The case for limiting the rise in global temperatures to 2°C was made many years ago and finally agreed at COP16 in Cancun in 2010. But the text noted the importance of an even more aggressive target, notably 1.5°C, proposed by the small island states who were deeply concerned about future sea level rise. While 1.5°C doesn’t guarantee to limit sea level rise such that certain island nations remain safe, it does further shift the global risk profile in terms of possible major changes in the ice shelves.

The idea of a 1.5°C goal has remained largely in the background since 2010, but COP21 has brought the issue to the forefront of negotiators minds, with a reported group of some 100 countries now willing to support such an objective. At a reception early in the second week, the UK Climate Minister was very upbeat about the 1.5°C goal and the government’s role in working with AOSIS (Alliance of Small Island States). At the COP Plenary on Wednesday night (9th December), many groups and nations spoke about the need for a 1.5°C goal. But while there is increasing enthusiasm for and talk about such a goal, there seems to be limited substantive discussion on the feasibility of achieving it.

As often discussed in my postings, the expected global temperature rise is closely linked with cumulative emissions over time, not the level of emissions in a certain year. This means that what might have seemed achievable in 2010, is all the more difficult in 2015 with higher emissions and continued upward pressure. In fact, between 2010 and 2015 another 60 billion tonnes of carbon has been released into the atmosphere. Total emissions since 1750 now stand at just under 600 billion tonnes carbon, with 1.5°C equivalent to some 750 billion tonnes carbon based on a climate sensitivity of 2°C per trillion tonnes. Even if emissions were to continue to plateau as we have seen over 2014-2015, the 1.5°C threshold would be reached as early as 2028.

There are always a variety of trajectories possible for any temperature goal, but 1.5°C offers little room for flexibility, given its stringency. One such pathway which adds up to ~750 billion tonnes carbon by 2100 is shown below (global CO2 emissions on the vertical scale). In this pathway, global net zero emissions must be reached in just 40 years (860 billion tonnes accumulation), followed by another half century of atmospheric carbon removal and storage (~100 billion tonnes removal). Some 10 billion tonnes of CO2 must be removed and stored each year by late in the century, either through bio-energy with carbon capture and storage (BECCS) or direct air capture of CO2 and subsequent storage (DACCS). Significant reforestation would also play a major role. With infrastructure in place, the 22nd century might even offer the possibility of drawing down on CO2 below a level that corresponds with 1.5°C.

Apart from massive reliance on CCS both on the way to net zero emissions and afterwards to correct the over accumulation, such a plan would require a complete rebuild of the energy system in just 40 years. This would include the entire industrial system, all transport and power generation. Alternatives would have to be found for many petroleum based products and a new large scale synthetic hydrocarbon industry would be needed for sectors such as aviation and shipping. While agriculture is largely a bio based emissions system, a solution to agricultural methane emissions would also nevertheless be needed.

A pathway that doesn’t involve future use of CCS would require net zero emissions in just 23 years – an option that isn’t even remotely feasible. Returning to the 40 year pathway, even this presents an immensely challenging task. While it might be feasible to have a zero emissions power sector in under 40 years, particularly given that all the necessary technologies to do so exist in one form or another, electricity still represents only 20% of final energy use. Solutions would have to be found for all other sectors, which in many instances involves electrification and therefore places a significant additional load on the redevelopment of the power generation system. Aviation would be particularly tricky.

Finally, there is CCS itself. The pathway above (and almost any other 1.5°C pathway) is completely dependent on it, yet the technology is hardly deployed today. It is certainly commercially ready, but the barriers to deployment are many, ranging from the lack of an economic case for project development to public concern about deep storage of carbon dioxide. The later that net zero emissions is reached, the greater the post net zero dependence on CCS becomes.

While the case for 1.5°C has certainly been made from a climate perspective, it has yet to be demonstrated from an implementation perspective.