Commentary on: Hsiang et al (2017) “Estimating economic damage from climate change in the United States.”
The authors of the above paper predict that by 2100 a good portion of the US will experience significant economic damage due to climate change. Their calculations assume a specific emissions trajectory known as Representative Concentration Pathway 8.5 (RCP8.5). RCP8.5 is the “worst-case” scenario of the four Representative Concentration Pathways developed by climate modelers. It projects a very high atmospheric concentration of greenhouse gases (GHGs) by the end of this century, with global warming reaching 2.6 to 4.8 °C by 2100. RCP8.5 comes with a “business-as-usual” storyline detailing socio-economic assumptions consistent with these projections. While this particular storyline isn’t the only one possible, it is the standard storyline and the one referenced by Hsiang et al in their paper.
The business-as-usual storyline: this century coal consumption will increase ten-fold; development of renewable energy technologies will be delayed; natural gas will decline as an energy source; improvements in agricultural land use will be delayed; and human population will rebound to 12 billion by 2100.
I addressed the assumptions about energy consumption and technology development in the last post. In brief: they constitute a reversal of current trends and are beyond implausible. For this post, I’ll be addressing trends in agricultural land use and population.
Now, let’s look at those trends. First, Arable Land Per Capita:
Arable Land needed to produce a Fixed Quantity of Crop Product:
Global Farm Area:
Source: FAOSTAT
So, pace the RCP8.5 storyline, there is no indication of any slackening of agricultural productivity. In fact, we may have already reached "peak farm" as “the ratio of arable land per unit of crop production shows improved efficiency of land use, the number of hectares of cropland [having] scarcely changed since 1990” (Ausubel, Wernick, & Waggoner, 2013). This despite an additional two billion mouths to feed in the last 25 years.
Speaking of global population, that's the one RCP8.5 plot point that is in the ballpark:
Source: http://www.businessinsider.com/un-world-population-projections-2015-7 "World Population Prospects: The 2015 Revision, Key Findings and Advance Tables."
Next: Putting it all together and then some: continued commentary on Hsiang et al (2017) “Estimating economic damage from climate change in the United States.”
References:
Ausubel, J. H., Wernick, I. K. and Waggoner, P. E. (2013), Peak Farmland and the Prospect for Land Sparing. Population and Development Review, 38: 221–242. doi:10.1111/j.1728-4457.2013.00561.x
“Estimating economic damage from climate change in the United States.” By Solomon Hsiang, Robert Kopp, Amir Jina, James Rising, Michael Delgado, Shashank Mohan, D. J. Rasmussen, Robert Muir-Wood, Paul Wilson, Michael Oppenheimer, Kate Larsen, Trevor Houser Science 30 Jun 2017: 1362-1369.