Harborview has explored several strategies to blunt downtown's summer heat island. Tree advocates emphasize canopy expansion for shade and evapotranspiration, while planners point to the extensive rooftop acreage that could be retrofitted for vegetation. Over the past two years, the city piloted green roofs on 24 mid-rise buildings in the commercial core and planted street trees on adjacent blocks. Midday air temperatures measured at pedestrian level on blocks with new green roofs registered, on average, 1.2 degrees lower than on matched control blocks without green roofs; blocks with newly planted trees did not show a comparable reduction. That disparity, officials argue, is not surprising: young trees provide minimal shade for several years, and green roofs start cooling immediately by shading roof membranes and releasing moisture. Critics counter that the buildings receiving green roofs also underwent energy-efficiency upgrades that included reflective roof coatings and improved insulation, which could have contributed to lower ambient temperatures by reducing heat flux into the canyon of streets below. The city's consultant acknowledges this possibility but notes that the albedo of the vegetated plots is lower than that of white coatings, suggesting that evapotranspiration, not reflectivity, explains the cooling. The consultant further points to sensor locations along sidewalks rather than on rooftops, arguing that the measured differences reflect air temperatures where pedestrians walk. While it remains difficult to segregate the many contributors to microclimate, the preponderance of the pilot's evidence supports a pragmatic conclusion for Harborview: in the near term, green roofs reduce downtown's peak summer air temperatures more effectively than adding street trees. Long-term investments in canopy will matter, but for immediate relief in the city's dense core, green roofs appear to be the more potent tool.
Which of the following, if true, most strengthens the claim made in the passage that, in Harborview, green roofs reduce peak summer downtown temperatures more effectively than adding street trees?
- In a randomized pilot, some buildings received green roofs while similar buildings received high-albedo roof coatings; only the green-roof blocks showed significant pedestrian-level cooling, and adjacent blocks with newly planted street trees showed no measurable change over the study period. (correct answer)
- Green roofs cost substantially more per square foot to install than planting street trees of comparable maintenance burden.
- Residents report feeling more comfortable on shaded sidewalks than in open plazas during peak summer afternoons.
- Street-tree survival rates in Harborview's downtown are only 55 percent after three years due to limited soil volume and vandalism.
- The city simultaneously painted several roofs white in the same district that received the green roofs to meet an unrelated building-code requirement.
Explanation: The randomized comparison isolates green roofs' effect and shows greater cooling than both reflective coatings and newly planted trees, directly supporting the claim. The other options concern costs, preferences, survival, or confounds without demonstrating superior temperature reduction by green roofs.