Analysing the 1.5°C limit in the Paris Agreement

A new analysis of the 1.5°C temperature limit in the Paris Agreement’s long-term temperature goal has identified a number of important areas that require more scientific research.

The analysis, written by a team of scientists who have published key research papers on the science, impacts and policy aspects of the 1.5˚C limit, is a centrepiece of a collection by Nature Climate Change, Nature Geoscience and Nature on ‘Targeting 1.5°C’  (http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate3096.html).

Much of the paper’s focus is on characteristics of presently available science on low emission pathways that could achieve 1.5˚C. The paper finds that most of such scenarios at least temporarily overshoot the 1.5˚C limit, where warming would rise above this level before returning to below 1.5˚C by 2100.

“Whether the presently available pathways are in line with the Paris Agreement’s temperature goal is not a scientific, but a political question.  Many vulnerable countries see 1.5°C as a limit that should not be exceeded,” said the paper’s lead author, Carl-Friedrich Schleussner, of Climate Analytics.  “Further research on the feasibility of pathways that limit warming to below 1.5°C is therefore a central element of a post-Paris science agenda.

The paper confirms that limiting warming to 1.5˚C significantly reduces risks and impacts compared with 2˚C, but also underscores the need for more research to improve the scientific understanding of impacts at 1.5°C of warming.  These include consequences of 1.5°C for vulnerable systems such as agricultural production in tropical regions, impacts on human health and natural systems such as coral reefs, ice sheets, and impacts such as sea level rise.

“In the light of our findings of discernible impact differences between 1.5°C and 2°C, we urgently require a better scientific understanding of the potential impact legacy of temporarily exceeding the 1.5°C limit,” said Michiel Schaeffer, co-author and Scientific Director at Climate Analytics.

The paper confirms that early peaking of global emissions, around 2020, and rapid decline toward zero emissions are critical steps towards achieving the Paris Agreement’s temperature goal. Governments’ near-term mitigation targets for the 2020-2030 period are insufficient to achieve the temperature limit. To address that, the Paris Agreement includes a mechanism aimed at increasing climate action over time.

“We outline how the legal and policy mechanisms in the Paris Agreement would, in principle, allow to achieve its goals:  through a regular, science-driven and synchronised five-year review of national emission reduction commitments that aims at enabling countries to regularly improve their pledges,” said Bill Hare, CEO of Climate Analytics.

“The first major review in 2018 of national mitigation commitments, which is meant to lead to Governments increasing their 2025-2030 emission reduction targets by 2020, could be a crucial first test of the Paris Agreement’s effectiveness,” he said.

In order to achieve the Paris Agreement’s mitigation and temperature goals, globally negative CO₂ emissions are required by the second half of the century. Achieving such negative emissions entails the deployment of uncertain and at present controversial technologies, including biomass energy carbon capture and storage.

“We find there is no clear difference in the assumed levels of bioenergy and negative emissions between 1.5˚C and 2°C pathways,” said Joeri Rogelj, a co-author and Research Scholar at the International Institute for Applied Systems Analysis, Austria. “Nevertheless, important and genuine sustainability concerns linked to these technologies have to be researched and addressed. Acting earlier and faster can substantially reduce the need for these technologies, but at this point not entirely eliminate it anymore.”

More research is also required into the risks and costs of negative emission technologies, such as bioenergy with carbon capture and storage (BECCS). Ensuring the environmental and social sustainability of bioenergy will be a key challenge under any future scenario and the scientific literature to date provides no conclusive evidence for increasing risks to food security from bioenergy systems between 1.5°C and 2°C scenarios.