Ask and ye shall receive. I recently wrote about what future GHG emissions scenarios included in terms of emission reduction requirements. I have maintained for some time now that most of the IPCC’s emission and concentration scenarios are essentially useless for practical planning purposes. Sure, they’re interesting academically, but we climate scientists can’t just study something for the sake of studying it in today’s tight federal budget environment.
In that post, I showed some graphics from a 2013 Nature paper which combined historical emissions as well as projected emissions. Due to the article’s age, I had to search for additional data which showed more recent emissions. I also showed a simple calculation of projected emissions assuming constant 2.1% annual emissions growth and how different emissions growth would have to be in order to achieve an emissions scenario many scientists characterize as ‘doable': RCP4.5.
Well, a new Nature Climate Change paper (26Feb2014) updates the 2013 graph I showed, with some small changes:
Figure 1. Historical (black dots) and projected CO2 emissions from a Nature Climate Change article (subs. req’d). Bold colored lines (red (RCP8.5), yellow (RCP4.5), green (RCP6), and blue (RCP2.6)) represent IPCC AR5 RCP-related emission scenarios.
Note that this figure shows exactly what I wrote about in my earlier post: historical emissions are tracking at or above the RCP8.5 scenario. They also exceed the other three scenarios so far in the early 21st century. These differences are relatively small so far (they will grow with time), but the trend difference between historical and RCP2.6 is already important. As the figure shows, if we wanted to match RCP2.6 (and keep 2100 global mean annual temperatures near 2C above pre-industrial), emissions would have to be declining for multiple years already. They aren’t. Our actual annual emissions already exceed the annual maximum assumed by RCP2.6. If we were to match RCP2.6 at some time in the future, emission reductions would have to be larger than RCP2.6 assumes, which is currently technologically impossible.
The figure also shows that if we continue at or along the RCP8.5 pathway, we will exceed the 2°C policy target by approximately 2046. The paper begins with this short and sweet abstract:
It is time to acknowledge that global average temperatures are likely to rise above the 2 °C policy target and consider how that deeply troubling prospect should affect priorities for communicating and managing the risks of a dangerously warming climate.
And it includes this well-written paragraph:
This global temperature target has brought a valuable focus to international climate negotiations, motivating commitment to emissions reductions from several nations. But a policy narrative that continues to frame this target as the sole metric of success or failure to constrain climate change risk is now itself becoming dangerous, because it ill-prepares society to confront and manage the risks of a world that is increasingly likely to experience warming well in excess of 2°C this century.
I wouldn’t have used the term `dangerous` because it conveys a judgmental aspect to an objective statement. But that’s personal style. I agree completely with the underlying message. If we have a small (I would say nearly zero) chance of keeping warming below 2°C this century, then 2°C shouldn’t be the target. We can make an infinite number of possible targets, but most of them will be unachievable. How much effort should we put into such targets? How supportive of additional climate policies will the public be if initial targets fail early? These aren’t simply academic questions. Many climate activists think they’re convinced of how important action is, but their rhetoric doesn’t support that conviction. They’re more ideological than they’d care to admit.
I met someone at a talk at the University of Colorado on Monday and ended up having lunch with them to exchange economic information for climate information. I tried to convince them of the need to switch targets now, to no avail. I ran into a basic problem of climate communication. This person has a worldview and I was in the unenviable position of trying to modify that worldview. Just as many climate communicators try to do with climate skeptics. It’s incredibly difficult to do this because you’re dealing with a lifetime of information and experience overlaying a biology that is predisposed to that very worldview.
I will continue to post about historical versus projected emission/concentration pathways. If activists really are supportive of the objective science as they claim, I think they will eventually shift their target. They will of course have to come to terms with what they will initially perceive as a failure. But the faster they can do that, the sooner we can set more reasonable and achievable targets and start making headway towards mitigation.