The likelihood (i.e. probability) components of climate-related risks in Samoa are evaluated for both present day and future conditions. Changes over time reflect the influence of global warming. The risks evaluated are extreme rainfall events (both six-hourly and daily), drought, high sea levels, extreme winds and extreme high air and water temperatures. Projections of future climate-related risk are based on the output of global climate models, for given emission scenarios. All the likelihood components of the climate-related risks show increases as a result of global warming, though for some the increases are small relative to the uncertainties. Best estimates of long term, systematic changes in the average climate for Samoa indicate that by 2050 sea level is likely to have increased by 36 cm, rainfall by 1.2%, extreme wind gusts by 7% and maximum temperatures by 0.7 C.

The observed long term trend in relative sea level for Apia is 5.2 mm/yr. But maximum hourly sea level is increasing by approximately 8 mm/yr, a rate far in excess of the observed local and global trends in mean sea level. For Apia an hourly sea level of 1.8 m above mean sea level is currently a 100-year event. It will likely be at least a four-year event by 2025. No significant long term trends are evident in the observed daily, monthly, annual or maximum daily rainfall. Currently a daily rainfall of at least 300 mm is a relatively rare event at Apia, with a return period of 14 yr. There is large uncertainty in the rainfall projections, with two models suggesting substantial increases in rainfall, one model suggesting only small increases, and one model indicating a large decrease in rainfall into the future. An extreme daily rainfall of 400 mm is currently a 60-year event. It will likely be a 40-year event by 2050. An extreme six-hourly rainfall of 200 mm is currently a 30-year event. It will likely become a 20-year event by around 2050.

A monthly rainfall below the ten percentile is used as an indicator of drought. Drought frequency is strongly linked to the occurrence of El Niño events. Six global climate models that were best out of 19 at simulating present day ENSO conditions show no significant changes toward El Niño-like conditions in the latter part of the current century. Therefore it is not yet possible to make any predictions about the future nature of El Niño events and the implications for the frequency, duration and intensity of droughts in Samoa. Currently an extreme wind gust of 70 kt at Apia has a return period of 75 years. This will reduce to approximately 40 years by 2050. There is relatively high confidence in projections of maximum air temperature. A maximum air temperature of 34 C is currently well in excess of a 100-year event. By 2050 it will likely have a return period of 40 years.