Dr Joeri Rogelj is a research scholar at the International Institute for Applied Systems Analysis (IIASA) in Austria. Rogelj was recently awarded the inaugural Piers Sellers Prize for world leading contribution to solution-focused climate research.
Last year’s Paris Agreement is considered a major global step in addressing the threat of climate change.
It was a diplomatic victory to bring 195 governments together to agree a goal to limit global average temperature increase to “well below 2C” relative to pre-industrial levels, and to “pursue efforts” to limit it to 1.5C.
It was a similar tour de force that 186 parties submitted “intended nationally determined contribution” (INDCs) in the run up to Paris. INDCs set out each country’s contribution to cutting greenhouse gas emissions in order to meet the collective global temperature goal. The INDC process succeeded in overcoming decades of hesitation across the international community.
Moreover, every five years, these contributions are to be updated or renewed, steadily increasing in ambition.
But accompanying this success came a set of puzzling questions: what do the INDCs add up to in terms of global emissions reductions? Are they sufficient to keep warming to well below 2C? If not, what temperature increase are we heading for? What about 1.5C? And, are the INDCs at least a step in the right direction?
An added complication is that INDCs don’t all follow the same approach. Some countries’ emissions pledges are conditional on receiving funding, for example, while others depend on the growth of their economy. This is why various studies (pdf) addressing these questions have often come up with different answers.
Our new paper, just published in Nature, provides some clarity on this issue by carrying out a “meta-analysis” to see what conclusions we can draw from ten of these studies.
And to cut a long story short, the INDCs alone do not keep warming to well below 2C.
Similar level of effort
The current INDCs cover the period up to 2030. To estimate warming over the entire 21st century, scientists have to make assumptions about what happens beyond the INDCs.
We identified three main possibilities for climate action after 2030: it stalls, it continues at a similar level of effort, or it accelerates.
It’s worth noting that the Paris Agreement states that future “nationally determined contributions” (NDCs) – the successors to INDCs – will need to increase in ambition. Stalling of emissions cuts would not be consistent with the ratchet mechanism within the agreement. This, therefore, leaves two options: continuing or accelerating action.
In our analysis, we first we assume that action continues after 2030 at a level similar to the INDCs.
How such a “similar level” is determined is another point of discussion – and will likely pop up as an issue when countries submit their future NDCs as part of the Paris Agreement. For our purposes, we assumed that the level of effort needed to achieve the INDCs would be continued until 2100.
Our findings suggest that extending the current INDCs out to 2100, global average surface temperature would rise to 2.6-3.1C, relative to pre-industrial levels.
This is the median estimate, which means there’s a 50-50 chance of keeping warming to below this range.
We also find a 66% probability that temperature rise can be kept below 2.9-3.4C, and a 90% probability that it stays below 3.5-4.2C.
This is an improvement on not having INDCs at all. With climate policies as they are around the world, there’s a 50% likelihood that warming would stay below 3.1-3.4C by 2100.
But the bottom line is that the level of climate action in the INDCs do not keep global temperature rise below 2C. And they present an almost 10% risk that temperature rise hits 4C by 2100.
Acceleration of action
The other possibility is to assume an acceleration in post-2030 emissions cuts.
However, here we hit a problem. Our analysis suggests that by 2030, the INDCs as they stand may have used up the entire carbon budget for a good chance – 66% probability – of keeping temperature rise below 2C this century.
This means, even if countries submit more ambitious emissions cuts in their future NDCs, we will have likely already blown the budget for 2C – and will have definitely blown it for staying below 1.5C.
Looking at emissions pathways that are in keeping with INDCs, those that come close to keeping temperature rise to 2C by 2100 depend on a rapid decline of CO2 emissions from energy and industry after 2030. This would require, for example, emissions cuts of 3.5% per year between 2030 and 2050.
Historically, some countries have managed to achieve annual emissions cuts of 2-3%, but for energy security reasons rather than to tackle climate change. Although these historical rates are not a perfect comparison, achieving this level of action at a global scale in future will be a tough nut to crack.
Therefore, if INDCs stay as they are, the likely option will be that we overshoot 2C or 1.5C, and then attempt to bring temperature back below those levels by 2100.
This option assumes a massive scale-up of negative emissions technologies, which remove CO2 from the atmosphere and store it on land, underground or in the oceans.
Our analysis suggests that the annual rate of negative emissions scale-up to the middle of the century will need to be roughly similar to the current expansion of renewable energy.
But renewable energy infrastructure is expanding because costs have reduced dramatically over the past decade and governments are starting to appreciate the benefits of a diversified and clean energy system. By comparison, the equivalent can’t be said for the roll-out of negative emissions technology around the world, or, in some cases, even the demonstration of it to work at scale.
It is premature, then, to conclude that large-scale take up of the negative emissions in the near future is achievable. And without a clear, strong, and stable long-term policy signal that makes emitting CO2 economically and politically unattractive, it thus seems very unlikely that such scale up of negative emissions technologies will ever occur.
That said, no matter what we assume post-2030, the INDCs aren’t quite enough to bring us in line with the Paris Agreement’s climate goal of keeping warming to well below 2C, let alone keeping it to 1.5C.
More needs to be done
The main conclusion of our study is, therefore, that more needs to be done before 2030, rather than waiting until later. There’s no silver bullet, but we identify the most promising ways to increase action before 2030.
One option is to expand the scope of the INDCs to include more sectors – such as international shipping and aviation – and other types of greenhouse gases. However, the bulk of the necessary reductions can only be achieved by extending emissions cuts in sectors already covered by INDCs, such as energy, transport and farming.
These INDCs thus need to be strengthened internationally or governments should aim to overachieve them domestically.
It is now up to governments to live up to the ambition of the Paris Agreement and make sure that increasingly ambitious emissions cuts are put forward. And this ambition needs to start before the Paris Agreement is even ratified.
Main image: Comité de Paris en salle pleinière Seine pendant la COP21.
Rogelj, J., et al. (2016) Paris Agreement climate proposals need a boost to keep warming well below 2 °C, Nature, doi:10.1038/nature18307.
Guest post: Why Paris climate pledges need to overdeliver to keep warming to 2C