IPCC review reveals climate impacts occur at lower temperatures than previously thought

Comparison of risk thresholds across intergovernmental Panel on Climate Change assessments. All burning embers are presented with the same colour and temperature scale, removing technical details that varied between the original publications. Dashed lines connect the midpoints between undetectable and moderate risk, and moderate and high risk. Risks transitions have generally shifted towards lower temperatures with updated scientific understanding. Source: Zommers et al., 2020.

An international review of IPCC reports since 2001 has found that as the science has improved with each report, a trend has appeared showing climate-related impacts like heatwaves, the collapse of the West Antarctic ice sheet and coral bleaching are likely to occur at lower global mean temperatures than estimated in earlier reports.

The review in Nature Reviews Earth & Environment found this trend of earlier climate impacts with each report was not related to methodological variations or author bias but was directly related to improvements in climate science and our understanding of climate processes.

This important finding was an incidental outcome of the main thrust of the review, which was to examine ways to improve the expert judgement process used by the IPCC to determine climate impacts and the way these are communicated to the public in easy to understand figures known as burning ember diagrams.

Comparison of risk thresholds across intergovernmental Panel on Climate Change assessments. All burning embers are presented with the same colour and temperature scale, removing technical details that varied between the original publications. Dashed lines connect the midpoints between undetectable and moderate risk, and moderate and high risk. Risks transitions have generally shifted towards lower temperatures with updated scientific understanding. Source: Zommers et al., 2020.

Comparison of risk thresholds across Intergovernmental Panel on Climate Change assessments. All burning embers are presented with the same colour and temperature scale, removing technical details that varied between the original publications. Dashed lines connect the midpoints between undetectable and moderate risk, and moderate and high risk. Risks transitions have generally shifted towards lower temperatures with updated scientific understanding. Source: Zommers et al., 2020.

“This was the first time that the risk levels at given temperatures had been compared in a standardised way across IPCC reports,” said co-author Prof Jason Evans from the ARC Centre of Excellence for Climate Extremes.

“While many climate researchers have instinctively felt that every bit of new science was suggesting serious climate change impacts may occur sooner than originally estimated, this shows that intuition is in fact the reality.”

The review itself has come up with a number of recommendations to improve the consistency and transparency of the process involved in assessing climate impacts and their timing as determined by the science.

A key recommendation is to create a consistent, expert-elicitation protocol where scientists are led by an independent expert moderator outside the IPCC process that brings together and anonymises scientific judgements based on the peer-reviewed literature. The review recommends the protocol is published openly in advance, literature is made available to all, and the scoring process is entirely transparent.

The review also recommended the burning embers diagrams will now use standardised colours graduating the risk at specific global average temperatures through five colours from white (no significant climate change risk) on to yellow, orange, red, and purple, with purple representing an immediate, significant risk to large parts of the world’s population. This standard format also includes confidence levels.

The review also noted that there have been calls for regional burning ember figures, to help policymakers to make informed decisions and take into account potential benefits, limits to adaption, and the residual risks from adaption and mitigation.

“The need to act on climate change is growing in urgency with each passing year. Policymakers need clear, concise information to make decisions, but they also need to be aware that the science shows impacts may be closer than we originally estimated,” Prof Evans said.

“This review highlights that we can’t take for granted the time left for us act to prevent the worst outcomes of climate change.”

Reference: Zommers, Z., Marbaix, P., Fischlin, A. et al. Burning embers: towards more transparent and robust climate-change risk assessments. Nat Rev Earth Environ 1, 516–529 (2020). https://doi.org/10.1038/s43017-020-0088-0.

Originally published by the ARC Centre of Excellence for Climate Extremes, 19 October 2020.