A new Nature Climate Change editorial (subs. req.) has a very useful graph (2 variants) that I have been looking for:
Note first the y-axis: global CO2-emissions in Petagrams of carbon per year. This unit is different from the other common unit used: CO2 concentrations. The emissions eventually lead to the concentrations. This is only the CO2 emissions, not CO2-equivalent, which might be a better variable but introduces more complexity in analysis.
Let’s go through the lines on the graph before we discuss them. The “IS92” lines (a-f; light blue dashed) were the emission scenarios developed for the 1992 Supplementary Report to the IPCC Assessment. There are 40 SRES scenarios shown (thin green lines) and 6 illustrative scenarios (thick green dashed lines) that the IPCC developed for the 4th Assessment Report (AR4). These are the scenarios most people discuss: A1B, B2, etc. For the upcoming AR5, CO2 emissions form the basis of the scenarios. There are ways to convert from one to the other, which is how all of these different scenarios can be plotted together. The AR5 scenarios are labeled according to the anomalous forcing value expected in the year 2100 and a “Representative Concentration Pathway”. Thus, RCP3 represents 3 W/m^2 forcing due to CO2 concentrations. You can see what has to happen to global emissions to achieve this relatively low forcing value by the end of the century. Alternatively, there is an RCP4.5, RCP6, and RCP8.5 pathway. As a side note, my work will likely utilize the RCP8.5 pathway because we will most likely continue to move down this pathway for the foreseeable future.
Historical emissions are the black dots/line. The estimate for 2012 emissions is the red dot. It is obvious to see that our historical emissions has tracked near the top of any set of emissions scenarios (IS92-E, IS92-F, A1FI, A2, and A1B) and not the middle or bottom. That has implications in climate policy because most scientific studies performed to date have focused on the low to moderate scenarios. The reason is simple: most climate scientists thought there would be no chance of inaction once people saw what was likely to happen using even low or moderate emission scenarios. In general, scientists were wrong. The world has continued to increase the amount of CO2 emitted into the atmosphere, with “average annual growth rates of 1.9% per year in the 1980s, 1.0% per year in the 1990s, and 3.1% per year since 2000,” as I’ve covered in 2011 and earlier in 2012. The post-2000 increase is largely due to China and India.
The lead author of the report posted different form of this graph and included yet another call to action that the world will ignore:
A quick note: the RCP3 scenario’s absurdity becomes more clear post-2060: emissions have to turn negative to achieve 3 W/m^2 by 2100! Is anyone aware of technologies that remove emissions from the atmosphere in excess of what we emit to the atmosphere? Put another way, emissions would have to decrease to near-zero in addition to deployment of removal infrastructure. I obviously wasn’t involved scenario development, but it strikes me as incredibly myopic to include this pathway in climate scenarios: it exists only as a fantasy, especially when you realize that important feedback processes are still not understood well enough to include them in modeling efforts.
The main point of this graph is valid though: on our current emissions trajectory, global warming of 4–6.1 °C is likely. Given recent studies showing more sensitivity to temperature changes one order of magnitude less than this that has already started to generate real-world changes, no one can say with certainty what a 4°C rise in global temperature above the pre-industrial average will cause.
Now, I must make a very important point here. This does not mean the end of civilization or the world. Our species is remarkably adaptive to a wide range of conditions. While our species has never lived in a world that warm, we have enormous advantages over our ancestors: technology. The world might not look like it does today, and we will of course not live in the same way, but I firmly believe that whatever changes we make will allow the great majority of us to continue to live.
That is not to say that we should do nothing at all. I have made quite clear that the current approach (UNFCCC & IPCC) has proven itself to not work. I do not know exactly what the correct approach will be, but I think remaining in a failed paradigm is a bad idea moving forward. We must make new efforts – the more the merrier in the short-term so we can evaluate what does and does not work. My line of thought has developed to this: I think groups must initiate smaller efforts, and indeed I think in some cases they already have. Regional cohesive groups generally know better what works for them and why. A good place to start on a larger scale would be to work to understand why certain actions work in some places better than others and put policies in place to exploit those opportunities.
But 2°C is not achievable by any means that I can see. Neither is 350ppm CO2 concentration. Scientists and activists alike should cast aside these hard to understand numbers. A focus on other goals: energy portfolios, land use, and adaptation plans make more sense (different numbers since we tend to operate that way).