What was done
The authors evaluated the thermal impacts of historical land cover and land use (LCLU) changes in San Juan, Puerto Rico over the last four decades of the 20th century via an analysis of air temperatures measured at a height of 2 meters above ground level within four different LCLU types - urban-coastal, rural-inland, rural-coastal and urban-inland - after which they estimated what the strength of the urban heat island might be in the year 2050, based on anticipated LCLU changes and a model predicated upon their data of the past 40 years.

What was learned
Velazquez-Lozada et al. say their analysis revealed "the existence of an urban heat island in the tropical coastal city of San Juan, Puerto Rico that has been increasing at a rate of 0.06°C per year for the last 40 years." In addition, they report that predicted LCLU changes between now and 2050 will lead to an urban heat island effect "as high as 8°C for the year 2050."

What it means
Clearly, past urban heat island warming of San Juan and other major cities of the world (see Urban Heat Island in our Subject Index) totally dwarfs any greenhouse-gas-induced warming that may have occurred concurrently; and it would appear that the same will hold true for the future. Hence, even modest-sized cities must have urban heat island signals that match or exceed the sought-after greenhouse-gas signal, making the latter anthropogenic effect almost impossible to detect with any degree of confidence, especially when it is realized that solar, volcanic and aerosol (both natural and anthropogenic) effects must also be factored into the analysis.