Removing Atmospheric Carbon Dioxide Using Large Land Or Ocean Areas Will Change Earth Albedo And Force Climate

Abstract: When large surface areas of the Earth are altered, radiative forcing due to changes in surface reflectance can drive climate change. Yet to achieve the necessary scale to remove the substantial amounts of carbon dioxide from the atmosphere relevant for ameliorating climate change, enhanced rock weathering (ERW) will need to be applied to very large land areas. Likewise, marine carbon dioxide removal (mCDR) must alter a large fraction of the ocean surface waters to have a significant impact upon climate. We show that surface albedo modification (SAM) associated with ERW or mCDR can easily overwhelm the radiative forcing from the decrease of atmospheric CO2 over years or even decades. A change in albedo as small as parts per thousand has a radiative impact comparable to the removal of 10 tons of carbon per hectare. SAM via ERW can be either cooling or warming. We identify some of the many questions raised by radiative forcing due to these forms of CDR.

• The paper demonstrates that slight increases in Earth’s albedo can produce radiative forcing effects comparable to removing 1 gigaton of CO2 per 0.002 increase.
• It employs a zero-dimensional model to assess how enhanced rock weathering and marine CDR techniques interact with surface albedo modifications.
• The analysis calls for comprehensive field experiments to quantify albedo impacts, optimizing carbon removal strategies while mitigating unintended climate effects.

Evaluating Radiative Forcing of Large-Scale Carbon Dioxide Removal on Earth's Albedo

The study, authored by Marston and Ibarra, explores the interconnected implications of carbon dioxide removal (CDR) techniques on Earth's radiative balance, centering on the often-overlooked element of surface albedo modification (SAM). This work explores the climate forcing implications of enhanced rock weathering (ERW) and marine carbon dioxide removal (mCDR)—methods proposed to counteract atmospheric CO$_2$ concentrations—and their potential to influence Earth's climatic system through albedo changes.

Core Concepts and Methodology

The necessity for large-scale implementation of CDR methods to impact atmospheric CO$_2$ levels substantially requires expansive land or oceanic areas to be treated. ERW involves distributing finely ground silicate minerals over managed lands, which, through chemical reactions, draws CO$_2$ from the atmosphere. Simultaneously, mCDR proposes altering ocean chemistry to facilitate additional atmospheric CO$_2$ uptake. However, these alterations in surface characteristics could result in significant SAM effects that may either mitigate or exacerbate climate change through changes in Earth’s reflectivity.

The authors employ a zero-dimensional model to assess how ERW and SAM interact over temporal scales, determining that SAM effects, caused by changes in albedo, can overpower the benefits gained from CO$_2$ reduction in the short term. The model simplistically suggests that even slight changes in albedo have comparable and possibly greater immediate radiative force when compared to the prolonged effect of CDR on CO$_2$.

Key Findings

• Albedo Changes Supersede CO$_2$ Removal Impact: The study provides calculations demonstrating that SAM can significantly outweigh the impact of atmospheric CO$_2$ reduction due to ERW or mCDR on a decadal timescale. For instance, a minuscule increase in Earth's albedo by just 0.002 could yield an effect similar to the removal of 1 gigaton of CO$_2$.
• Potential for Dual Climate Outcomes: The albedo effect can be dual-faceted—cooling or warming—depending on the mineral types and soil background albedo. Materials like basalt might reduce albedo, enhancing warming, whereas light-colored wollastonite could increase reflectivity, thereby aiding in cooling.
• Field Experimentation Urgency: There is a call for comprehensive field studies to measure albedo outcomes from ERW, especially over large-scale implementations. Such data is essential for optimizing the dual goals of reducing greenhouse gases while managing unintended climate impacts.

Implications and Future Directions

This analysis underscores the vital need for integrating albedo effects into the assessment of CDR strategies. As ambitious goals towards carbon neutrality necessitate exploring all feasible methods, understanding these interactions becomes crucial. The authors suggest initiatives for extensive field research and sophisticated climate modeling to better understand and manage the climatic outcomes from SAM, specifically related to ERW.

Moreover, addressing these questions will inform how ERW and mCDR could be deployed sustainably without exacerbating regional climates through unintended alterations in weather patterns or precipitation due to localized increased radiative forcing.

Conclusion

The paper by Marston and Ibarra advances the discourse on CDR by intricately connecting large-scale interventions with broader climate dynamics via albedo modulation. This calls for heightened rigor in evaluating geoengineering techniques, ensuring that strategies to mitigate global warming do not inadvertently alter Earth's radiation budget in unforeseen ways. The implications stretch beyond theoretical modeling, hinting at a broader need for interdisciplinary approaches marrying geosciences, atmospheric physics, and ethical policy formulations to responsibly manage climate interventions.