Abstract

We believe that enhanced weathering (EW) with rock dusts on croplands will become a substantial tool to work against global warming. Currently measuring this process in open nature is complex and expensive which hinders development of EW. A cheap and scalable method of monitoring the EW process on croplands is necessary to optimize the potential combinations of soils, rocks, flora and fauna to reach an optimum of crop yield, soil-health and carbon dioxide removal (CDR). In this short paper we would like to explain why the ultimate goal of our involvement with Project Carbdown is to develop cheap, electronic sensors for this. 

Introduction

In 20-30 years we need negative emissions at gigaton/year scale (IPCC). We believe that enhanced weathering, likely on croplands, can be a substantial part of this. The advantages are: 

1. relatively little energy investment (mostly for grinding the stone), 

2. a global abundance of the necessary rock materials (e.g. basalt), 

3. no competition for space with food-production, 

4. no need for development/building of new technologies (e.g. mining/spreading machinery),

5. several co-benefits for agriculture (by improving yields, soil fertility and animal husbandry), 

6. outstanding carbon permanence (mineralized weathering products are stable forever at human timescales),

7. and the fact that we use an almost non-reversible, natural chemical process that runs exothermically by itself -- we just create a better setup to speed it up.

For enhanced weathering to succeed we need to show that:

1. We can reach a reasonable scale (e.g. megatons/year in Germany by 2035-2050).

2. It is possible at a reasonable cost (< €200-500 per ton of CO₂).

3. Weathering performs carbon dioxide removal at reasonable speed (substantial part of the carbon uptake from the air/atmosphere is happening in <20 years).

4. We can measure the weathering and the measuring can be scaled quickly/cheaply. 

Problem #1: Looking at scale

Here is a sample calculation: Annual CO₂ emissions of Germany are 640 megatons now and should go down by 90% due to emissions reductions by 2050, so let’s use 60 megatons of remaining hard-to-mitigate emissions per year that require negative emissions (we would at least reach carbon-neutrality with this, afterwards we need to lower the CO₂ level to 350 ppm).

Let’s assume we are able to apply 40 tons of basalt to half of the 17 million hectares of agricultural land in Germany every 10 years. That’s 340 megatons of basalt which could at max collect 170 Megatons of CO₂. If it takes 20 years for most of the weathering we will gain 8,5 megatons of CO₂ per year, that would be ~13% of the remaining annual German emissions. If (optimistically) it takes only 10 years, we could reach 17 megatons of CO₂ per year.

=> The order of magnitude is ok, sequestration will be even higher in wetter/warmer areas of the world, so it is worthwhile to follow this path further.

Problem #2: Looking at cost

One ton of very fine, commercially mined basalt dust (<100 microns) today costs about €60 to the field edge in Germany and sequesters ⅓ to ½ ton of CO₂, so capturing a ton of CO₂ would cost €120-180 (plus spreading, measuring, accounting). That’s much cheaper than DAC (which already sells at hundreds of €/ton today, see Climeworks et al.) for the next few years and in the range of CO₂ sequestration with biochar today. With e.g. waste materials from quarries and coarser grains there is substantial potential to further lower cost.

Additional benefits from spreading basalts on croplands are potential yield increase, soil improvements, and being a lime replacement - all could mean profit improvements for the farmer.

=> Cost is not a problem at this point, but if we need to apply large security buffers into certification models for EW already a factor of 5-10x could kill the business model. So there is little room for these buffers -- or we need a way to make buffers smaller by measuring “in-situ”, which points to problem #4.

Problem #3: Looking at speed

From our team member Professor Jens Hartmann’s (University Hamburg) experiments we know that in his soil column experiments basalt sequesters 0,5-1,5%/year of its weathering potential (0,3-0,5 tons of CO₂ per ton of basalt) in a European climate setting. Thus it would take 50-70 years to reach 50% of the potential. This is too slow and would render EW on croplands almost useless. 

BUT: These are lab experiments without plants, fungi, bacteria, fauna at ambient air CO₂ levels. We expect much higher rates (>10x ?) in soils in open nature, e.g. due to a much higher CO₂ levels in soils and due to various effects of fungi, bacteria, etc. 

=> Relying on lab experiments is not enough: It will be much harder to sell negative emissions from EW as a climate solution if it takes decades to reach a substantial impact according to lab results. We need to find out how this can be made faster (testing with many variations/combinations of location, rock-type/size, soil, crop, water, temperature, climate, farm-work, etc.?) and for that we need massive field experiments (hundreds/thousands), which require a cheap and technically simple solution for problem #4.

Problem #4: Looking at fast/easy measurement

Currently we can only measure weathering with high effort in the lab (total alkalinity, ion balances, isotopes, etc.) or use largely unverified model estimations.

=> This is a problem: A lot of manpower/time per sample/plot creates high cost which makes scaling experiments to thousands of plots in hundreds of places in a few years impossible, which means we will likely have no answers to problem #3 and #2 soon which means EW is not scaled and we will not be able to tackle problem #1.

Conclusion

It all comes down to measuring the weathering cheaply and at scale. This is “the root of all problems” for EW on croplands. If we can find a measurement method that would enable us to 

• A: cut the the safety buffers necessary for certificates by e.g. 50% or 70% or 90% (?) and 

• B: help us to quickly understand how we can speed up the EW process in the field (through many variations/experiments) and

• C: (maybe) massively simplify certificates after measuring many variations and having enough understanding of EW and

• D: learn how rock dust can improve agriculture by substantial co-benefits (additional revenue or cost-reductions) for the farmer,

then we will have succeeded. Cheap and easy measuring is in our perspective an important way to show that this will be a viable business model for the farmers and this will make it scale by itself. Looking at an approach that uses cheap, scalable sensors seems like the right way to go on. 

The ultimate goal is to provide farmers with a list of technical and cost benefits of rock dust application that will make the idea sell itself. From Carbon Drawdown Initiative’s side we will keep this long term goal as our guiding light for all experiments and decisions.