The roles of paleoclimatology in estimating the effects of enhanced weathering
As carbon dioxide concentrations continue to rise in Earth’s atmosphere so does the amount of research concerned with sequestering this greenhouse gas. This is because it is assumed that greenhouse gasses will endanger society’s survival so if people want to live without preventable environmental catastrophe they should care about geoengineering. The scientific problem explored in this paper is whether or not enhanced weathering can effectively remove carbon dioxide from the atmosphere given a limited amount of time and resources without causing other problems or side effects. One geoengineering technique or hypothesis known as enhanced weathering or “stimulated weathering” (Hartmann & Kempe, 2008, p. 1159) stands out since it reportedly not only removes CO₂ from the atmosphere cheaply but it also adds nutrients to soils and waters for living creatures, decreases ocean acidification, and produces clean power. Much is already known regarding the dissolution rates of specific ultramafic rocks in laboratory conditions although research lacks real world or “field” analyses of enhanced weathering when applied to Earth’s dynamic systems (Moosdorf, Renforth, & Hartmann, 2014, p. F). This paper assumed that past weathering rates influenced climate greatly. Also this paper cited paleo-instances that constrained the theoretical potential of enhanced weathering in regards to removing carbon dioxide from the atmosphere in a timely manner. Evidence for this physical science foundation is based on both past precedents in the proxy record and small scale experimental results which supports the theoretical chemistry of weathering. Paleoclimatology offers an alternative to large scale experimentation as the only way to gain insights regarding enhanced weathering techniques acting on the real world before it is applied. This is thanks to evidence from the past which recorded enhanced natural weathering events and its effects on atmospheric CO₂ concentrations and therefore Earth’s climate.
The goal of this paper is to review the literature on enhanced weathering which can be shown by the chemical equations found in [Figure. 1]. More specifically it will highlight when and how the Earth’s geological past was invoked to support the enhanced weathering theory that is it is an important negative feedback that regulates Earth’s climate throughout history (Hartmann et al., 2012, p. 113; Schuiling, 2013, p. 1; Hartmann & Kempe, 2008, p. 1159; Schuiling & Krijgsman, 2006, p. 349; Renforth, 2012, p. 229). Researchers did this by supplying examples that show when natural enhanced weathering occurred in the past and comparing it with the suggested anthropogenic efforts. These earlier natural events were due to natural tectonic and glacial inputs. For example this research looked at relatively early (120,000 years ago) past evidence that represented glacial weathering and its lag times
that affected the climate (Vance, Damon, Teagle, & Foster, 2009, p. 495). Vance et al.’s research suggested that “the pulse of rapid chemical weathering initiated at the last deglaciation (18 kyr ago) has not yet decayed away and rates remain about twice the average for an entire late Quaternary glacial cycle” (p. 494). For another example within the last 65 million years increased weathering is due to glacial “rhythms” and tectonic “trends” the latter include forcings such as the Tibetan Uplift (Zachos, 2001, p. 686) [Figure 6 & 7] which might mimic enhanced weathering efforts since it is assumed both of these natural events accelerated CO₂ transport between carbon reservoirs. Finally the implications of increased comminution efficiencies, optimum particle size, and energy capture will be examined by comparing old crushing methods with new cavitation and sonic methods also with distribute waste heat capture.
