What on earth is an RCP?

A quick guide to carbon dioxide emissions scenarios used by the IPCC Assessment Report 5

If you’re reading beyond the headlines about the recent climate change report, you’ll quickly hit lots of references to emissions scenarios called Representative Concentration Pathways (RCPs). I’d heard of these, but they were different from the scenarios used in the previous reports by the IPCC in 2001 and 2007. Here’s my summary of these scenarios based on an excellent guide by Graham Wayne at skepticalscience.com

Figure 1. Global carbon dioxide emissions (gigatonnes of carbon per year) under 4 scenarios with different population and economic growth and climate policies (van Vuuren etal, 2011)

RCPs are scenarios that describe alternative trajectories for carbon dioxide emissions and the resulting atmospheric concentration from 2000 to 2100. They encompass the range of possible climate policy outcomes for the 21st century. By agreeing on a limited set of scenarios, researchers (especially climate modelers) can be more sure they are comparing apples with apples when conducting their research and communicating their results.

The RCPs describe 4 different scenarios based on different assumptions about population, economic growth, energy consumption and sources and land use over this century. Details can be found at skepticalscience.com or the source for much of Wayne’s document ie van Vuuren etal (2011).

Figure 2. Atmospheric concentration of carbon dioxide (parts per million) under 4 scenarios

The scenarios are named after the level of “radiative forcing” that each scenario produces (measured in watts per square metre). While crucial to describing the mechanics of climate change, I’ve found that an understanding this term is not needed to comprehend the problem of climate change, its scale and its implications.

Notice how concentration continues to increase even after emissions slow and then drop. Carbon dioxide accumulates in the atmosphere and stays there for decades. Even if emissions start reducing in 2020, the concentration continues increasing and starts falling very slowly only after 2050. Understanding this relationship between our emissions today and the CO2 concentration experienced by our grandchildren is key to grokking the problem of climate change.

So now a brief look at each of the scenarios.

RCP 2.6

This scenario might be described as the best case for limiting anthropogenic climate change. It requires a major turnaround in climate policies and a start to concerted action in the next few years in all countries, both developing and developed.

Global CO2 emissions peak by 2020 and decline to around zero by 2080. Concentrations in the atmosphere peak at around 440 ppm in mid century and then start slowly declining.

Global population peaks mid century at just over 9 billion and global economic growth is high. Oil use declines but use of other fossil fuel increases and is offset by capture and storage of carbon dioxide. Biofuel use is high. Renewable energy (eg solar & wind) increases but remains low.

Cropping area increases faster than current trends, while grassland area remain constant. Animal husbandry becomes more intensive. Forest vegetation continues to decline at current trends.

RCP 4.5

Emissions peak around mid century at around 50% higher than 2000 levels and then decline rapidly over 30 years and then stabilise at half of 2000 levels. CO2 concentration continues on trend to about 520 ppm in 2070 and continues to increase but more slowly.

Population and economic growth are moderate but slightly lower than under scenario RCP 2.6

Total energy consumption is slightly higher than RCP 2.6 while oil consumption is fairly constant through to 2100. Nuclear power and renewables play a greater role.

Significantly, cropping and grassland area declines while reforestation increases the area of natural vegetation.


In this scenario, emissions double by 2060 and then dramatically fall but remain well above current levels. CO2 concentration continues increasing, though at a slower rate in the latter parts of the century, reaching 620 ppm by 2100.

Population growth is slightly higher peaking at around 10 billion. This scenario assumes the lowest GDP growth of the four.

Energy consumption increases to a peak in 2060 then declines and levels out to finish the century at levels similar to RCP2.6. Oil consumption remains high while biofuel and nuclear play a smaller role than in the other 3 scenarios.

Cropping area continues on current trend, while grassland area is rapidly reduced. Natural vegetation is similar to RPC4.5

RCP 8.5

This is the nightmare scenario in which emissions continue to increase rapidly through the early and mid parts of the century. By 2100 annual emissions have stabilised at just under 30 gigatonnes of carbon compared to around 8 gigatonnes in 2000.

Concentrations of CO2 in the atmosphere accelerate and reach 950 ppm by 2100 and continue increasing for another 100 years.

Population growth is high, reaching 12 billion by centuries end. This is at the high end of the UN projections. Economic growth is similar to RCP6 but assumes much lower incomes and per capita growth in developing countries.

This scenario is highly energy intensive with total consumption continuing to grow throughout the century reaching well over 3 times current levels. Oil use grows rapidly until 2070 after which it drops even more quickly. Coal provides the bulk of the large increase in energy consumption

Land use continues current trends with crop and grass areas increasing and forest area decreasing.

Climate forecasts

With this information on board I can try to make at least some sense of the forecasts included in the IPCC’s latest report. Figure 3 below is a chart showing forecast temperature change under the best (RCP2.6) and worst (RCP8.5) scenario.

Figure 3. Projected global surface temperature change under different emissions scenarios. Zero is set at the average of 1986-2005 levels (Figure SPM.7(a). IPCC Working Group I Assessment Report, Summary for Policy Makers, 2013)

If we get it together and emissions peak by 2020 and reduce to zero this century (ie RCP2.6), global temperature could be stabilised at around 1°C above levels in the late 1900's. The IPCC say it is unlikely (<33% probability) that the rise will exceed 2°C.

On the other, hand if we carry on as if there is no problem without even a slow down in emissions growth until late in the 21st century (ie RPC8.5) , the forecast outcome is not pretty. Temperatures are forecast to continue increasing and by 2100 and reach around 4°C higher than late 20th century levels. The likely range of outcomes for 2100 is approximately 3°C to 5.5°C higher.

While these projections are similar to those produced by the IPCC in 2007, the prospect of a +4° or higher world seems more possible or even probable now 5 years later. The colossal impacts and implications of a +4° or +6° world are better understood now than in 2007. The IPCC’s Working Group II will release their report on impacts, adaptations and vulnerability in March 2014. I don’t expect it to be fun reading.


Working Group I Contribution to the IPCC Fifth Assessment Report. Climate Change 2013: The Physical Science Basis. Summary for Policymakers (draft 27 September 2013) (pdf)

Wayne, G.P. (2013) The Beginner’s Guide to Representative Concentration Pathways, Version 1.0 August 3013 www.skepticalscience.com

van Vuuren, D.P. et al (2011), The representative concentration pathways: an overview. Climatic Change, Vol 109, Issue 1-2, pp 5-31 Springerlink Open Access