Noting that “major increases in crop productivity and changes in regional climate are generally collocated in time and space over the central United States” [Alter 2018: 1587], the study tested the hypothesis that there is a causal relationship – that historical agricultural intensiﬁcation has affected regional summer climate in this area.
… from 1950 to 2010, the amount of corn harvested annually in the Corn Belt increased by 400%, from 2 billion to 10 billion bushels (National Agricultural Statistics Service, 2016) [Alter 2018: 1586].
From 1910 to 1949 (pre-agricultural development, pre-DEV) to 1970–2009 (full agricultural development, full-DEV), the central United States experienced large-scale increases in rainfall of up to 35% and decreases in surface air temperature of up to 1°C during the boreal summer months of July and August, when crop water use in the Corn Belt is at its peak [Alter 2018: 1586].
The authors used a regional climate model to test their hypothesis by comparing a set of simulations where “enhanced photosynthesis over cropland [serves] as a proxy for agricultural intensiﬁcation” [Alter 2018: 1589] to a control simulation with no agricultural intensiﬁcation. They found that:
Over the region that has experienced signiﬁcant increases in observed rainfall (region of signiﬁcant change—ROSC), the mean rainfall increase is ~7% (0.20mm/d) for the simulations and ~15% (0.37mm/d) for the observations. Thus, it seems that agricultural intensiﬁcation has been a major contributor to the observed increase in summer rainfall in the central United States [Alter 2018: 1589].
Strikingly, these increases in rainfall are also very consistent: Agricultural intensiﬁcation enhances simulated rainfall across the aforementioned swath in the central United States during at least 62% of the 150 ensemble years (signiﬁcant at the 5% level using the chi-square test). In the observational data, a similar consistency in precipitation enhancement is evident when comparing the pre-DEV and full-DEV time periods. This suggests that the changes in rainfall due to agricultural intensiﬁcation are not the result of occasional increases but instead are indicative of a more systematic change in the summer rainfall regime of the central United States [Alter 2018: 1589].
This study usefully contributes evidence that vegetation cover affects local and regional climates, while drawing conclusions, however, that are not necessarily helpful to understanding how to mitigate and adapt to climate change. The study’s findings suggest that agricultural intensification can potentially mitigate local climate change effects in the future, but it is unlikely that the methods that drove agricultural intensification in the 20th Century will continue to work in a changing climate. The reason that these methods are now obsolete is that they strip the soils of the organic material and living organisms necessary for the resilience of plants, and their ability to cope with droughts, floods, heat and other challenging conditions.
The model here uses “enhanced photosynthesis” as a proxy for agricultural intensification. While the increase in yield between early and late 20th Century Corn Belt production represents an increase in photosynthesis, high-input agriculture is but one pathway to enhanced photosynthesis. Moreover, it is an extremely problematic one with respect to climate change, given the high energy costs of fertilizer, pesticides and fuel, and the damage to the soils from these practices.
Instead, a useful lesson to draw from this study is simply that enhanced photosynthesis itself can mitigate climate change regionally. In the context of agricultural production in the era of climate change, enhanced photosynthesis might best be accomplished through ecological intensification, a strategy for improving resilience within an agro-ecosystem, and thereby greater photosynthesis and more reliable crop production.
Alter, Ross E., et al., 2018, Twentieth Century regional climate change during the summer in the central United States attributed to agricultural intensification, Geographical Research Letters 45-3: 1586-1594 , https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017GL075604