What was done
According to the authors, daily temperature (T) measurements of Tmin and Tmax that were collected during 1969-2007 from twelve different stations scattered across the island of Oahu were downloaded from their repository at the U.S. National Climate Data Center, after which they computed the trends of each parameter over the 39-year period of 1969-2007 and the 25-year period of 1983-2007.

What was learned
Safeeq et al. report that over the longer 39-year period, island-wide minimum temperature increased by 0.17°C/decade, while there was no detectable trend in the corresponding maximum temperature. And during the more recent 25-year period, they found that annual maximum temperature actually showed a decline, while minimum temperature continued to increase. And they thus calculated that the trend in the diurnal temperature range (DTR) "shows a decline during the past 39 years with a stronger decreasing trend during the recent 25 years."

What it means
What are the implications of the three researchers' findings? Perhaps one of the most significant implications - which they do not mention, however - is the finding of Yang et al. (2013), who while working in Guangzhou City (the largest metropolis in Southern China) discovered "a linear DTR-mortality relationship, with evidence of increasing mortality with DTR increase," where "the effect of DTR occurred immediately and lasted for four days," such that over that time period, a 1°C increase in DTR was associated with a 0.47% increase in non-accidental mortality, and who also found this effect to be most prevalent among "the elderly, females and residents with less education." Thus, with Oahu's decreasing DTR trend, and its increasingly decreasing value, many of the inhabitants of Oahu should be able to expect a modest increase in the livability of their island home.

Reference
Yang, J., Liu, H.-Z., Ou, C.-Q., Lin, G.-Z., Zhou, Q., Shen, G.-C., Chen, P.-Y. and Guo, Y. 2013. Global climate change: Impact of diurnal temperature range on mortality in Guangzhou, China. 2013. Environmental Pollution 175: 131-136.