Researchers assess the feasibility of enhanced weathering assuming constraints e.g. rock amounts, time and biological side effects. At first some climate scientists were skeptical of enhanced weathering techniques because the amount of rock needed was believed too great considering the amounts needed to have similar effects in the past. They said “realistic maximum net-CO₂-consumption potential is expected to be much smaller than 0.1% of anthropogenic emissions” (Hartmann & Kempe, 2008, p. 1159) and “5.0Gt of olivine per year is an impractical task and we suggest that the maximum achievable haulage would be between 5 and 10% of this figure. This implies that the maximum possible CO₂ uptake by beach reaction of olivine would be between 1.5 and 2.8% of 1990-level emission rates” (Hangx & Spiers, 2009, p. 764). More recent research (post-2009) in the area of enhanced weathering has found that the potential for carbon capture is becoming more realistic and the amount of CO₂ created in the effort is actually relatively small compared to the CO₂ sequestered [Figure 2]. Researchers also framed the amount of rock needed to consume the CO₂ in more bite sized junks. For example it was reported that it only required 1.2 mm of olivine powder spread over the entire surface of the earth to theoretically “remove all the CO₂ currently in the atmosphere” (Schuiling & Krijgsman, 2006, p. 350-351). Perhaps the fear of climate change has ushered in an acceptance stage since an American Geophysical Union paper said “to sequester a significant amount of carbon dioxide from the atmosphere, an Enhanced Weathering program would need to process 1 Gt to 10 s of Gt of rock per year. This would make it one of the largest global industries” (Hartmann et al., 2012, p. Appendix A).
The skepticism, friction or second thoughts now come from the unknown effects of spreading large amounts of ultrafine material on biological and climate systems (Hartmann et al, 2013, p. 117; Renforth, 2012, p. 240; Moosdorf, Renforth, & Hartmann, 2014, p. G) although some say environmental consequences are minimal (Schuiling & Krijgsman, 2006, p. 352). Paleoclimatology could address some of these issues with past precedents if matching natural past and future man-made weathering scenarios can be identified and compared.
The most outstanding proponent of enhanced weathering is Roelof Schuiling who has appeared in the Dutch and United States media (Hangx & Spiers, 2009, p. 757; Fountain, 2014, p. A1 & A6)
expounding the benefits of enhanced weathering including clean power production (Schuiling, 2013, p. 1). Although many other researchers are starting to agree that this carbon dioxide removal (CDR) “technique has certain geoengineering potential” (Köhler, Hartmann, & Wolf-Gladrow, 2010, p. 20231; Moosdorf, Renforth, & Hartmann, 2014, p. F-G). For example models showed “enhanced weathering via the dissolution of fine-grained olivine powder on land in the humid tropics…might sequester up to 1 Pg of C per year directly” (Köhler, Hartmann, & Wolf-Gladrow, 2010, p. 20231). It was found that “if terrestrial enhanced weathering were used to sequester 10% of the 9.1 Gt CO₂—C emitted by fossil fuel combustion and cement production in 2010, 0.9 (1.7) Gt of ultramafic rock material would need to be weathered…for comparison, the estimated present total mass movement by humans in 20-45 Gt a” (Moosdorf, Renforth, & Hartmann, 2014, p. E). Those critical of enhanced weathering found problems with coastal spreading techniques i.e. cold waters and high pH but found terrestrial applications more promising (Hangx & Spiers, 2009, p. 765). Still almost all researchers said more work was needed in areas regarding the side effects for example river alkalization (Köhler, Hartmann, & Wolf-Gladrow, 2010, p. 20232), social impacts, and other environmental consequences. For example dam building has “starved” the oceans of dissolved silicon (DSi) thus making toxic “dinoflagellates” instead of diatoms which affect the carbon pump (Hartmann et al., 2013). This is an area where paleoclimatology could help since one could see past effects of increased weathering on biological systems and at what rate of time the effects are present. Although the effects might be short lived and localized since at “10 um, a complete dissolution within 1-2 years seems possible” (p. 20232). No climate scientist researched argued that the advanced weathering approach would fail because of lack of time to complete the reaction. The fast reaction rate of ultrafine olivine is relatively fast [Figure 3] especially considering the warm places where the spreading of the rock flour is supposed to take place.
The roles of paleoclimatology in enhanced weathering research. Climate scientists focusing on enhanced weathering techniques use paleoclimatology as a cornerstone supporting this potential CDR scheme. For example some said “global biogeochemical cycles have shaped the Earth’s climate and surface environment since the earliest days of the planet. A profound case in point is the consumption of CO₂ during the chemical weathering of silicate rocks that has regulated the global carbon cycle and in so doing Earth’s climate over several eons [Arvidson et al., 2006; Berner, 2004; Kempe and Degens, 1985; Walker et al., 1981] (Hartmann et al., 2013, p. 113). Within the last 65 Ma increased weathering due to the Tibetan uplift has transferred carbon from the atmosphere into the oceans (Zachos et al., 2001). Humans are now planning to mimic this natural phenomena and past lessons regarding weathering rates and effects on climate and biological systems could be used to construct future projections on the possible upcoming results.
