The US Global Change Research Program just released Volume II of the Fourth National Climate Assessment (NCA4-II) last week. NCA4-II focuses on the observed and projected impacts of climate change in the US, with special consideration given to risk reduction. The news media reacted to NCA4-II with alarm. Typical headline: Climate change will shrink US economy and kill thousands, government report warns.
NCA4-II did not say anything “will” happen. The report’s predictions were carefully hedged with qualifiers, e.g., “without substantial and sustained global mitigation and regional adaptation efforts, climate change is expected to cause” x, y, and z. The most alarming predictions were based on worst-case scenarios of extreme warming and minimal adaptation. These were the “dire” and “grim” predictions of the headlines.
The specific worst-case scenario used throughout NCA4-II was based on “Representative Concentration Pathway” 8.5 (RCP 8.5), a greenhouse gas concentration trajectory under which the global climate could warm over three times the 1.5°C goal set by the Paris Agreement. RCP8.5 has been criticized as unrealistic because, among other things, its “business-as-usual” storyline* assumes such a huge increase in coal consumption that coal reserves would be depleted long before the end of the century (Rutledge, 2011). Here’s more on RCP8.5 as well as the other RCPs**:
Per Climate Action Tracker (CAT), current US emission trends are actually within “striking distance” of the initial Paris Agreement targets for 2020 and 2025, despite Trump’s rhetoric and the US not even being a signatory to the Agreement. This unexpected progress is thanks to “subnational” and nongovernmental actors, such as states, cities, businesses, nonprofits and others. Yes, it would be great if the federal government were on board. And, yes, we need to do a lot more - but those worst-case scenarios aren’t going to happen. We won’t be living in an RCP8.5 world by 2100 - unless current trends are reversed and humanity goes crazy over coal, spurns renewables, cuts down all the trees, and decides that energy efficiency is for suckers.
But mitigation - reducing emissions and thus warming - is only half the story. The other half is adaptation. Even if the climate got hella hot by 2100, as in RCP8.5 hot (which it won’t), the worst-case scenarios in NCA4-II assumed minimal adaptation to climate change. This would be another reversal of current trends. The US is already doing lots in the adaptation department. We are pointed in the right direction.
Next: Care of the Fourth National Climate Assessment Report, Volume II, a list of climate change adaptations in place or in development.
* RCPs don’t dictate a specific storyline. A storyline serves as a possible explanation for how the RCP may be reached. Some researchers simplify the RCP8.5 storyline to one of “no mitigation” without going into detail about coal use, etc. However, the “business-as-usual” storyline (e.g., maximal coal/minimal renewables) associated with RCP8.5 is the standard one.
**These RCPs predate the Paris Agreement. Researchers have added RCP 1.9 to model a 1.5°C rise in global temperature and use RCP 2.6 to model a 2°C rise.
References:
Rutledge, David (2011-01-01). "Estimating long-term world coal production with logit and probit transforms". International Journal of Coal Geology. 85 (1): 23–33. doi:10.1016/j.coal.2010.10.012.
USGCRP, 2018: Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II [Reidmiller, D.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, K.L.M. Lewis, T.K. Maycock, and B.C. Stewart (eds.)]. U.S. Global Change Research Program, Washington, DC, USA. doi: 10.7930/NCA4.2018. https://nca2018.globalchange.gov/
van Vuuren, D. P., J. Edmonds, et al. (2011). "The representative concentration pathways: an overview." Climatic Change 109(1): 5. https://link.springer.com/article/10.1007/s10584-011-0148-z