Balancing Benefits and Challenges of Regreening in Semi-Arid Climates.

Land surface characteristics significantly influence regional weather patterns, with the surface heat budget being governed by factors such as surface albedo, emissivity, heat fluxes, and evaporation.  In this study, we investigate the impact of regreening on regional temperature regimes and livability factors in the semi-arid NEOM region in northern Saudi Arabia. We conduct numerical experiments using a high-resolution (1.5x1.5 km grid spacing) Weather Research and Forecast (WRF) regional model to study the effect of converting the surface type from desert to savanna trees with 45% density across a 3.2E5-hectare area. We evaluate the effects of regreening using simulations over three summer months.

Our results indicate that regreening reduces surface temperature by approximately 0.6°C, primarily due to enhanced evapotranspiration. However, irrigation and increased moisture fluxes contribute to a rise in wet-bulb temperature, an important metric for heat stress. Specifically, the wet-bulb temperature increased by 0.7°C, potentially exacerbating heat stress in the region. Notably, maintaining this regreened area requires about 1.2 billion tons of water for irrigation during the summer period.

In semi-arid regions used in this study, where natural water sources are absent, irrigation relies on desalinated water. Although desalination ensures a reliable water supply, it requires substantial energy and generates emissions that contribute to atmospheric warming and negatively impact regional air quality.

These findings highlight the trade-offs associated with regreening in semi-arid regions, where reductions in surface temperature due to evapotranspiration may be offset by increased heat stress, energy demands, and environmental costs of desalination. This emphasizes the need for integrated and sustainable approaches to such interventions.