The main scientific problem asked at the beginning of this paper was answered because paleoclimatology supports the general idea of enhanced weathering because there is evidence that rocks
have sequestered atmospheric CO₂ thanks to the proxy records [Figures 6-10] and the climate models [Figure 4]. The past helps researchers get a feel for the magnitude of this negative feedback and backs up the idea that it could be used within a suite of differing geoengineering technologies to stop the climate from changing within a few decades of implementation. The time it takes for the reaction to take place is small if we consider the effects of increased surface area [Figure 3]. Furthermore it was found the amount of CO₂ it takes to initiated the enhanced weathering is small compared to the net output of CO₂ sequestered [Figure 2]. This research also found that paleo-instances of increased natural weathering and increased sedimentation harmed biological systems before therefore caution should be taken to make sure that man-made weathering does not do the same thing again (Algeo & Twitchett, 2010). Enhanced weathering should be undertaken right away using crucible like devices (Innovation Concepts, 2014) instead of using the natural environment until further knowledge is gained regarding the effects of increased weathering on biological systems. The question remains whether or not the side effects of heavy metals, other toxins, and river alkalization will be a problem worse than the solution? This was pointed out by almost all the papers on enhanced weathering for example “ecosystem assessments for the expected impacts of the alkalinity rise are necessary before considering implementation” (Köhler, Hartmann, & Wolf-Gladrow, 2010, p. 20232) and “dramatic changes in dissolved matter fluxes with unknown consequences for aquatic ecosystems (e.g., increase in pH or additional release of heavy metals) (Hartmann & Kempe, 2008, p. 1164). Disciplines such as biology should be employed for further understanding regarding enhanced weathering on living creatures.
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(Source: Hartmann & Kempe, 2007, p. 1160)
Figure 1: Scientists have understood the mechanisms for how olivine (Mg₂SiO₄) and other ultramafic rocks sequester CO₂ as they weather since “1845” (Hartmann et al., 2013, p. 115). Energy is released during the reaction and is represented by the equation △Gᵣ= -27 kJ mol –1. The “△Gᵣ is the free energy of reaction (a negative value suggest that the reaction is thermodynamically likely to proceed) (Renforth, 2012, p. 230). Although other researchers cited a higher energy released -239.2 kJ/mol (Innovation Concepts, 2014, Slide 15). This freed energy can be captured by a waste heat steam engine. The Cyclone Waste Heat Engine (WHE) operates at a maximum 600 degree F (Cyclone, 2014) which matches the temperature of a “perfectly isolated” reaction and “a higher end temperature could be reached” (Schuiling, 2013, p. 1). The WHE weighs 18 pounds and produces 15.8 hp (Cyclone, 2014) which could be used to make hydrogen for energy storage or electricity for homes. It would be interesting to see if many distributed modular systems could meet energy requirements of the entire world given the amount of potential energy stored in rocks revealed in [Figure 2]? According to researchers the amount of energy stored in the rocks is very significant. For example an olivine power plant built in the hole of an abandoned open pit mine could make “50% of the energy produced by burning the lignite that was mined” from the pit (Schuiling, 2013, p. 4) compared with the olivine powder powered power plant. Innovation Concepts has a patented CO₂ Energy Reactor™ currently in development.
