipient of multiple awards from the National Academy of Science and Technology and the National Research Council of the Philippines as well as commendations for three highly cited papers in Institution of Chemical Engineers (IChemE) journals. E-mail him at [email protected].