Abstract
The 7 August 2019 IPCC special report on land and climate change predicts that the average global temperature will rise more than 1.5 C if human production of greenhouse gases (GHGs) continues at the 2019 rate to 2030, significantly compromising land productivity and the world food supply. Given a relationship between GHGs and land productivity, the World3 simulator can help to bound estimates of the sensitivity of human population dynamics to GHG reduction of land productivity. World3 projects that in the worst case the peak size of the human population could be reduced by 4% - 37% by GHG reduction of land productivity, compared to World3’s “benchmark scenarios”, during calendar years 1900-2100. In particular, World3’s “business-as-usual” (BAU) scenario implies that in the worst case the peak size of the human population would be reduced by ~20% by reduction of land productivity. This BAU-specific result is consistent, to within a factor of two, with GHG/wheat-productivity relations described in the literature.
Highlights
The 7 August 2019 IPCC special report on land and climate change predicts that the average global temperature will rise more than 1.5 C if human production of greenhouse gases (GHGs) continues at the 2019 rate to 2030
A fundamental question in IPCC 2019 is how we should model the relationship between GHGs and land productivity
In the absence of any other considerations, one might suppose that human population dynamics, human population size, varies linearly with GHG (Note 2)
Summary
The 7 August 2019 IPCC special report on land and climate change predicts that the average global temperature will rise more than 1.5 C if human production of greenhouse gases (GHGs) continues at the 2019 rate (mass GHG per capita per unit time) to 2030. In the absence of any other considerations, one might suppose that human population dynamics, human population size, varies linearly with GHG (Note 2). Such a model is mathematically simple, but is inadequate as a basis for policy guidance for at least the following reasons: i. In order to assess the relationship between the land-productivity-loss effect of GHGs on human population dynamics, we need a model that embraces (i)-(ii) and whose behavior is reasonably well understood. Given a relationship between GHGs and land productivity, it is reasonable to hypothesize that World could help to bound estimates of the sensitivity of human population dynamics to GHG reduction of land productivity. It should be noted that this kind of concern applies to any effort to assess the effect of GHG-induced land productivity loss on human population dynamics
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