The highlight of the Paris Agreement is without question the ambition embodied within it. This had its foundation with the Alliance of Small Island States (AOSIS) and their deep concern regarding future sea level rise. But the issue snowballed as the conference progressed, supported by a strong dose of techno-optimism that was prevalent throughout the halls of the Le Bourget Conference Centre. The text that was agreed upon is important, with the goal embodied in to distinct sections;
Holding the increase in the global average temperature to well below 2 °C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 °C above pre-industrial levels, recognizing that this would significantly reduce the risks and impacts of climate change;
Parties aim to reach global peaking of greenhouse gas emissions as soon as possible, recognizing that peaking will take longer for developing country Parties, and to undertake rapid reductions thereafter in accordance with best available science, so as to achieve a balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the second half of this century . . .
In a post written before the conclusion of COP21, I assessed that a 1.5°C goal would require a rapid forty year transition to net-zero anthropogenic emissions and a period until at least the end of the century with negative emissions via BECCS (bioenergy and CCS) and DACCS (direct air capture and CCS). But the pathway proposed by the Agreement itself isn’t quite as ambitious, even while it aspires to a 1.5+°C outcome. Rather, it proposes achieving a balance between anthropogenic emissions and removals by sinks in the second half of the century. This may not be sufficient to achieve the 1.5+°C goal, with a key deciding element being the role of natural sinks.
The 1.5+°C pathway issue is highlighted in a paper published by the MIT Joint Program in July 2013. MIT deliberately avoided the use of negative emissions technologies, partly due to concerns about their scalability but also preferring to test the impact of natural sinks on the outcome. Of these, the ocean is the major short term sink because of the imbalance between levels of CO2 in the ocean and the atmosphere.
MIT analyzed four pathways that result in net zero anthropogenic emissions. These are shown in the chart below (fossil energy CO2 emissions only) against a business as usual trajectory based on the 2010 post-Copenhagen national pledges.
- An immediate drop to net zero by 2015, starting in 2010 (Natural only after 2015).
- A very rapid drop to net zero by 2035, but with growth from 2010 to 2030 (Natural only after 2035).
- A more extended drop to net zero by 2060, with the decline commencing in 2010 (Alternative).
- The IEA 450 scenario, with emissions peaking around 2020 and reaching net zero by 2070 (IEA 450).
Pathway 3 is of particular interest. In this case anthropogenic emissions are at net zero by 2060, although starting to decline from 2010 when energy emissions are at 30 Gt CO2 per annum (it is now 2016 and they are at ~33 Gt). This scenario sees temperatures rise above 2°C by mid-century, but then decline as the ocean takes up significant quantities of CO2 from the atmosphere but with nothing being added from anthropogenic sources. After some 20-30 years, as the ocean’s upper layer comes into balance with the atmosphere, uptake of CO2 slows. Mixing into the deep ocean is much slower but will continue for hundreds to thousands of years.
Back in 2010 the cumulative emissions from 1750 (to 2010) stood at some 532 billion tonnes carbon, which means that Pathway 3 approximates a 1.5°C outlook as the area under the curve from 2010 to 2060 (energy, cement and land use) represents an additional 250 billion tonnes of carbon emissions, giving a total of some 780 billion tonnes. The relationship between carbon emissions and temperature is about 2°C per trillion tonnes. The chart below shows the modelled pathway which results in an end-of-century temperature rise of 1.5°C.
The natural sink is therefore very important, offering some 0.5°C (see the light blue line in the chart above) of temperature reduction following an overshoot. This is possibly the only way in which 1.5°C can be met, although significant anthropogenic sinks may also be developed (including reforestation) later which could offer the same drawdown. As such, with the Paris Agreement potentially not making use of this and instead only providing for emissions to fall to a level which matches the ability of sinks to take up carbon emissions, the task of meeting 1.5°C becomes considerably more difficult.
The same is true of the IEA 450 Scenario. With 2010 now behind us, the future equivalent of the Alternative pathway which saw reductions from 2010 onwards is probably the red 450 line (reductions from 2020), which overshoots to 2.7°C before achieving something of a plateau at 2°C. But to bring this down further by the end of the century and therefore comply with the Paris Agreement would also require the major application of anthropogenic sinks, such as via CCS and rapid reforestation.
This discussion may be something of a moot point today because the job of rapidly reducing emissions hasn’t even started and arguably we have at least 40+ years to think about where the endpoint should be. Nevertheless, as nations begin to reflect on the Paris outcome in the coming months and relook at their respective reduction pathways, the long term end point does become relevant because energy infrastructure planning requires a multi-decadal outlook. In its initial formulation of a long term carbon budget, the UK did need to look forward to 2050 but that was from a 2008 starting point. With a new starting point of 2020 or thereabouts, a 2060 or even 2070 end-point may well be considered.
There is of course a disturbing flip side to this story – continued rapid uptake of CO2 by the ocean also gives rise to increasing levels of ocean acidification.