(Source: Moosdorf, N., Renforth, P., & Hartmann, J., 2014, p. E)
Figure 2. Researchers “mapped…736,000 km²” of ultramafic rocks “globally” of this between “14,700,000” to “11,800,000 km²” was deemed suitable for potential enhanced weathering applications (Moosdorf, N., Renforth, P., & Hartmann, J., 2014, p. E). They also found “rock flour spread on the application areas can potentially sequester up to 1.1 (0.8) t CO₂ t-¹ (t-¹ means ‘per tonne of rock’) in the optimistic and pessimistic scenario.” (p. 8). Researchers said “efficiency improvements and renewable energy usage could reduce the associated CO₂ emissions below the optimistic scenario” (p. E). The most pessimistic projections found “efficiency reduction of olivine” to sequester CO₂ due to mining, grinding and transport “around 15-30%” (Hangx & Spiers, 2009, p. 763). A question remains on whether or not sonics could help with the grinding of the rocks (Momber, 2004).
(Source: Hangx & Spiers, 2009, p. 760)
Figure 3. Researchers found that grain size and temperature were two of the big factors when it comes to the dissolution rates of olivine. (Hangx & Spiers, 2009, p. 760). They assumed 1 t of olivine “dissolved… traps 1.25 t of CO₂ (CO₂: olivine uptake ratio Q=1.25) (p. 760). Smaller grain size means increased surface area which speeds up the reaction. Grain size affects the lag time concerning CO₂ ratios in the atmosphere which then affects the climate.
(Source: Köhler, Hartmann, & Wolf-Gladrow, 2010, p. 20231)
Figure 4. Each graph shows how the climate model predicts effects from the two differeent scenarios. The red A2 line simulates what will happen when nothing is done to sequester CO₂ anthropogenic emissions and B1 implies “C sequestration rates of 1 or 5 Pg of C per year. The first is the potential of olivine dissolution on tropical land, and the latter is an upper limit in a scenario with additional dissolution in tropical open ocean surface waters” (Köhler, Hartmann, & Wolf-Gladrow, 2010, p. 20230-20231).
(Source: Hartmann et al., 2013, p. 118)
Figure 5. Considering time it easy to see why certain rocks are more conducive to short-term enhanced weathering strategies.
(Source: Peizhen, Molnar, Downs, 2001, p. 893)
Figure 6. Paleoclimatology provides clues when trying to find the carbon over time. According to researchers the last 5 million years shows a large increase in “sediment accumulation in the main oceans” (Peizhen, Molnar, Downs, 2001, p. 891-892).
(Source: Peizhen, Molnar, Downs, 2001, p. 893)
Figure 7. Graphs d & e shows the sedimentation rate around the Tibet basins (Peizhen, Molnar, Downs, 2001, p. 893). There is approximately 600 Gt C currently in Earth’s atmosphere and “CO₂ consumption fluxes by chemical weathering range from .22 to .29 Gt C a. This is smaller than the fluxes between other reservoirs, e.g., 10 Gt C a are emitted to the atmosphere through anthropogenic activities” (Hartmann et al., 2013, p. 116). Also did increased carbon dioxide levels increase pressure and temperature? (Kasting, & Ackerman, 1986; Levenspiel, Fitzgerald, & Pettit, 2014).
(Source: Innovation Solutions, 2014, Slide 6)
Figure 8. Weathering or sedimentary carbon is not the only place carbon is deposited. Most of the carbon is deposited in limestone and dolomites.
(Source: Holland, H. D., & Zimmermann, H., 2000, p. 484)
Figure 9. The ratio of limestone to dolomite in the accumulation of carbonates has varied throughout time. Carbonate and sedimentary carbon deposition rates along with the amount of carbon in the atmosphere were some of the main variables considered when exploring the potential of enhanced weathering from a paleoclimatology perspective. For example from 65 to 120 million years ago there was a large amount of carbonate mass accumulation. This explains how the carbon was regulated and sequestered by the rocks even though sedimentary weathering did not kick in until around 65 million years ago [Figure 6].
Figure 10. The decline in atmospheric CO₂ correlates inversely with the increase in natural weathering from [Figures 6 & 7]. This supports the theory that enhanced weathering can draw down CO₂ levels. Although one would expect to see a sharper increase in the amount of CO₂ sequestered since 5 million years ago but the reaction rate could slow down as CO₂ became depleted.