LAUSR

Funding information University of Hawai‘i at Hilo Based on a revised and extended multi-station Hawaiʻi Temperature Index (HTI), the mean air temperature in the Hawaiian Islands has warmed significantly at 0.052 C/decade (p < 0.01) over the past 100 years (1917–2016). The year 2016 was the warmest year on record at 0.924 C above the 100-year mean (0.202 C). During each of the last four decades, mean state-wide positive air temperature anomalies were greater than those of any of the previous decades. Significant warming trends for the last 100 years are evident at low(0.056 C/decade, p < 0.001) and high-elevations (0.047 C/decade, p < 0.01). Warming in Hawai‘i is largely attributed to significant increases in minimum temperature (0.072 C/ decade, p < 0.001) resulting in a corresponding downward trend in diurnal temperature range (−0.055 C/decade, p < 0.001) over the 100-year period. Significant positive correlations were found between HTI, the Pacific Decadal Oscillation, and the Multivariate ENSO Index, indicating that natural climate variability has a significant impact on temperature in Hawaiʻi. Analysis of surface air temperatures from NCEP/NCAR reanalysis data for the region of Hawaiʻi over the last 69 years (1948–2016) and a mean atmospheric layer temperature time series calculated from radiosonde-measured thickness (distance between constant pressure surfaces) data over the last 40 years (1977–2016) give results consistent with the HTI. Finally, we compare temperature trends for Hawaii's highest elevation station, Mauna Loa Observatory (3,397 m), to those on another mountainous subtropical island station in the Atlantic, Mt. Izaña Observatory (2,373 m), Tenerife, Canary Islands. Both stations sit above the local temperature inversion layer and have virtually identical significant warming trends of 0.19 C/decade (p < 0.001) between 1955 and 2016.