What was done
Noting that "breezy cities on small tropical islands ... may not be exempt from the same local climate change effects and urban heat island effects seen in large continental cities," the authors describe the results of their research into this topic conducted in and about San Juan, Puerto Rico.  In this particular study, a NASA Learjet carrying the Airborne Thermal and Land Applications Sensor (ATLAS) that operates in visual and infrared wavebands flew several flight lines, both day and night, over the San Juan metropolitan area, the El Yunque National Forest east of San Juan, plus other nearby areas, obtaining surface temperatures, while strategically-placed ground instruments recorded local air temperatures.

What was learned
Gonzalez et al. report that surface "temperature differences between urbanized areas and limited vegetated areas [were] higher than 30°C during daytime," creating an urban heat island with "the peak of the high temperature dome exactly over the commercial area of downtown," where noontime air temperatures were as much a 3°C greater than those of surrounding rural areas.  In addition, they report that "a recent climatological analysis of the surface [air] temperature of the city has revealed that the local temperature has been increasing over the neighboring vegetated areas at a rate of 0.06°C per year for the past 30 years."

What it means
In the words of Gonzalez et al., "the urban heat island dominates the sea breeze effects in downtown areas," and they say that "trends similar to those reported in this article may be expected in the future as coastal cities become more populated."  Indeed, we may assume that this phenomenon has long been operative in coastal cities around the world; and that it has helped to erroneously inflate the instrumental surface air temperature record of the planet over this period, contributing to the infamous hockeystick representation of world temperature history employed by the Intergovernmental Panel on Climate Change.