“The planet is doing us a favor by taking up our excess carbon emissions,” affirms Caltech professor Christian Frankenberg. But how does the Earth accomplish that, and how much longer will the Earth take to absorb carbon emissions emitted by human activities? Those are the questions at the center of revolutionary research revolutionizing our understanding of carbon sequestration.
Over the past 150 years, the human species has emitted more than 2,000 gigatons of carbon dioxide (CO2) into the air and increased its concentration 50% over pre industrial levels. The majority of it stays in the atmosphere warming the planet, but almost one third gets taken up by the earth based systems in so called “land sink.” The process is critical to the carbon accounting of the planet, but how effectively it can be done over the long term has been a mystery.
Recent studies have shown that the majority of the carbon in terrestrial ecosystems is not in vegetation or trees but in dead pools such as sediments and soils. This matters because the carbon in these passive reservoirs can be sequestered for decades, centuries, or millennia while the transient storage in leaves or other perishable short lived plant material cannot. Caltech geobiology professor Woody Fischer has this to say “There’s this fundamental question If we’re putting more CO2 into the air, is it promoting the growth of more trees and plants? We know one third of that CO2 goes back into the land. Where specifically is it going?”
The answer is in the intricate carbon allocation patterns of ecosystems. Trees, for example, use excess CO2 to grow leaves, roots, or bark, each with a different effect on carbon permanence. Leaves are transient and emit carbon back into the air, while roots store carbon into the ground, where it can persist for hundreds to thousands of years. Such a sophisticated understanding contradicts broad assumptions that forests are the largest carbon sinks. To be true, recent studies have revealed that forests are only getting a negligible percentage of the earth’s carbon uptake—a paltry 6%—while sediments and soils do infinitely better. And human activities complicate it all the more.
The decomposition of wood and paper trash in landfills is a good case in point as organic matter has become monolithic carbon sink around the world. While the process is imperfect, decomposition of organic waste in landfills is found to release methane, a stronger greenhouse gas whose global warming potential is several orders of magnitude greater than CO2. Similarly, damming and building retention ponds are found to enhance carbon sequestration because these suppress anaerobic decomposition. These findings are a warning regarding the fine line between natural sink carbon utilization and evading undesirable side reactions. One of Caltech’s most surprising findings by their team, headed by Yinon Bar On, is the rate of acceleration in carbon sequestration in terrestrial ecosystems.
Since nearly three decades of ground measurements and satellite observation, scientists have realized that carbon sequestration has risen from 0.5 gigatons annually in early 1990s to an astronomical 1.7 gigatons annually in recent years. Acceleration has principally resulted from the increase in nonliving sinks like sediments and soil organic matter, and not from plants. But though the planet temporarily halts the increase by holding excess carbon, the health of the sinks themselves is compromised.
Disruptions like fire, drought, and deforestation caused by global warming are destroying forests’ capacity to act as long term carbon sinks. In places like the Amazon Rainforest, deforestation and changes in rain are already compromising carbon sequestration when concerns are rising that this precious system will begin as a carbon source in the upcoming decade. All these implications cause land use practice and climate policy to have a cataclysmic impact. As MIT environmental engineering professor César Terrer has pointed out, “While plants are indeed absorbing more CO2 and buying us some time in the fight against climate change, it’s crucial to remember that they can only sequester a fraction of human emissions.” This highlights the necessity for a broad strategy that addresses both living and nonliving carbon pools in mitigation.
There are new technologies being developed to upscale the capacity to sequester nonliving sinks. For example, Graphyte and companies like these are encapsulating biomass waste in bricks that are sealed with polymers so that they will not decay and will sequester carbon for thousand of years. Likewise, work on the use of subsoil sees the potential of underground layers of soil that are mineral rich and capable of stabilizing organic carbon.
Controlling the complex dynamics of carbon storage is no experiment it’s a duty. Environmentalists and policy makers need to reconsider saving and restoring healthy ecosystems for maximum sequestration of carbon. Whether it’s sustainable forest management or soil science, the future requires policy making at the highest level coupled with evidence based decision making.
As Fischer so eloquently states, “What are the limits of the land sink to take up excess CO2?” The solution to this will determine the fate of greenhouse gas mitigation and the resilience of the natural systems that support our world. With the revelation of the mysteries of Earth’s secret carbon vaults, scientists are breaking new ground for innovative solutions in concert with the gravity of the climate crisis.
