If we continue like this, the Paris Agreement will fail

At the 12th of December, 2015 in Paris, 196 countries agreed on the common goal of limiting global warming to well below 2.0, preferably well below 1.5 degrees centigrade compared to pre-industrial levels. The so-called Paris Agreement went into force on November 4th, 2016 (ratified now by 191 parties), so we are approaching fifth anniversary.

It's clear what is needed to reach the goal: end blowing greenhouse gases into the atmosphere, which means stop burning fossil fuels, and save or raise the earths carbon binding capacity. And rapidly so. Science tells us that net global carbon emissions need to fall by about 45% from 2010 levels by 2030 and reach "net zero" by around 2050 to meet the 1.5 degrees target (20% and 2075 for the 2 degrees target), as illustrated below ("overshoot" means a temporary increase of temperature above target before it settles): 

So, how is mankind doing? Let's have a look at the data. Spoiler: well, let's say there is room for improvement.

But let's clarify the game that is played under the Paris agreement first. Essentially, it is what social psychologists and economists call a Voluntary Contribution Mechanism, or VCM [2]: each contracting party ("player") decides independently and voluntarily how much to contribute to the goal in the form of greenhouse gas abatement. Actually, the players merely pledge contributions, as abatement requires economic and social transformations that do not realize instantaneously. The technical term for those pledges is "nationally determined contributions" (NDCs). The contributions are voluntary as there are no formal sanctions, neither rewards for contributing a lot, nor punishments for contributing little (or nothing). The parties are not even bound to their pledges. Thus, the success of the Paris Agreement depends on, first, voluntary pledges and, second, voluntary compliance with those pledges. Indeed, a lot of voluntariness in there, but that was the price of having an agreement in the first place. 

The expectation is that publicity somehow creates incentives for the players to be ambitious: NDCs are published openly (see also this nice spreadsheet), and are therefore subject to public and scientific scrutiny. In addition, revised NDCs are to be submitted in a five-year cycle (i. e. end of 2020, 2025, 2030, and so on), creating opportunity to check whether pledges are in line with the sub-1.5 or sub-2.0 targets. Again, the parties are free not to revise, but they are "encouraged" to be increasingly ambitious with their NDCs.

The first revision cycle ended just a few months ago in December 2020, the first opportunity to take stock. In a nutshell: it's just not enough, by far. The United Nations published an initial synthesis report (another to come at a yet unspecified date) in February that examines the combined consistency of pledges with the goal of the Agreement [3]. 48 countries submitted a new or updated NDCs at the time the report was compiled (by the time of writing this post, the number was at 59). The combined emissions cuts of those pledges are around 3% lower by 2030 than in the previous round of pledges submitted in 2015. However, assuming that all (new and old) pledges will be fully implemented, combined emissions would be just 0.5% lower in 2030 than in 2010 (or 0.7% lower than in 1990). That's not even close to the 45% that would be required to keep up with the sub-1.5 target (or the 20% for the sub-2.0 target). At least, with the new pledges, emissions are projected to peak before 2030 in the most optimistic scenarios, which was not the case with the previous set of NDCs submitted in 2015 (figure taken from p. 22 of the UN report):

There is a great lot hiding behind the world "optimistic", however. Recall that NDCs are pledges, nothing less, nothing more. Setting a goal of, say, cutting greenhouse gas emissions by 55% until 2030 (like the EU) is fine for indicating direction. But every sportsman and sportswoman know: a goal without a plan is just a wish – and committing to an ambitious plan requires leaving the comfort zone, entering the unknown, going for risks, and bearing a cost – and eventually success is still not guaranteed. Likewise, even if we assume all the best intentions, that is, that the Paris Agreement parties want to implement their pledges, there is no guarantee that they can and will. There are technical constraints, and perhaps more importantly, there are political constraints to overcome. Taking these into account, optimism is difficult to justify [4].

Indeed, Peiran Liu and Adrian Raftery, statisticians from the University of Washington in Seattle, recently quantified the chances of meeting the Paris Agreement targets and the likelihoods that the parties will actually meet their NDCs in the form of statistical probability margins [5]. They use the Coupled Model Intercomparison Project Phase 5 (CMIP 5) ensemble of climate models and place it into a dynamic statistical model that explicitly accounts for bias and measurement error in the climate models that make up the ensemble. They then analyze the NDCs to produce conditional probabilistic forecasts of future global temperature with respect to the commitments made by countries in the NDCs. They find that the probability of staying below 2.0 degrees, if all countries meet their NDCs and continue to reduce emissions at the same rate after 2030, is 26%. But the really interesting part is illustrated in the figure below, showing the probabilities that countries will actually achieve their pledges as shades between green (very likely) and red (very unlikely), given projected dynamics of population, GDP per capita, and carbon intensity (carbon emissions per unit of GDP):

Evidently, the odds are not good for many of the key parties. On current trends, the probabilities of meeting their NDCs for the largest emitters are low, e. g. 2% for the USA, 16% for China, 10% for Japan, 13% for Germany, and 2% for France. An exception is Russia with 93% probability. Taking account of those risks of failure, the probability of meeting the sub-2.0 target falls to just 5%. The pivotal role of the United States is highlighted by the finding that even if all other parties fully meet their pledges, failure of the USA alone reduces the probability of success from 26% to 18%. 

This does not mean that the Paris Agreement is completely useless in mitigating climate change. The median forecast of cumulative carbon emissions by 2100 (from 2015) is 2,083 gigatons under the current set of NDCs, compared to 3,108  gigatons without the Agreement. But to have any chance of meeting the Agreement's bare minimum goal of staying below 2 degrees warming, pledges must increase by 80%, yes eighty percent, and those pledges must be made real. The former part is scary enough, but I think it is the latter part that is currently underrepresented in public discourse. It's time to be clear about what "minus x% by the year y" and "net zero" actually mean. It's time to put the instruments in place that eventually make burning fossil fuels unattractive. It's time to realize that this implies a world that is radically different from todays. And it's time to trust the uniquely human capacity to adapt by the power of thought and imagination. Start with yourself, commit to change, press for strong pledges, you're in good company [6].

[1] Intergovernmental Panel on Climate Change (2018). Special Report: Global Warming of 1.5 ºC. URL: https://www.ipcc.ch/sr15/.

[2] Isaac, R. M., and Walker, J. M. (1988). Group size effects in public goods provision: The voluntary contributions mechanism. The Quarterly Journal of Economics 103(1), 179-199. DOI: https://doi.org/10.2307/1882648.

[3] United Nations Climate Change (2021). Nationally Determined Contributions under the Paris Agreement. Synthesis Report by the Secretariat. Document FCCC/PA/CMA/2021/2. URL: https://unfccc.int/documents/268571.

[4] Stammer, D., Engels, A., Marotzke, J., Gresse, E., Hedemann, C., and Petzold, J., eds. (2021). Hamburg Climate Futures Outlook 2021. Cluster of Excellence Climate, Climatic Change, and Society (CLICCS), University of Hamburg. URL: https://www.cliccs.uni-hamburg.de/results/hamburg-climate-futures-outlook.html.

[5] Liu, P. R., and Raftery, A. E. (2021). Country-based rate of emissions reductions should increase by 80% beyond nationally determined contributions to meet the 2 °C target. Communications Earth & Environment 2, 29. URL: https://www.nature.com/articles/s43247-021-00097-8.

[6] United Nations Development Programme (2021). The People's Climate Vote. Results. URL: https://www.undp.org/publications/peoples-climate-vote.

The headline image is based on a photograph by Hannah Busing under non-commerical license.