What would it take to limit climate change to 1.5°C?

Potsdam, May 21, 2015 – A new study analyzes the required climate policy actions and targets in order to limit future global temperature rise to less than 1.5 degrees Celsius by 2100.

This level is supported by more than 100 countries worldwide, including those most vulnerable to climate change, as a safer goal than the currently agreed international aim of 2 degrees Celsius – an aim which would already imply substantial greenhouse-gas reductions. Hence the interest for scrutinizing the very low end of greenhouse-gas stabilization scenarios.

Limiting temperature rise by 2100 to less than 1.5 degrees Celsius is feasible, at least from a purely technological standpoint, according to the study published in the journal Nature Climate Change by researchers at the International Institute for Applied Systems Analysis (IIASA), the Potsdam Institute for Climate Impact Research (PIK), and others.

The new study examines scenarios for the energy, economy, and environment that are consistent with limiting climate change to 1.5 degrees Celsius above pre-industrial levels, and compares them to scenarios for limiting climate change to 2 degrees Celsius.

“Actions for returning global warming to below 1.5 degrees Celsius by 2100 are in many ways similar to those limiting warming to below 2 degrees Celsius,” says IIASA researcher Joeri Rogelj, one of the lead authors of the study. “However, the more ambitious 1.5 degrees Celsius goal leaves no space to further delay global mitigation action and emission reductions need to scale up swiftly in the next decades.”

The authors note, however, that the economic, political, and technological requirements to meet even the 2 degrees Celsius target are substantial. In the run-up to climate negotiations in December 2015, such information is important for policymakers considering long-term goals and steps to achieve these goals.

Key elements: accelerated energy efficiency gains and CO2 removal

The study identifies key elements that would need to be in place in order to reach the 1.5 degrees Celsius target by 2100. One fundamental feature is the tight constraint on future carbon emissions.

“In 1.5 degrees Celsius scenarios, the remaining carbon budget for the 21st century is reduced to almost half compared to 2 degrees Celsius scenarios,” explains PIK researcher Gunnar Luderer, who co-led the study. “As a consequence, deeper emissions cuts are required from all sectors, and global carbon neutrality would need to be reached 10-20 years earlier than projected for 2 degrees Celsius scenarios.”

Faster improvements in energy efficiency also emerge as a key enabling factor for the 1.5 degrees Celsius target. In addition, all the scenarios show that at some point in this century, carbon emissions would have to become negative at a global scale.

That means that significant amounts of CO2 would need to be actively removed from the atmosphere. This could occur through technological solutions such as bioenergy use combined with carbon capture and storage – a technology that remains untested on a large scale, increases the pressure on food supply systems and in some cases lacks social acceptance – or through efforts to grow more forests, sequestering carbon in tree trunks and branches. Afforestation, however, just like bioenergy plantations, would have to be carefully balanced against land use requirements for food production.

Overshooting the limit – and declining to 2100

In contrast to many scenarios examined in recent research, which set 2 degrees Celsius as the absolute limit and do not allow temperature to overshoot the target, the current set of scenarios looks at a long-term goal, and what would need to happen to get temperature back down to that level by 2100.

“Basically all our 1.5 degrees Celsius scenarios first exceed the 1.5 degrees Celsius temperature threshold somewhere in mid-century,” explains Rogelj, “before declining to 2100 and beyond as more and more carbon dioxide is actively removed from the atmosphere by specialized technologies.”

The recent IPCC fifth assessment report did not describe in detail the critical needs for how to limit warming to below 1.5 degrees Celsius as the scenarios available to them did not allow for an in-depth analysis. Yet over 100 countries worldwide – over half of the countries in the United Nations Framework Convention on Climate Change (UNFCCC), including the Alliance of Small Island States (AOSIS) and the Least-Developed Countries (LDCs) – have declared their support for a 1.5 degrees Celsius target on climate change. The target itself is also up for debate at the upcoming climate negotiations. This new study fills this gap.

The authors make clear that an increase of international efforts to curb greenhouse gases is imperative to keep the 1.5 degrees Celsius target achievable.

“The 1.5 degrees Celsius target leaves very little leeway,” says Luderer. “Any imperfections – be  it a further delay of meaningful policy action, or a failure to achieve negative emissions at large scale – will make the 1.5 degrees Celsius target unattainable during this century.”

By the International Institute for Applied Systems Analysis and the Potsdam Institute for Climate Impact Research

Article: Rogelj, J., Luderer, G., Pietzcker, R.C., Kriegler, E., Schaeffer, M., Krey, V., Riahi, K. (2015): Energy system transformations for limiting end-of-century warming to below 1.5°C. Nature Climate Change [DOI: 10.1038/NCLIMATE2572] Weblink to the article once it is published: http://dx.doi.org/10.1038/NCLIMATE2572