Misuse of Scenarios in the Fourth U.S. National Climate Assessment

Updated: 25 Feb 2020

This post is simply to collect some notes for my future use. Details after the break …

The U.S. National Climate Assessment misused scenarios — and in particular RCP8.5 — in its 2017 and 2018 reports.

1. From the 2017 report: “RCP8.5 implies a future with continued high emissions growth, whereas the other RCPs represent different pathways of mitigating emissions.” (2017, p. 16) And also “all of the three lower RCP scenarios (2.6, 4.5, and 6.0) are climate-policy scenarios” and “At the higher end of the range, the RCP8.5 scenario corresponds to a future where carbon dioxide and methane emissions continue to rise as a result of fossil fuel use…” (2017, p. 136).

Similarly from the 2018 report: “The higher scenario (RCP8.5) represents a future where annual greenhouse gas emissions increase significantly throughout the 21st century before leveling off by 2100, whereas the other RCPs represent more rapid and substantial mitigation by mid-century, with greater reductions thereafter” (2018, p. 41).

The clear implication here is that RCP8.5 is a baseline scenario (“continued high emissions growth”) and the others are climate policy scenarios (“different pathways of mitigating emissions” and “climate-policy scenarios”). This implication is wrong.

According to the RCP Scenario Database: “The RCPs are four independent pathways developed by four individual modeling groups. The socioeconomics underlying each RCP are not unique; and, the RCPs are not a set or representative of the range of potential assumptions. For instance, the RCPs with lower radiative forcing (RCP 6.0, RCP 4.5 and RCP 2.6) are not derived from those with higher radiative forcing (RCP 8.5, or even RCP 6.0). The differences between the RCPs can therefore not directly be interpreted as a result of climate policy or particular socioeconomic developments.”

According to Moss et al. (2010): “The RCPs cannot be treated as a set with consistent internal logic. For example, RCP8.5 cannot be used as a no-climate-policy reference scenario for the other RCPs because RCP8.5’s socioeconomic, technology and biophysical assumptions differ from those of the other RCPs.”

Confusingly, the USNCA says as well “RCP8.5 reflects the upper range of the open literature on emissions, but is not intended to serve as an upper limit on possible emissions nor as a business-as-usual or reference scenario for the other three scenarios” (2017, p. 136).

2. From the 2017 report: “The observed increase in global carbon emissions over the past 15–20 years has been consistent with higher scenarios (e.g., RCP8.5) (very high confidence).” (2017, p. 31)

Similarly, from the 2018 report: “Current trends in annual greenhouse gas emissions, globally, are consistent with RCP8.5.” (2018, p. 41)

This statement is largely correct with respect to total (FF&I plus LUC) carbon emissions, but it is at the same time highly misleading. Below is a graph from @Peters_Glen showing FF&I carbon dioxide emissions, and it is clear that even by 2016 the trajectory of emissions growth had fallen well below RCP8.5. This matters because the IPCC has projected future emissions to be dominated by FF&I with LUC playing a lesser role.

Below is another graph from @Peters_Glen showing total carbon dioxide emissions versus the RCP projections. Even factoring in LUC, by 2016 it was clear that total emissions had fallen below a RCP8.5 trajectory.

In fact, the USNCA cited data indicating that observed emissions had fallen below RCP8.5:

“The evidence that actual emission rates track or exceed the higher scenario (RCP8.5) is as follows. The actual emission of CO2 from fossil fuel consumption and concrete manufacture over the period 2005–2014 is 90.11 Pg. The emissions consistent with RCP8.5 over the same period assuming linear trends between years 2000, 2005, 2010, and 2020 in the specification is 99.24 Pg.” (2017, p. 152)

The USNCA also included a table showing that FF&I emissions had fallen below RCP8.5 (2017, 152). (Note: the numbers in the table do not match the numbers in the corresponding paragraph above, so there may be an error or typo here — regardless, from 2011 to 2015 there was clear evidence of a growing divergence between observations and RCP8.5)

The USNCA notes awareness of the divergence: “Medium confidence in recent findings that the [emissions] growth rate is slowing.” (2017, p. 153)

The misuse of short-term trends versus IPCC scenarios was noted in Manning et al. (2010) who argued that misinterpretations of scenarios was leading some to “to incorrectly conclude that current emissions are higher than the values used in climate change projections. This may be spreading into general reviews of climate change science …”

Indeed, van Vuuren and Riahi (2008) concluded: “We find that there are good reasons to assume that the rise in emissions in the long term will be less rapid than in recent years.”

The USNCA did not cite either of thee papers.

3. The 2018 frequently uses RCP8.5 as a reference scenario — functionally equivalent to what is commonly called “business as usual” — in its economic analysis, contrary to claims made elsewhere in the USNCA.

For instance in the 2018 report: “Figure 1.21 provides an illustration of the type of economic information that is integrated throughout this report. It shows the financial damages avoided under a lower scenario (RCP4.5) versus a higher scenario (RCP8.5).” (2018, p. 71)

Also in the 2018 report: “Comparing outcomes under RCP8.5 with those of RCP4.5 (and RCP2.6 in some cases) not only captures a range of uncertainties and plausible futures but also provides information about the potential benefits of mitigation. ” (2018, p. 1415)

This is a completely improper use of the RCP scenarios as they are not supposed to be compared in this manner in order to assess the benefits of mitigation (see Moss 2010 cited above). This is utilizing RCP8.5 as a baseline and RCP4.5 to represent climate policy, and then expressing differences between them as benefits of mitigation. This is an erroneous application of the RCPs and is in fact the use of RCP8.5 as a BAU scenario.

4. The NCA 2018 characterizes sea level rise under RCP8.5 as an “intermediate” projection. This is erroneous.

“Recent economic analysis finds that under a higher scenario (RCP8.5), it is likely (a 66% probability, which corresponds to the Intermediate-Low to Intermediate sea level rise scenarios) that between $66 billion and $106 billion worth of real estate will be below sea level by 2050; and $238 billion to $507 billion, by 2100.” (2018, p. 330)