Addressing the Grand Challenge of atmospheric carbon dioxide: geologic sequestration vs. biological recycling

Human activities are impacting the natural world at a global scale leading to accelerating non-linear system response. Among the Greatest Engineering Achievements of the 20th Century, electrification captured the number one spot, followed by two transportation methods; the automobile and the airplane [1]. While these technologies clearly have irreversibly changed the way we conduct our lives on a daily basis in a positive sense, it has been made possible only through the combustion of cheap, carbon-based fuels.Unfortunately, extraction and utilization of fossil fuels have changed the environmental landscape. From the air we breathe, to the water we drink and the land we rely upon for all of our material resources, changes (some might say irreversible) to our global ecosystems have been documented that suggest future generations cannot continue "life as we know it". Entire libraries could be filled with publications that have been dedicated to documenting the physical, chemical and biological impacts that by-product emissions have had on global climates and ecosystems. As a call to action in response to these and other critical issues facing the global community, the National Academy of Engineering unveiled their list of Grand Challenges for Engineering, including the management of carbon in our atmosphere [2].A few comprehensive reviews have cited hundreds of scientific reports for those interested in understanding historic and current trends and impacts [3-5]. Carbon dioxide (CO2) measurements in the atmosphere have increased from 280 ppm in pre-industrial years to almost 390 ppm today [6], and the vast majority of stored CO2 is currently sequestered in the deep oceans [7]. Figure 1 is a simplified representation of the global carbon cycle where the numbers in parentheses are estimates of the primary carbon reservoirs in gigatons carbon (Gton C = billion metric tons of carbon). Natural fluxes are shown in the diagram as yellow numbers, while human contributions are