Limitations on carbon dioxide uptake by enhanced weathering

Managing atmospheric carbon dioxide

Despite widespread recognition by the scientific community that avoiding dangerous climate change requires significant reduction in anthropogenic CO₂ emissions, emissions continue to increase at a rate of more than 1% per year and now are at 38 Pg yr^-1. Unabated emissions growth has led to a search for methods that could partially offset emissions by increasing the role of carbon dioxide removal (CDR).  One method that has received attention is Enhanced Rock Weathering (ERW) that exploits weathering reactions where CO₂ dissolved in soil water reacts with Ca-Mg-Na bearing rock, releasing cations and converting CO₂ into dissolved inorganic carbon or alkalinity, essentially as bicarbonate ion (HCO₃^-). This process occurs naturally, as decomposition of organic matter in soils produces high pCO₂ at levels that can exceed tens of thousands of ppm. Dissolved Ca⁺⁺, Mg⁺⁺ and HCO₃^- can be transported to the oceans where they can buffer pCO₂ and ultimately remove it as carbonate minerals.  Terrestrial weathering plays a critical role in the long-term stability of Earth’s climate, offsetting the 0.2 to 0.3 Pg yr^-1 of volcanic and metamorphic CO₂ emissions.  If weathering did not consume CO₂ our climate would have long ago resembled that of Venus. 

The premise of ERW as a CDR strategy is that grinding natural basaltic rock, which has high levels of Ca and Mg, and applying it over wide areas of row crop agriculture could increase the net weathering consumption of CO₂.   The method has the advantage of relative ease of implementation compared to some CDR alternatives and could offer some additional agronomic benefits if rock addition improves soil pH or provides additional nutrients. Initial and widely quoted estimates of the potential CDR based primary on model studies suggested up to 10 tons of CO₂ per hectare per year in some regions, and if applied to large cropland areas have been proposed to offset > 1 Pg CO₂ yr^-1.

Volcanic Weathering

We knew that the entire global CO₂ uptake from basaltic weathering was around 0.12 Pg CO₂ yr^-1, well below the proposed ERW impacts.  We re-examined the literature to assess CO₂ uptake rates from a suite of natural and engineered volcanic systems and the reasons why systems that are entirely composed of basalt yielded much less CO₂ uptake than the proposed ERW systems that would have, at most, only a few % of basaltic rock after amendment.  We confirmed that CO₂uptake rates in volcanic systems were much lower than most ERW estimates, even in highly active systems with wet and warm tropical climates like in the Philippines.  Proponents of ERW have emphasized grinding of rock to generate small particles with high surface area that should in principle react rapidly, thus increasing CO₂ uptake rates over natural levels. We found that a) volcanic tephras had grain size distributions similar to engineered ERW applications, and b) there was a weak relationship between particle size and common measures of surface area until quite small grain sizes, which pose both a cost problem and an aerosol inhalation hazard.

Buffering reactions in the Critical Zone

Research on the Critical Zone, Earth’s surface reactive boundary layer where air, plants, rocks and water meet in highly dynamic ways, has emphasized how processes that take place in the shallow soil zone, including rock dissolution reactions, are just the first step among many that control the export of weathering reaction products to groundwater, rivers, and the oceans. For ERW to have a major impact, cations and alkalinity need to be exported from the proximal shallow soil weathering zone.  We find that the initial products of dissolution generated in the shallow soil with high organic matter content, high pCO₂ and organic acid levels, are significantly modified by a series of reactions as they migrate through the Critical Zone.  These include precipitation of secondary minerals, including clays, oxides and carbonates, which either remove cations, generate acidity that consumes alkalinity and returns dissolved CO₂ to the atmosphere, or both.  These reactions are not simple to model effectively as they often occur far from equilibrium, can produce hard-to-quantify amorphous or poorly crystalline products, and have critical but still incompletely understood kinetic controls. They occur along the transport continuum, from shallow soils to the deeper vadose zone, groundwater, and in streams. The net result is that only a modest fraction of dissolution products of weathering reactions (natural or engineered) is actually exported through the Critical Zone.

Water

Transporting weathering products through the soil to groundwaters and streams requires water.  The highest observed weathering rates are from wet tropical system with runoff of 2 to >3 meters per year.  But most regions of large-scale crop production are much drier.  The upper Mississippi watershed, a target for ERW, has runoff of < 0.25 m yr^-1. Capacity for transport of weathering products including alkalinity is strongly limited by water in those regions. For example, for an optimistic soil DIC level of 3 millimole L^-1 this water flux could maximally transport CO₂ equivalent to an uptake rate of around 0.3 t CO₂ ha^-1 yr^-1, more than an order of magnitude below some proposed model estimates. And only a fraction of the soil DIC would come from reaction with the basalt amendment.

Our paper briefly lays out some of the controls on weathering rates that the geochemistry and hydrology communities have developed based on decades of research. We find that the high CDR rates proposed by some ERW proponents are quite unlikely to be realized in practice.  It is important to direct limited resources in the fight against climate change to where they can have the greatest impact.  While ERW could have beneficial local impacts we don’t think it can play a major role in offsetting emissions.  It is well-accepted by workers in the field of climate that only major reductions in emissions can make a substantial difference to our climate future.  What roles various CDR strategies can play in marginal improvements to this goal remain the focus of much work.