Shifting storms, sweltering summers: Tehran faces future ‘Day Zero’ when the taps run dry

2025

Choi, Y.-W. & Eltahir, E. A. B.

Bulletin of the Atomic Scientists

During the summer of 2025, Iran experienced an exceptional heatwave, with daytime temperatures across several regions, including Tehran, approaching 50 degrees Celsius (122 degrees Fahrenheit) and forcing the temporary closure of public offices and banks. During this period, major reservoirs supplying the Tehran region reached record-low levels, and water supply systems came under acute strain. By early November, the reservoir behind Amir Kabir Dam, a main source of drinking water for Tehran, had dropped to about 8 percent of its capacity. The present crisis reflects not only this summer’s extreme heat but also several consecutive years of reduced precipitation and ongoing drought conditions across Iran. As a result, the capital of Iran is now facing a potential “Day Zero” when taps could run dry.

The drought quickly disrupted Tehran’s urban systems. With dry soils and high evaporation, rivers and wetlands shrank. Falling reservoir levels led to disruptions in hydropower generation, and water shortages prompted strict saving measures across parts of the capital. Amid these escalating pressures, officials warned that the capital city may even have to be evacuated if water supplies fail to recover. In November, President Masoud Pezeshkian said the capital would have to be moved. These cascading impacts exposed how vulnerable Tehran’s infrastructure, economy, and communities have become under compounding heat and drought stress.

These cascading impacts stem from a prolonged shortage of precipitation in recent years (Figure 1a). Precipitation around Tehran typically peaks between December and April, replenishing reservoirs behind dams before the onset of the dry summer. Over the past five years, precipitation during this wet period has remained consistently below the long-term climatological baseline, with the 2024/25 season showing the most pronounced and prolonged deficit across the entire rainy season. When such prolonged dryness was followed by an exceptionally hot summer, it amplified hydrological stress across the region.

This prolonged precipitation deficit was not confined to Tehran but was part of a broader regional anomaly extending across much of Iran (Figure 1b). Satellite-based estimates for November 2024 to April 2025 reveal a pronounced north–south precipitation dipole, with enhanced precipitation north of latitude 40° N but markedly reduced precipitation across central and southern Iran. The precipitation deficit was particularly evident along a broad corridor extending from the eastern Mediterranean through Iran, indicating reduced storm activity across the region. This weakening of storm activity led to marked reductions in snowpack accumulation and reservoir inflows, aggravating the ongoing water scarcity crisis.

In maps of global projections of climate change impacts on precipitation, the region over and around the Mediterranean basin stands out because of the magnitude and significance of its precipitation decline. MIT researchers Alexandre Tuel and Elfatih Eltahir have explained why this region stands out as a hotspot for climate change. A more recent follow-up study by our group projects future declines in winter and spring precipitation extending to Mesopotamia and surrounding regions under a high-emission scenario by the end of the century. The projected change of the air circulation over the Central and Eastern Mediterranean, where most storms originate during winter, inhibits the formation of storm systems and consequently limits their eastward propagation, thereby reducing precipitation over Mesopotamia and adjacent regions eastward, including the area around Tehran.

Another contributing factor is the poleward displacement of storm tracks. During the spring season, the projected changes in regional air circulation due to global climate change move northward from the Mediterranean into southern Europe pushing the storm tracks further north and creating a dipole pattern (more precipitation in the north, less precipitation to the south) that reduces precipitation around Tehran. Consistent with this theory, simulations by IPCC (Intergovernmental Panel on Climate Change) models of future climate in this region project a pattern of change that resembles the regional pattern observed this last year, especially during the spring (Figure 1c). This similarity between observed and projected patterns suggests that the dry conditions observed this year may offer a glimpse of relatively dry conditions in the future, especially in spring season.