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Climate Change & Carbon Dioxide Removal

The earth is predicted to warm 2 degrees Celsius by 2100 if if emissions remain high over the next few decades.  This global increase in temperature will  have devastating impacts  our current food systems including .  It is well known by now that the Earth’s atmospheric levels of carbon dioxide are causing many issues such as sea level rise, ocean acidification, loss in ice sheet volume, increase in global temperatures, vanishing coral reefs - the list goes on. The solution to this warming is firstly to cut emissions created by human activity. But, even if those emissions are drastically cut in a short amount of time, there will still be too much carbon in the atmosphere to slow the Earth’s warming trend. Therefore, carbon dioxide removal (CDR) techniques come into play to help lower CO2 levels. There are many techniques to CDR, including direct air capture (DAC), reforestation, biochar, etc. The technique that we are focused on is enhanced rock weathering (ERW).

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Agricultural emissions contribute to 13% of greenhouse gas emissions globally; however, if climate-smart technologies are applied, farms can be a part of the solution by reducing their carbon footprint, improving soil health, minimizing CO2-emitting inputs such as agricultural lime, and ultimately improving their crop yields.

Earth’s Natural Carbon Cycle

In part of Earth’s natural carbon cycle, carbon from the atmosphere is locked into rain droplets as it falls throught the atmosphere. This “carbonated” water (much the same way as we carbonate beverages to drink) then falls to Earth’s surface, and ends up dissolving minerals in specific types of rocks on the surface. When these minerals dissolve, they realease ions such as calcium (Ca) and magnesium (Mg) into the soil and water within the soil. These ions then are transformed into a different mineral called a “carbonate mineral” via a chemical reaction.  These “carbonate minerals” are then carried by groundwater flow to rivers, and then eventually, to the ocean. In the ocean, these carbonate minerals will build up on the ocean floor and create sedimentary rocks such as limestone.

What’s been described above is a part of a process that Earth goes through over very large spans of time. However, since humans are releasing more carbon dioxide than the Earth can sequester by itself, we are tasked with figuring out a way to help Earth by sequestering carbon at a faster rate than it can by itself.

Enhanced Rock Weathering

One of the ways we have figured out how to do this is by “Enhanced Rock Weathering” (ERW) which takes the process detailed above, and “enhances” it, or rather speeds it up, by making these rocks be more available on the surface of the Earth.

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The process of sequestering carbon doesn’t neccesarily happen when rain falls on Earth’s surface. The carbonated rain water has to fall on specific types of rock that have the capability to sequester carbon.

Luckily, we know exactly what type of rocks these are -- They are called “silicates” which are minerals with the element silica (Si) present in them. Silicates are broadly available and actually make up 95% of Earth’s crust.

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The ERW process gathers these types of silicate rocks (such as basalt, olivine, or wollastonite) and crushes them into dust and then spread onto farm land. The reason for crushing is to increase the surface area of the rock, so that greater amounts of the rock are exposed to the carbonated rain water, which means that more carbon can be sequestered.

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Increased Yields

Field trials at Zumwalt Acres in Illinois demonstrated improved yields in oats by up to 50% after rock dust application

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Enhanced Rock Weathering

When rain falls on the field, the minerals in the rock dust are dissolved and then react with the CO2 in the rainwater to  lock away the carbon for thousands of years.

Image by Joshua Hoehne

Photo courtesty of Zumwalt Acres

Improved Soil Health

ERW improves soil pH and replenishes soil micro nutrients needed by plants. Rock dust releases silica, which improves both crop strength and pest resistance.

Image by Avinash Kumar

Agricultural Lime vs Basalt Rock Dust

Basalt is a type of volcanic rock that is extremely abundant on Earth's surface. About 90% of all volcanic rock is basalt. Half of the minerals that make-up basalt are silicate minerals.

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Agricultural lime is pulverized limestone that is used as a soil additive to increase soil pH. Lime is a carbonate mineral, which means it is composed of calcium carbonate (CaCO3).

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So why swap using agricultural lime for the use of basalt rock dust?

Weathering of silicate minerals is twice as effective for removing CO2 than carbonates.

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Liming is commonly a carbon source, whereas silicates provide the same agricultural co-benefits while also being a carbon sink.

Transforming agriculture for CO2 Removal

Case Study 1

U.S. Corn Belt Trials

  • Yield response: Basalt increased corn and soybean yields by ~12-16%. Lime was also used, but did not return as high of a yield increase.

  • Soil pH: ERW made soil pH increase significantly, and  prevented soil acidification that occurs from nitrogen fertilization.

Case Study 2

U.K. Agricultural Trials

  • Removal potential: There are ~6 millihectares of potential agricultural land in the U.K. On this land, enhanced rock weathering could remove up to 30 Mt (30,000,000 metric tonnes) of CO2  per year by 2050.

  • Co-benefits: ERW provides substantial mitigation of nitrous oxide, the third most important greenhouse gas and
    major cost savings from reduced fertilizer usage.

Case Study 3

Australia Sugarcane Trials

  • New, more accurate approach to measuring the amount of carbon dioxide sequestered

  • Removal potential: Calculated an average initial CDR value of 1.47 ± 1.09 tonnes of CO2 equivalent per hectare.

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