Public understanding of climate change-related sea-level rise

majority of respondents (68%; n = 518) felt somewhat or pretty well informed about sea-level rise, only 38% (n = 519) felt somewhat or extremely clear about the rate/timing/extent of sea- level rise to 2100. Some of the most extreme projections selected by our respondents go beyond the 5 m of sea-level rise by 2100 that some media have reported. For example, the 6.8% of respondents who thought sea-level rise could reach ‘15 m or more’ by 2100 and the 33.1% who believed that all the planet’s ice could melt over a period of ‘decades,’ selected options that are unprece- dented in the geological record and defy physical laws around how fast ice can melt, even under extreme temperature forcing [44, 45]. Beyond the more descriptive analysis of the answers, we also examined the extent to which overestimation was associated with level of concern about sea-level rise. Results from t-tests confirmed that respondents who were more likely to overestimate the amount of sea-level rise and how fast this would occur were significantly more likely to express greater concern about sea-level rise to 2100 in New Zealand, and globally, compared to their counterparts (i.e., t > 5, p < .001; Cohen’s d > .50; see Section 1.4 of the S1 File for detail). When assessing the severity of environmental problems for geographically distant places, individuals tend to rate the severity of these problems as worse ‘there’ than ‘here’ [46–48]. Hence, we also examined whether such spatial bias would be observed in relation to concern

for sea-level rise. The spatial bias effect was operationalized as the difference between ratings of national concern and global concern (i.e., average Aotearoa New Zealand minus average global). A difference score equal to zero represents judgement of no nation–global distinction, while values other than zero represent biased comparative judgements. Values above zero rep- resent the expected spatial bias effect with global sea-level rise judged as more concerning than national sea-level rise, and values below zero represent an opposite effect. For most respon- dents (78.8%) a biased judgement was absent (i.e., the difference score was zero) but results from a one-sample t-test indicate the mean difference score of .08 (SD = .59) was statistically different from zero, t(1034) = 4.28, p < .001, Cohen’s d = .14. A spatial bias effect was thus observed for sea-level rise: respondents judge global sea-level rise as more concerning than national sea-level rise. The public’s association of melting sea ice with sea-level rise, as well as its selection as the top-ranked cause of sea-level rise, provides further evidence of public confusion on this issue, which may be due to the significant media coverage given to melting sea ice in the Arctic. It is possible, though, that some respondents might have been influenced by science that says the flow of ice sheets can be modulated by the presence or absence of sea ice [49–51]. Research into how publics understand sea-level rise is still an emerging field [8] and there

are few international studies with which to compare our results. However, we note that our findings that respondents tend to overestimate the amount of sea-level rise possible, contrast with a 2019 Australian study showing that New South Wales coastal users and businesses tended to underestimate the amount and rate of sea-level rise along the NSW coast with 45% of respondents predicting a rise of 1–25 cms in the next 20–50 years [52]. Similarly to our results a 2016 American study found that individuals were concerned by the impacts of sea- level rise but were uncertain about its timing [53]. This survey did not ask respondents to esti- mate the projected amount of sea-level rise, but asked “When do you believe the effects of sea- level rise will significantly impact the county, if ever?” Finally, a 2016 study of residents of New Jersey following Hurricane Sandy showed that while participants believed sea-level rise was occurring, they did not view sea-level rise as a current threat to themselves [54]. This survey did not ask about the projected amount of sea-level rise or its timing, but asked participants whether they believed sea-level rise was occurring and whether it was affecting them. These PLOS ONE Public understanding of climate change-related sea-level rise PLOS ONE | https://doi.org/10.1371/journal.pone.0254348 July 9, 2021 7 / 12