Human Choice, Warming & Emissions:
The Representative Concentration Pathways
More CO2 and other greenhouse gases in the atmosphere equals more warming for our climate. This relationship is straightforward. Less clear is just how much more emissions we will add to our atmosphere and how much warmer things will get. This isn’t a scientific uncertainty. It’s a social, political, and economic uncertainty based on whether we collectively cut our carbon footprints by upholding international agreements, such as the Paris Agreement, and by moving away from fossil fuels, or whether we continue on our high greenhouse gas path by keeping things business as usual.
To account for emissions uncertainty in climate projections, researchers have developed the Representative Concentration Pathways (RCPs). Designed by the international climate research community to support the work of the Intergovernmental Panel on Climate Change, the world’s climate science assessment organization, RCPs are added to global climate models to account for a range of human choices, from cutting emissions right away to doing nothing. Because more emissions will create more warming, the range of emissions pathways (sometimes called trajectories) also represents a range of temperature pathways from mild warming to extreme warming; trajectories, in other words, from the moderate to the menacing.
There are four RCPs to represent this range from low to high warming. CIRC primarily employs just two: RCP 4.5 and RCP 8.5. RCP 4.5 is a kind of middle-of-the-road scenario that assumes emissions will start being cut by the middle decades of this century, and hence warming will continue but will slow from its current rate. RCP 8.5 on the other hand is a high-warming scenario that assumes we continue on our current high emissions path. (The numbers in the names, if you’re curious, represent the amount of radiative forcing—trapped energy from the sun—that can be expected by the end of this century under each scenario.) Because we don’t know which path humanity will take, whenever possible CIRC researchers use both RCP 4.5 and RCP 8.5 in their climate projections. Keep this in mind as you read our findings.
A Note on the Paris Agreement
The RCPs are meant to represent a range of possible warming under climate change. This range extends outside the temperature goals set by the Paris Agreement, which seeks to hold global mean temperature below 2 degrees Celsius (3.6 degrees Fahrenheit) of warming above preindustrial levels by the year 2100. In fact, three of the four RCPS are likely to see increases of global mean temperature relative to preindustrial levels beyond 2 degrees Celsius (3.6 degrees Fahrenheit) by the end of this century. The exception is RCP 2.6, the lowest scenario in terms of emissions and warming.
IPCC, 2013: “Summary for Policymakers.” In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
Moss, Richard H., Jae A. Edmonds, Kathy A. Hibbard, Martin R. Manning, Steven K. Rose, Detlef P. Van Vuuren, Timothy R. Carter et al. “The next generation of scenarios for climate change research and assessment.” Nature 463, no. 7282 (2010): 747–756.
Rupp, David. E., John T. Abatzoglou, Katherine C. Hegewisch, Philip W. Mote. “Evaluation of CMIP5 20th century climate simulations for the Pacific Northwest USA,” Journal of Geophysical Research: Atmospheres 188 (2013). doi:10.1002/jgrd.50843.
Schleussner, Carl-Friedrich, Joeri Rogelj, Michiel Schaeffer, Tabea Lissner, Rachel Licker, Erich M. Fischer, Reto Knutti, Anders Levermann, Katja Frieler, and William Hare. “Science and policy characteristics of the Paris Agreement temperature goal.” Nature Climate Change (2016).
Van Vuuren, Detlef P., Jae Edmonds, Mikiko Kainuma, Keywan Riahi, Allison Thomson, Kathy Hibbard, George C. Hurtt et al. “The representative concentration pathways: an overview.” Climatic Change 109 (2011): 5–31.