The ability to summon rain from the clouds is the stuff of ancient legends and futuristic aspiration. But given what we know now, and what we might credibly learn in coming years, how can we best integrate water in the atmosphere to our debates over limited transboundary water resources on the ground? Perhaps nowhere is that question more salient than in the Middle East, a region beset by water conflicts from time immemorial [1].

Traditionally, aquifers and lake and river basins shared by two or more countries are considered transboundary waters [2]. Here, we propose that atmospheric water vapor should also be recognized as a dynamic shared resource with its own unique characteristics. This is motivated by recent controversies around weather modification projects and large water development initiatives in the region.

In recent decades, weather intervention projects have sparked political disputes, despite ongoing scientific debate over the effectiveness of strategies like cloud seeding. While Israel, a pioneer in cloud seeding, recently discontinued its longstanding program due to limited gains, the UAE has substantially invested and emerged as a regional leader in this technology [3, 4]. Other countries have followed this uptrend as water supply challenges grow, particularly because cloud seeding costs much less than other options like desalination. Politically, a sense of competition has emerged among countries along shared weather pathways, typically from west-southwest to east-northeast. The UAE has considered even more ambitious endeavors, such as constructing an artificial mountain to uplift water vapor during its travel and facilitate rainfall over its territory [5]. Meanwhile, leaders of downwind states like Iran, suspicious of upstream modifications, sometimes blame these projects for reducing their water resources during droughts [6]. Although consistently dismissed by the scientific community, such claims give rise to new disputes over sky water, regardless of cloud seeding’s effectiveness.

In addition to moisture transport from remote sources, local evaporation has also been a topic of interest in the context of shared waters, such as in the Southeastern Anatolia Project (GAP in Turkish) at the headwaters of the Tigris-Euphrates Basin. This macro hydropower and irrigation development has been the source of longstanding disputes over the Tigris and Euphrates rivers between Türkiye and downstream countries [7]. In response, defenders of the GAP have at times pointed to the potential for irrigation to enhance precipitation downwind—a subject of research interest for several decades [8]—to make up for some of the water appropriated through withdrawals in GAP.

Moreover, air pollution in upstream countries, as carried by weather systems, can affect precipitation quantity and quality in downstream nations in different ways. One key factor is the abundance of dust aerosols, which modulate precipitation processes. Dust aerosols can act as cloud condensation nuclei (CCN), or more effectively as ice nuclei (IN), facilitating cloud formation and precipitation [9]. However, these aerosols suppress precipitation under certain conditions. Elevated dust concentrations can result in the formation of smaller cloud droplets, hindering their growth necessary for efficient precipitation. Additionally, dust particles absorb or scatter solar radiation, causing localized atmospheric heating or cooling that alters static stability and potentially limits rainfall [10].

Air pollution can also impact precipitation quality by altering its chemical composition. Pollutants like sulfur dioxide (SO₂) and nitrogen oxides (NOx), from burning fossil fuels and industry, can react with atmospheric water vapor to form acid rain. As polluted air masses from upstream regions reach downstream countries, they can lead to acid rain events, posing risks to ecosystems, soil, and water resources. The Convention on Long-Range Transboundary Air Pollution (LRTAP) is a good example of an intergovernmental treaty that addresses these harmful but uncertain atmospheric processes [11]. This agreement has been ratified by 51 countries from Europe and North America, including Türkiye, Azerbaijan, and Armenia.

We recognize substantial uncertainty in these areas, due to their complex nature and limited observations. However, we expect that sky water will soon introduce a new dimension to existing transboundary water issues, as already signaled by the ongoing debates about weather intervention projects [12]. Those issues are likely to intensify with climate change, further water limitations, and lack of robust agreements. Therefore, a collaborative international and transdisciplinary initiative is needed to formulate the problem, identify knowledge gaps, and outline a roadmap for future work.

The first step is to recognize water, in all its forms, as a common resource that necessitates a regional approach for effective management. This approach should begin with establishing multinational cooperation for monitoring and sharing data, including field campaigns for specific transboundary extreme events such as atmospheric rivers and dust storms. Such data can support the research on how activities in upwind countries influence precipitation and water resources in downwind nations by altering air moisture and aerosols. That research includes the net effect of dust aerosols on precipitation, a topic that we know little about—particularly in the Middle East, which provides an ideal natural laboratory for investigation of these processes [13]. Regional climate models could help by comparing the real world with alternative scenarios such as reduced dust emissions or removal of the GAP project. These prescribed conditions would allow us to isolate their impacts on the region and provide decision-makers with unbiased and independent scientific insights.

This information can guide comprehensive agreements that tie multiple cooperative efforts for better managing shared waters [14]. As interstate agreements increase, countries can forge deeper interdependence, leading to win-win scenarios and mitigating the risk of cooperation failure. For instance, Iraq could facilitate field campaigns for monitoring extreme events like atmospheric rivers, which caused record floods in Iran in 2019 [15]. In exchange, Iran can maintain river outflows to Iraq at a level required for sustainability of the Tigris-Euphrates Basin: a notion of “you help me in the sky, I’ll help you on the ground”. Similar agreements could be established between other countries.

These collaborative endeavors can contribute to regional stability and peace, an important factor recognized by the United Nations as it emphasizes on transboundary cooperation through its Sustainable Development Goal (SDG) Target 6.5. The multinational collaboration on science and technology would be a more realistic response to the current state of knowledge. However, if an intergovernmental scientific report inspires the negotiation of a treaty, the formulation of such a treaty would be facilitated when guided by a standardized framework supported by the U.N., similar to the Internationally Shared Aquifer Resource Management (ISARM) initiative for groundwater. Such a platform for atmospheric water does not currently exist. Therefore, we call for initiating a dialogue, at least within the scientific community, about regulating practices that can affect the quality and quantity of transboundary water vapor in the region.

The combined effects of climate variability and change, mismanagement, and regional disputes has led to frequent water shortages in the region with consequences such as intensified dust storms and forced migration. These issues can affect regions far beyond the Middle East, thus, they must be recognized as a global concern that demand global attention in the context of a rapidly changing world.