Tag Archives: The Beginners Guide to Representative Concentration Pathways

Chart of the Day, 30 Jan 2015: Pick a Pathway (to Climate Nirvana or Climate Hell)

After yesterday’s post on China’s emissions, I will try to keep in a ‘cup half full’ frame of mind today.

The Intergovernmental Panel on Climate Change (IPCC) is an organisation for which I have great respect. But while their research may be applauded for its rigour,  communication with the wider world frequently lacks clarity (to put it mildly). Take, for example, the emissions scenarios, which in the Fifth Assessment Report (AR5) are called Representative Concentration Pathways (RCPs). Here are the RCPs and the change in temperature that accompanies them (Source: IPCC AR5, WG1).

Global Mean Temperature Change jpeg

I have spent many an hour grinding through IPCC reports trying to find clear explanations of what sits behind these pathways, but it is a painful process. Eventually, Skeptical Science saved the day by publishing “The Beginner’s Guide to Representative Concentration Pathways“.

There are four Representative Concentration Pathways: RCP 8.5, RPC 6, RCP 4.5 and RCP 2.6. The numbers refer to what is termed the ‘radiative forcing’, the change in net energy flow as measured in watts per square metre. Moreover, RCP 8.5 is expected to keep on increasing past 2100, RCP 6 and 4.5 will peak in 2100 and RCP 2.6 will have peaked prior to 21oo. Simplistically, a larger forcing means the globe will reach a higher mean temperature, as you can see in the chart above.

Surprisingly, AR5 is not particularly concerned with the socioeconomic assumptions that lie behind the RCPs. In this respect, the climate scientists behind the RCP concept are thinking the way economists often do: they are saying “imagine if we had condition X, what would be the output Y”. In this way, you can explore the model, and, hopefully, you obtain some insight which you can then take back to the real world.

I’m still a little uncomfortable with this. I think the IPCC should have chosen RCPs with highly transparent assumptions and realistic story lines. Instead, two of the four RCPs look utterly unrealistic to me. For example, to get to RCP 2.6 would require a transition away from fossil fuels that now looks impossible. And the good news? Well, RCP 8.5 looks barking mad to me too. Here are the emissions trajectories (from the Skeptical Science RCP report; click for larger image):

RCP Emission Trajectories jpeg

And if you concentrate on the blue line in the CO2 chart, you can see that around 24 giga tonnes of carbon are expected to be emitted in 2060. In yesterday’s blog post I was on working in units of tonnes of CO2. In the above chart, while the subject is CO2, the y-axis is in carbon. For those who have forgotten high school chemistry, you have to remember this:

CO2 jpeg

So when we move from calculations working in tonnes of carbon to tonnes of CO2 we have to multiple by 3.67 (44 ÷12) and vice versa. Joe Romm had a great piece in Climate Progress a while ago highlighting the number of people who get caught out through mixing up CO2 and carbon units. Accordingly, the 24 giga tonnes of carbon in 2060 is equivalent to about 88 giga tonnes of CO2. To put this in perspective, what are the big emitters putting out today:

Regional Emissions to 2019 jpeg

European emissions are already in decline and US emissions are flatlining. China’s emission growth will decelerate because its fixed-investment driven GDP growth model will come to the end of its natural life. China is also just about to enter its own demographic transition, and we have all see what such a transition did to Japanese economic growth (and by extension its emissions).

Obviously, India and other developing nations will increase their emissions, but they are unlikely to be able to replicate China’s export-led growth model. Further, with every passing year, the grid parity of renewables falls. Prime Minister Modi announced a push toward renewables when meeting President Obama. This was not just as a diplomatic gesture ahead of the Paris climate talks, but also as a pragmatic measure to buttress India’s energy independence and reduce the country’s exposure to volatile fossil fuel price movements.

So the cup half full is that RCP 8.5 looks unlikely–but the cup half empty is that RCP 6.5 is pretty awful climate-wise all the same.