The World Resources Institute has put together a very helpful infographic showing the four example emissions pathways (RCPs, or representative concentration pathways) used in the latest IPCC reports. Well worth using if you’re giving a talk on the area – I’m adding it to mine. I took the opportunity to look a bit more into the carbon budget numbers, and found a few things which initially confused me, so wanted to set them out here to caution others.
The standard figure given for the total budget is a nice round figure: a total 1000Gt (gigatons, or billions of tons) of carbon since the start of the industrial revolution: if we stay under that we have a 2/3rds chance of staying under a 2C temperature rise. (Here’s the first thing to avoid: that’s carbon, not carbon dioxide, which some sources quote instead, multiplying all figures by about 3.67.) We’ve emitted a total of 515Gt so far, so that’s 485Gt left, right? (See the WRI’s previous infographic.) On this basis, the WRI’s forecasts look reasonably friendly: we stay under budget on the low pathway (RCP2.6) and don’t exceed it until the 2040s or 2050s on the other three. Plenty of time to take action. (For reference, current emissions are close to 10Gt per year.)
However: this doesn’t allow for two other factors. Firstly, emissions of other greenhouse gases, which contribute a substantial proportion of warming. The IPCC Working Group I report from which the 1000Gt budget is taken notes (see page 27 of the summary for policymakers) suggests an alternative figure of 790Gt – or a budget down from 485Gt to 275Gt – if we are looking at the low pathway (RCP2.6) – there is a further complication as the other greenhouse gases tend to be much more potent but not stay in the atmosphere so long, so it’s impossible to reduce everything to CO2-equivalents as one might like. Other calculations you can find online use this figure (or 800: the figure was revised just before publication) so coming up with much more pessimistic forecasts.
Secondly, the way the WRI puts things suggest that it is possible to get near the time the budget is due to run out, take action, and all will be fine. What this doesn’t take into account of course is the massive amount of inertia in the system: we simply can’t just stop emitting carbon altogether (barring complete socio-economic collapse). One report I’ve seen suggests that the last date we can seriously start taking action is 2017: beyond this we will have built sufficiently much carbon-using infrastructure that simply using everything to the end of its natural lifespan will be too much.
There’s a final issue, which is the large uncertainties in the IPCC’s figures. They don’t put error bars on the budget figures, but they do on the emissions to date: 515 [445-585], which in their notation means a 90% chance of being within that range. 70Gt either way makes a substantial difference.
So, onto the WRI’s figures. On the larger 485Gt budget, the lowest pathway (RCP2.6) stays under, the medium one (RCP4.5) passes it in 2056, the high (RCP 6.0) in 2057, the highest (RCP8.5) in 2045.
How about the lower budget, to give us a 66% chance of staying under 2C. Our emissions are currently tracking RCP8.5 reasonably closely, with an average emissions rise of about 2% a year. If we continue on this path, we exceed the 275Gt budget in 2033. (Note: I’m using my own approximations rather than the official figures for ease of calculations so we may be slightly out, but my answers are within a year of WRI’s where we’re doing the same model.) How about if we cut emissions from 2020? Even with a 4.2% a year cut from then on, which is close to the RCP2.6 model, we still exceed the budget by 2043 and end up in the territory of between 33-50% chance of avoiding a 2C rise. A rather more swingeing cut of about 6.5% pa is needed to stop exceeding the 275Gt/66% budget. I think that’s at or about the top end of what is considered possible.
One other thing to note: in the WRI figures, the medium pathway seems slightly worse than the high one when it comes to meeting the carbon budget (2056 v 2057). This is an artefact of the pathway models which I hadn’t spotted before: the four are based on four independent models from different research groups, and the “high emissions” one has lower emissions than the other three initially, and lower than the medium one until the late 2030s. A bit unfortunate.
What do we conclude from all this? Firstly, due to the various uncertainties it is difficult to communicate climate figures in a way which is both simple and accurate. Secondly, the situation is urgent (or very likely so) – 2020 isn’t too far off, and the need for strong concerted action is greater than ever. Even the lowest emissions pathway has quite strong negative consequences – and the infographic brings that across well, even if, as I hope is clear, I have difficulty with their figures for carbon budgets.