By Land and by Sea, Harnessing Natural Carbon Sinks to Battle Climate Change

Written by: Adin Richards
Edited by: Elizabeth Zhang

The goals and commitments of the 2016 Paris Climate Agreement have been lauded as monumental first steps in rallying a global effort to address climate change and mitigate its disastrous effects [1]. Yet, all available indications about the effects of greenhouse gas emissions point to a grim reality: even if world leaders were to fulfill every commitment laid down in the accords, it would not be enough to keep warming below the goal of two degrees celsius over the next century [2]. Under these dire circumstances, new life has been breathed into the once distant prospect of geoengineering. ​

Geoengineering is an umbrella term that encompasses any large scale scheme geared toward deliberately altering the systems which govern our planet’s climate [3]. Despite their large variety, geoengineering techniques fall under the two main approaches of carbon dioxide removal (CDR) and solar radiation management (SRM) [4]. A planet’s temperature is dictated by three factors: 1) the amount of solar energy it receives, 2) the amount of that solar energy which is reflected back by its surface and atmosphere, and 3) the amount that is subsequently stored in the atmosphere by greenhouse gases [5]. Schemes to adjust the first two factors fall under an SRM approach, while CDR strives to alter the third. 

Probably the most well known geoengineering proposal—spraying the atmosphere with reflective sulfur aerosols—is an SRM technique. The similar scheme of introducing particles that induce cloud formation (“cloud seeding”) also falls under the SRM approach, since both of these increase the amount of incoming solar radiation that is reflected back to space by our atmosphere [6]. Although the effects of implementing such proposals are not fully known, major concerns include their potential to endanger crop production and alter global precipitation patterns. Mechanical techniques to reduce atmospheric greenhouse gases such as CO2 avoid these concerns, but can only mitigate emissions from specific sources such as power plants rather than global levels of CO2 [7]. In order to tackle this limitation, we need a separate class of schemes which make use of our planet’s existing thermostatic infrastructure: plants. 

At the most modest level, efforts to remove CO2 from the atmosphere can consist simply of large scale tree plantings [8]. Although a valuable tool in keeping global CO2 levels in check, existing forest biomass stores just a little over a third of the amount of carbon in the atmosphere. Soils, however, contain more carbon than all of our planet’s biomass and the atmosphere combined [9]. This makes the ground beneath our feet a potent means of mitigating greenhouse gas emissions. 

Not all soils have an equal capacity to store carbon. Underlying our iconic forests is a vital partnership between trees and nitrogen fixing mycorrhizal fungi, organisms which are responsible for the variation in soil carbon storage potential. Certain lineages of trees are associated with different types of nitrogen-fixing fungi. For example, while ectomycorrhizal mycorrhizal fungi excrete a chemical which harms other nitrogen fixing organisms, arbuscular fungi do not. Moreover, other nitrogen fixing organisms are largely bacterial autotrophs which are not as effective at storing carbon in soil. As a result, where ectomycorrhizal fungi dominate, more carbon is sequestered [10]. This difference provides an opportunity to take advantage of and promote natural stores of CO2 in our soil. 

Increasing soil carbon sequestration is one of the several bio-based approaches to climate mitigation involving terrestrial ecosystem management, but efforts do not need to be limited to this domain. In fact, oceans have far greater biological reservoirs of carbon than those on land [11]. Much of the ocean’s absorption of atmospheric carbon results in ecologically damaging acidification. However, a second mechanism of ocean carbon intake known as the biological pump involves the incorporation of carbon into organic matter. This organic matter is then subsequently transported to the deep ocean where it can be stored for centuries to millions of years [12, 13]. 

One proposal to increase the uptake of carbon by ocean organisms involves fertilizing environments with iron, a key nutrient for many marine primary producers [14]. Nevertheless, this could be problematic because providing just one key nutrient could lead to algae blooms that deplete other nutrients such as silica, nitrate, and phosphate [15]. One way to avoid this issue is to transfer nutrient-poor water concentrated deeper in the ocean up to the surface where more sunlight is available for photosynthesis [16]. This artificial upwelling would promote algal growth without altering ocean chemistry, and is famously supported by James Lovecraft, the originator of the Gaia hypothesis that promotes an integrated view of Earth and living systems as dynamically intertwined [17, 18].

​Enthusiasm for climate interventions has always been dampened by the precautionary principle, which requires a scientific consensus on all the ramifications of an action before it is taken [19]. However, as opportunities to mitigate climate change through preventative measures dwindle, more interventionary measures have continually been put forth. What has been presented here are some of the more modest proposals. For example, by using already existing biological carbon sinks, many of the more contentious dilemmas that have plagued classic geoengineering schemes can be sidestepped. Nevertheless, at present humans are the dominant force on Earth, making null the question of whether we ought to intervene in our climate. The real question is, should we use our knowledge of geoengineering to take the reins and actively alter biological systems to steer Earth in the direction we want?

Works Cited: 
[1] The United Nations Framework Convention on Climate Change. The Paris Agreement [Internet] [Cited 2020 Feb. 20] Available from: https://unfccc.int/process-and-meetings/the-paris-agreement/the-paris-agreement

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[11] Frape D. The functions and sizes of the five carbon sinks on planet Earth and their relation to climate change Part I Their present sizes and locations. 2016. [Cited 2020 Feb. 20] Available from: http://www.world-agriculture.net/article/the-functions-and-sizes-of-the-five-carbon-sinks-on-planet-earth-and-their-relation-to-climate-change-part-i-their-present-sizes

[12] University of California, San Diego. The Biological Carbon Pump. 2020. [Cited 2020 Feb. 20] Available from: http://earthguide.ucsd.edu/virtualmuseum/climatechange1/06_2.shtml

[13] House KZ, Schrag DP, Harvey CF, Lackner KS. Permanent carbon dioxide storage in deep-sea sediments. 2006. [Cited 2020 Feb. 20] Available from: https://www.pnas.org/content/103/33/12291

[14] Boyd PW, Jickells T, Law CS, Blain S, Boyle EA, Buesseler KO, et al. Mesoscale Iron Enrichment Experiments 1993-2005: Synthesis and Future Directions. 2007. [Cited 2020 Feb. 20] Available from: https://science.sciencemag.org/content/315/5812/612.full

[15] Powell H. Fertilizing the Ocean with Iron. 2007. [Cited 2020 Feb. 20] Available from: https://www.whoi.edu/oceanus/feature/fertilizing-the-ocean-with-iron/

[16] Yiwen P, Fan W, Huang T-H, Wang S-L, C-T A Chen. Evaluation of the sinks and sources of atmospheric CO2 by artificial upwelling. 2014. [Cited 2020 Feb. 20] Available from: https://www-sciencedirect-com.revproxy.brown.edu/science/article/pii/S0048969714016544

[17] Lovelock JE, Rapley CG. Ocean pipes could help the Earth to cure itself. 2007. [Cited 2020 Feb. 20] Available from: https://www.nature.com/articles/449403a?error=cookies_not_supported&code=fcec3246-e6df-4057-b960-e56dbbba0a93


​[18] Gaia Theory. The Gaia Theory offers insights into climate change, energy, health, agriculture, and other issues of great importance. 2020. [Cited 2020 Feb. 20] Available from: http://www.gaiatheory.org/

​[19] Kriebel D, Tickner J, Epstein P, Lemons J, Levins R. The precautionary principle in environmental science. 2001. [Cited 2020 Feb. 20] Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1240435/

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