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 the poorest third of US counties, mostly in the south, will experiences significant damage due to climate-related effects on agriculture, crime, coastal storms, energy, human mortality, and labor. They project this damage will occur “under business-as-usual emissions (Representative Concentration Pathway 8.5)”. In other words, the authors’ analysis is based on a particular global emissions scenario unfolding. If we are to put stock in their projections, we need to consider the plausibility of their assumptions. The output is only as good as its input.
First, a brief introduction to Representative Concentration Pathways (RCPs). RCPs describe four possible climate futures, each depending on how much greenhouse gases are emitted in the years to come. The four RCPs, RCP2.6, RCP4.5, RCP6, and RCP8.5, are named after a possible range of “radiative forcing” values in the year 2100 relative to pre-industrial values. A simplified definition of a “forcing” is the difference between the energy radiated out into space and energy absorbed by the earth. RCP8.5 models a set of conditions that could lead to a world where climate forcing by the year 2100 reached 8.5 watts per meter squared. Of the four RCPs, RCP8.5 represents the worst-case scenario, as the following chart makes clear:
Each RCP also comes with a story line that would result in the target effect. The story line of RCP8.5 has been described as "conservative business as usual". Here are some of its key plot points:
- Delayed development of renewable energy technologies
- Rebound of human population to 12 billion by 2100
- Wide and increasing international disparities in productivity, energy efficiency, and GDP
- Delayed improvements in agricultural land use
- 10-fold increase in the use of coal as a power source
- Move away from natural gas as an energy source
- Little change in environmental and economic policies across the world
In brief, RCP8.5 paints a picture of “… low income, high population and high energy demand due to only modest improvements in energy intensity” throughout the 21st century.
Next: Is RCP8.5 truly a “business as usual” scenario? Does it make sense to use RCP8.5 as the basis for estimating the economic effects of climate change?
Reference:
“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.