The trouble with tweets (and carbon models too!)

Saw an interesting tweet in #agchat on twitter:

The link in the tweet is to a neat paper in PNAS (Burney et al., 2010, PNAS, doi:10.1073/pnas.0914216107): “Greenhouse gas mitigation by agricultural intensification.”

The thing about tweets is that you can only fit so much information in them. The tweet above highlights a point from the paper about how yield improvements may have prevented the conversion of native ecosystems to agricultural production. But, a yield-first approach is not always resilient and often has unintended consequences. To some extent, the authors recognize this fact:

careful and efficient management of nutrients and water by precision farming, incorporation of crop residues, and less intensive tillage are critical practices in pursuit of sustainable and increased agricultural output…

yield gains alone do not necessarily preclude expansion of cropland, suggesting that intensification must be coupled with conservation and development efforts

In the same way the above tweet leaves out information from the paper it links to, carbon models must also simplify the world. Here are important things that I think their model leaves out:

1) The authors “assume that after a conversion to agricultural land, soil organic carbon content remains the same” (emphasis mine). While it is true that most carbon is generally lost in the first 5-20 years, conventional tillage and related erosion processes continue to cause small losses of carbon (see Davidson & Ackerman, 1993).

2) Maximizing potential yield (which is incentivized by America’s crop insurance program) doesn’t necessarily increase average yield. This is especially true in the face of record setting droughts which have caused record crop losses in the Great Plains during the summers of 2011 and 2012. Because potential yield maximization often causes of soil organic matter loss and can negatively affect other ecosystem services, it can not only destabilize the resilience of the landscape, with the assistance of other factors (like erosion) it can lead to a totally degraded landscape.

3) The model assumes yield increases in the west prevented the conversion of new cropland in the developing world. However, into the 1980s (when we reached a plateau of food production per capita), yield increases went beyond global food production needs and instead of merely preventing agricultural expansion, depressed prices and put third world farmers out of business. Taking croplands out of production is well and good, but can leave behind a vulnerable economy. Also, without active restoration efforts, carbon stocks on these landscapes may be difficult to recover.

On current marketplaces it’s a lot harder to buy/sell/eat carbon than food. In addition (I’m making assumptions here, please comment if you disagree) smallholder producers are likely to have less of their food wasted because they are closer to their consumers. Far outside the scope of their is economic effect of more farms in more places compared to fewer highly productive farms concentrated in the developed world.

In summary, there is something to be said for the yield gains we’ve made in industrialized agriculture. It’s true that boosting soybean yields in the Great Plains can make it less attractive to convert Amazonian rainforests into soy production. There’s even more to be said for new moves toward precision farming and conservation tillage. However, when we summarize these benefits in a tweet it’s impossible to account for how complex these issues are. In the same way, it’s important to question the models and the assumptions they make, before you use them to build your world view.


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