Agriculture. the green revolution and its role in emissions avoidance

Burney J A, Davis S J and Lobell D B (2010). Greenhouse gas mitigation by agricultural Intensification, PNAS.

This paper estimates the net effect on GHG emissions of historical agricultural intensification between 1961 and 2005. It finds that emissions from factors such as fertilizer production and application have increased, the net effect of higher yields has avoided emissions of up to 161 gigatons of carbon (GtC) (590 GtCO₂e) since 1961. It also estimates that each dollar invested in agricultural yields has resulted in 68 fewer kgC (249 kgCO₂e) emissions relative to 1961 technology ($14.74/tC, or ~$4/tCO2e), avoiding 3.6 GtC (13.1 GtCO2e) per year. In order to calculate ‘avoided’ emissions, the paper describes two hypothetical alternative scenarios, AW1 and AW2.

In AW1, it looks at what would happen if the green revolution hadn’t happened but that population growth and current patterns and levels of consumption are what they are today.

In AW2, it attempts to answer the challenge that the green revolution has in fact made current patterns of consumption possible. It therefore keeps per capita consumption patterns (including per capita intakes of meat and dairy) at 1961 levels, but assumes pop growth to today’s levels. Population projections derived from pre-1961 fertility and mortality rates are used which coincidentally result in very similar 2000 populations, albeit with different age structures.

In each of the three scenarios the calculations include N₂O from agricultural soils; CH₄ from rice cultivation; C released from both biomass and soil by conversion of forest, shrub, and grassland to cropland; and N₂O, CH₄, and CO₂ from the production and use of nitrogenous, phosphate, and potash fertilizers. In scenario AW2 as well as in AW1, emissions are still higher than they really are today.

The paper notes that “The AW scenarios presented [...] do not attempt to dynamically describe what might have occurred in the absence of yield improvements; rather, they demonstrate the range of possibilities by calculating, on the one hand, the area required to support modern global living standards with 1961 yields (AW1) and, on the other hand, the area required to maintain 1961 living standards (with 1961 yields through 2005 (AW2).”

It also says: "In the AW1 scenario, this study demonstrates the incredible environmental cost modern living standards would have exacted without yield improvements (or unprecedented humanitarian crises). Although the GHG impacts of yield improvements in the RW are lower compared with the AW2 scenario, the deepest troubles in a world like AW2 would have appeared mainly after 2000 and thus are somewhat masked in this analysis. The population projections in the AW2 scenario (1950–1955 mortality and fertility rates projected forward) result, coincidentally, in population totals similar to the RW in 2000 (24). Thus, the constant per capita production in AW2 would result in less dramatic land expansion than in AW1. However, population in AW2 would strongly diverge from the RW population after 2000, resulting in a much greater future GHG impact without gains in yield NB: this is because of the age structure in AW2 – the population is on average younger and so future population growth will be greater".

The conclusions are as follows:

"Our results demonstrate the importance of land use change emissions over direct emissions of methane and nitrous oxide from agricultural systems, and suggest that the climatic impacts of historical agricultural intensification were preferable to those of a system with lower inputs that instead expanded cropland to meet global demand for food. Enhancing crop yields is not incompatible with a reduction of agricultural inputs in many circumstances, however. To the contrary, 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. Furthermore, it has been shown in several contexts that yield gains alone do not necessarily preclude expansion of cropland, suggesting that intensification must be coupled with conservation and development efforts. Nonetheless, for mitigating agriculture’s future contributions to climate change, continuing improvement of crop yields is paramount. The global population is expected to reach 8.9 billion by 2050, with food demand expected to rise by 70%. Even if yield gains over the next four decades are smaller than those of the previous four decades, the potential to avoid future emissions may be larger and more cost-effective than the 161 GtC of emissions avoided thus far, given that current cropland expansion often occurs in tropical forests and that the remaining forests are carbon-rich relative to many cleared forests. Improvement of crop yields should therefore be prominent among a portfolio of strategies to reduce global greenhouse gas emissions; in order to speed the adoption of agronomic advancements that improve crop yield, mechanisms for connecting investments in yield gains to the global carbon markets should be explored".

Note that the paper does not mention shifts in patterns of consumption although presumably it is implied in the statement quoted above that ‘this study demonstrates the incredible environmental cost modern living standards would have exacted without yield improvements.’ The study reveals (although doesn’t explicitly mention this) the important contribution that the sheer growth in the number of people have made to the increase in agricultural emissions.

The abstract is open access and can be downloaded here.