Meteorology

As is typical with oceanic islands, the climate on Norfolk Island is more moderate than on mainland Australia. The highest summer temperature recorded is 28.4° and the lowest winter temperature 6.2° – frosts are unknown.

Many species of plant and animal are highly sensitive to small variations in temperature, rainfall, day length or other parameters, which trigger stages in their life cycle – germination of seeds, flowering, nesting behaviour, as the case may be. The critical thresholds differ for every species.

The Bureau of Meteorology has tabulated a range of parameters – rainfall, temperature, winds et cetera – and these are publicly accessible on its website; click here: http://www.bom.gov.au/climate/averages/tables/cw_200288.shtml. (After clicking on the link make sure to select “All available” data in the tool bar at the head of the table – otherwise it defaults to “Main statistics”). You can select “Plot” to graph the data. NIFFS thanks the Bureau’s onsite meteorologist Adam Jauczius for assistance in accessing these data.

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Climate change

Between 1915 and 2019 Norfolk Island had a mean annual rainfall of 1263 mm. Although rainfall on average has been spread throughout the year, historically the totals have been highest during the cooler months (between April and August).

Reductions in annual rainfall and changes to the seasonal patterns on Norfolk Island are consistent with broader patterns of observed global change. Notably there has been a decline in annual rainfall since about 1970, which manifested in recent decades as long runs of dry years:

• Relative to the period 1915 to 1969 (1334 mm/year), between 1970 and 2020 mean annual rainfall was 11% lower (1184 mm/year).
• Relative to 1945 to 1969 (1386 mm/year), between 1995 and 2019 mean annual rainfall was 17% lower (1145 mm/year).
• The decline in rainfall has occurred in all seasons except summer, with the largest decreases in autumn and winter.
• There has been an increase in modelled crop/plant water demand since about 1990, in part due to an increase in temperature but more notably from an increase in wind speed over Norfolk Island.

There is strong agreement among 21 global climate models (~80%) that mean annual rainfall over Norfolk Island will decrease in future and that potential evaporation will increase (~90%):

• All models agree on a decrease in Norfolk Island’s spring rainfall and 90% agree on a decrease in winter rainfall. About 40% and 25% of models project decreases of greater than 20% in spring and winter respectively. There is a lack of agreement between models on the direction of rainfall change for summer and autumn.
• There is a substantial warming trend in air temperature and sea surface temperature at Norfolk Island and strong agreement among the global climate models that this will continue into the future.

CSIRO (2020). Norfolk Island Water Resource Assessment. A summary report. CSIRO, Australia.