There is now something of a religious fervor around the international goal of 2°C, to the extent that it is almost impossible to discuss other trajectories or outcomes. The only contrast that seems possible with 2°C is something that nobody wants, which is the “do nothing” scenario of 4°C or more.
Yet the 2°C pathway is hardly clear cut in itself. A recent series of discussions in a business group I attend has highlighted the range of myth, confusion and misinformation that surrounds the current goal. Given that this is an international goal that most nations subscribe to, exactly where are we headed? The number itself has been around for a while, but it was finally agreed at the Cancun COP16 after first appearing in the text emanating from Copenhagen. The Ad-hoc Working Group on Long Term Cooperative Action agreed the following in Cancun:
Further recognizes that deep cuts in global greenhouse gas emissions are required according to science, and as documented in the Fourth Assessment Report of the Inter- governmental Panel on Climate Change, with a view to reducing global greenhouse gas emissions so as to hold the increase in global average temperature below 2 °C above pre- industrial levels, and that Parties should take urgent action to meet this long-term goal, consistent with science and on the basis of equity; also recognises the need to consider, in the context of the first review, as referred to in paragraph 138 below, strengthening the long-term global goal on the basis of the best available scientific knowledge, including in relation to a global average temperature rise of 1.5 °C;
The text itself lays out an intention, but translating this into something tangible is easier said than done. It also turns out to be quite a divisive process and requires a deep dive into some reasonably complex statistics. This was perhaps best highlighted by the paper Greenhouse-gas emission targets for limiting global warming to 2°C, Malte Meinshausen, Nicolai Meinshausen, William Hare, Sarah C. B. Raper, Katja Frieler, Reto Knutti, David J. Frame & Myles R. Allen, Nature Vol 458 / 30 April 2009 (a copy is currently available here). Meinshausen et. al. showed that the uncertainty of the climate response combined with a variety of emission pathways delivers given probabilities for staying below 2°C, depending on the cumulative emissions over the period 2000-2049, with some indication of eventual outcome also given by emissions in 2020.
Excerpts from the table in the paper, giving probabilities of exceeding 2°C are shown below:
This is all very well, but the next step is the tough one. The call at Cancun was to “hold the increase below 2°C”, but this means different things to different people. At the meeting I attended recently, some interpreted this as meaning a “reasonable probability”, which was then interpreted as 75%. The table above shows that this means a limit on cumulative emissions between 2000-2049 of 1,000 Gt CO2. But with emissions from 2000-2013 already totalling about 470 billion tonnes, that leaves a remaining budget until 2050 of just 530 billion tonnes. That’s about 14 years of full on emissions, or for example, a trajectory that requires an immediate peak in emissions followed by year on year reductions of about 1.2 billion tonnes until emissions are near zero. Delaying the peak until 2020 pushes up the reduction rate to nearly 3 billion tonnes per annum.
By contrast, accepting a 50% probability gives a very different outcome. Emissions can peak in 2020 and a reduction pace of 1 billion tonnes per annum is then required. Alternatively, should emissions plateau in 2020 and start reducing in 2025, the annual effort rises to 1.5 billion tonnes. These are still very challenging numbers, but almost a world apart from the 75% probability case. The 75% case is most likely unachievable given where the world is today.
What was clear from the meeting I attended was that two people who may both talk about 2°C have very different perspectives on likelihood, usually without any thought as to the reduction implications behind their assumption. The EU is at least clearer on this in its main publication on the 2°C Target, where it notes in the key messages, “In order to have a 50% chance of keeping the global mean temperature rise below 2°C relative to pre-industrial levels . . . . .”.