Urban Greening Under Rapid Expansion: Benchmarking Riyadh Against Four Decades of Arid-City Evidence

Riyadh is implementing one of the world’s largest urban greening programmes, yet whether measurable vegetation is increasing relative to the city’s expansion has not been assessed against comparable cities. Using the GHS Urban Centre Database within a fixed 2025 boundary, we benchmark Riyadh against 170 desert-climate urban centres from 1985 to 2025. Riyadh’s vegetated share of built-up area fell from 4.78 to 2.91%, while vegetated built-up area rose 10.6% and built-up surface rose 81.7%. Mean greenness reached a minimum in 2010 and then rose by 30%, so vegetation intensity and spatial coverage diverged. Every peer city also declined from 2000 to 2025, indicating a general condition rather than a Riyadh-specific failure. A conditional model estimated across epochs shows Riyadh at or above expectation until 2015 and below it from 2020, coinciding with a 27% rise in built-up surface; the 2025 observed-to-expected ratio of 0.89 lies within the model prediction interval, so it is not statistically distinguishable from expectation. When expressed in the harmonised database metric of vegetated share of built-up surface, the improvement implied by the 2030 ambition corresponds to 25.3% compound annual growth, a gain no peer city achieved in any of 1339 valid transitions.