Australia’s first climate risk assessment has the stated purpose of guiding adaptation responses to protect people and property in a  heating climate, but what happens if the reality is worse than some low-ball projections of future risks?

Australia has just released its first-ever _National Climate Risk Assessment_, and some big numbers drew fairly predictable media coverage: _First climate risk assessment finds 1.5m Australians at risk from sea level rise by 2050_, _No Australian spared_ and _Apocalyptic report a ‘wake-up call’ on climate impacts: PM_.

One NCRA finding that grabbed attention was on rising sea levels: “By 2050, the number of coastal communities located in high and very-high risk areas will increase from 8% to 18% due to sea-level rise. If current populations remained as they are today, this would represent an increase to 1.5 million people at risk.”

The NCRA drew much of the data from Australia’s Future Climate and Hazards Report, prepared by the Australian Climate Service. It provides the following metrics used in sea-level rise assessment in the report, drawn from climate models:

• +0.14 metre:  Approximate projected sea level rise at 2050;
• +0.32 metre: Approximate projected global sea level rise at 2090, assuming +2.0°C global warming is reached in 2090;
• +0.54 metre: Approximate projected global sea level rise at 2090, assuming +3.0°C global warming is reached in 2090; and
• +0.94 metre: Plausible higher-end projected sea level rise consistent with jurisdictional planning used in Australia.

Note that these figures are relative to present day sea level, which is 0.2 metres above the 1880 levels.

This is also shown in Table 5.

So for planning and adaptation benchmark 0.94 metres above 2020 is the plausible high-end case? Well no, and that’s the problem, because in climate politics numbers mean more than words; in general, both the media and policymakers go with the headline-grabbing numbers presented, and ignore the fine print. I saw no media coverage or public discussion of any number above 0.94 metres.

Yet both the NCRA and the Hazards report actually point to something different and much bigger. After giving the above numbers, the Hazards report then notes:

“Under higher emissions scenarios, sea levels will rise by metres over the coming centuries. In addition, thresholds (tipping points) for the Greenland and Antarctic ice sheets could be crossed during the 21st century… Please see the Tipping points and abrupt responses section of this report for discussion of higher mean global sea levels this century and beyond.”

And then this on tipping points:

• “Future changes in the Southern Ocean will reduce its efficiency to absorb heat and carbon from the atmosphere, thereby accelerating the rate of global climate change.”
• “The collapse of the Antarctic ice sheet could result in rapid sea level rise – approaching two metres by 2100 and five metres by 2150.”
• “Reaching potential climate and ecological tipping points is very likely to result in abrupt changes.”
• “Global climate models, such as those used in this report, do not capture tipping points or abrupt changes.”
• “Due to the currently limited understanding of potential tipping elements, there may be some which have not yet been identified or may already be unavoidable.”

To summarise, abrupt (and difficult to project) changes in the Antarctic and Arctic could produce sea-level rises of two metres this century, but the NCRA process will ignore that risk.

The reason given is that this outcome has “low confidence”. But that is not true: repeatedly, over the last decade, peer-reviewed scientific papers have concluded that glaciers of the West Antarctic Ice Sheet have already passed their tipping points, for example here and here and here and here. That is, the rapid disintegration of Thwaites and other West Antarctic glaciers this century is more a probability than a possibility.

For example, the 2023 _State of the cryosphere_ report from the International Cryosphere Climate Initiative lays out the “catastrophic global damage” that would result from sustained warming at 2°C and the “catastrophic global damage” that would result., including the commitment to “between 12 and 20 metres” of sea level rise.

And in 2017, a US government report concluded that: “Emerging science regarding Antarctic ice sheet stability suggests that, for higher scenarios, a GMSL rise exceeding eight feet (2.4 metres) by 2100 is physically possible, although the probability of such an extreme outcome cannot currently be assessed.”

A decade ago, scientists warned that ​​the long-term sea-level rise associated with a 2°C warming would submerge parts of Australia on which 25-50% of the population currently lives. Australian Antarctic scientist Andrew Klekociuk says five metres is possible this century. And our scientists have also warned that current greenhouse gas levels will produce 5-25 metres of sea-level rise.

So why did the NCRA not identify that a plausible worst-case sea-level-rise scenario could be two metres or more this century? Was it scientific reticence or political nudging? Who knows, but the reality is that this is the opposite of good risk-assessment methodology.

The best approach in such cases is as follows:

• In the physical climate, many systems exhibit fast, non-linear change which is difficult to model or project, and which is often associated with a tipping point.
• Many climate systems exhibit tipping points or thresholds and the change may be abrupt and irreversible on relevant time frames, possibly leading to cascading events.
• An unforeseen chain or cascade of events may occur when one event in a system has a negative effect on other related components.
• A full climate change risk assessment should take into account the full range of outcomes, including tipping points and cascades. This must not be based only on historic experience, which may well be irrelevant to the future state, and must incorporate methods to understand unprecedented climate impacts.
• Hence a fundamental rule of risk assessment is to focus on the “fat-tail” risks and the plausible worst case scenarios, especially when the damages are so great and the risks are existential, such that there is no “second chance” to learn from one’s mistakes.

This is not just a theoretical issue, but a practical one, because in Australia we have repeatedly underestimated the pace and impact of climate disruption, and ended up with event impacts — fire, flooding, extreme heat — worse than was planned for, which have devastated communities. Extreme floods and rain events are often oddly described as a “once-in-a-hundred-years” or a “one-in-five hundred-year” events, suggesting they are unlikely to recur. But then they happen again, within a few years.

This shows the assessments of future climate risks are often too conservative, and so vulnerable communities and governments are underprepared. The NCRA, by failing to clearly identify a plausible high-end scenario to be used as a guide for future planning on sea levels, is falling into the same trap.

The views expressed in this article may or may not reflect those of Pearls and Irritations.