The Shifting Rainfall Patterns

In recent decades, Earth's climate has undergone significant changes, particularly in relation to temperature—a trend that is expected to persist into the foreseeable future. Humanity is emitting greenhouse gases at an unprecedented rate, driving warming that is now measurable across most of the planet.

In the past, the prevailing view was that the effect of greenhouse gases was limited to temperature increases. However, it is clear today that these emissions influence every aspect of Earth's climate, including global wind and precipitation patterns. These changes contribute to droughts, floods, and shifts in seasonal cycles.

How are precipitation patterns in the Eastern Mediterranean region expected to change? The answer to this question holds critical implications for agriculture, infrastructure, and ecosystems. To explore this, we will examine the expected changes using basic scientific principles: the impact of global warming on atmospheric water vapor and the global wind patterns.

 Precipitation patterns worldwide are expected to change. A rye field on a hot and dry summer day | rsooll, Shutterstock

Increase in Humidity

How can we understand the connection between rising greenhouse gas emissions and rainfall? One way is through the Clausius-Clapeyron equation, developed nearly two centuries ago by two scientists whose names it bears. This equation demonstrates that warmer air can hold more water vapor than cooler air. 

Consider a pot of water with a lid: the hotter the water and air inside the pot, the more water vapor the pot can contain. Two processes occur continuously in this system: water evaporates, while, at the same time, water vapor condenses back into liquid. As the temperature rises, and the water heats up the rate of evaporation increases. However, as the amount of vapor grows, the rate of condensation also rises. After a period of heating, the increasing rate of condensation eventually balances the rate of evaporation, and the amount of vapor in the pot stabilizes - —a state called equilibrium.  The Clausius-Clapeyron equation calculates the amount of vapor in the air at equilibrium as the air temperature changes.

Applying this principle to the atmosphere and the ocean instead of a pot, we find that a one-degree Celsius rise in Earth's surface temperature enables the air to hold up to seven percent more water vapor. This doesn’t mean warm air will always have higher humidity, but rather that it has the capacity to hold more humidity. For example, deserts remain hot and dry despite their high temperatures. According to the equation, as temperatures continue to rise, the air will be able to hold more water vapor. This additional atmospheric water vapor can condense into clouds and eventually precipitate as rain, leading to more intense rainfall events. 

The Hadley cell contributes to the rising air in the equatorial region, and this air cools, condenses, and falls as rain. Air cells on Earth’s surface | Designua, Shutterstock

Winds, Rain, and Deserts

Thus, looking ahead, we anticipate that in the future there will be more water in the atmosphere and increased precipitation. But where will the rain fall? The air contains water vapor and can also hold floating water droplets—the clouds. This represents a state of equilibrium: water vapor condenses into water droplets, and if they are not heavy enough to fall to the ground, they will evaporate and revert back to gas, that is, vapor. Rain, on the other hand, is an example of instability: the floating water droplets grow until their weight causes them to descend as rain.

This instability can develop in several ways. An important component is the vertical movement of air upwards. The air cools rapidly, causing the water to condense and turn into droplets. The droplets grow until they are heavy enough to fall as rain. For instance, the Eastern Mediterranean region is situated at the meeting point between a moderated Mediterranean climate zone and a desert climate zone, where the instability and vertical movement of the air explain why rain can fall there during the winter but not in the summer. To understand what causes these differences between the seasons, we must become familiar with the wind system known as the Hadley cell.

The Hadley cell is a global-scale atmospheric air circulation system driven by the temperature difference between the tropical region around the equator, extending up to latitude 23°, and the subtropical regions between latitudes 23° and 35°. Within the cell, air circulation causes humid air to rise in the tropical region around the equator and descend as dry air in the subtropical regions. This circular and closed circulation pattern is why the system is called a "cell." Vertical air movement within the Hadley cell links it to global precipitation patterns.  In the equatorial region, rising humid air cools, condenses, and precipitates as rain, supporting the existence of rainforests such as the Amazon rainforest, in the tropical regions. Conversely, in the subtropical regions, descending dry air suppresses rainfall, contributing to the formation of desert belts, such as the Sahara Desert in Africa.

The Hadley cell exhibits seasonal shifts. In summer, its descending and dry branch extends to approximately latitude 30°, stabilizing the atmosphere and preventing rainfall in regions such as the Eastern Mediterranean, around latitude 31°. This results in almost completely dry summers. In winter, the descending branch of the Hadley cell shifts equatorward, affecting a smaller area. This shift allows other weather systems to bring rainfall to the region.

In the winter, the Hadley cell does not reach the  Eastern Mediterranean, allowing rainfall there. A rainy day in Tel Aviv | Shabtay, Shutterstock

Stormy and Short Winters

Due to the increase in the concentration of greenhouse gases in the atmosphere, the Hadley cell is expanding, and its descending branch, which creates the global desert belt, is expected to shift northwards. How will this phenomenon affect precipitation in the Eastern Mediterranean?

In the summer, the expansion will not have a significant impact because summers in the region are already almost completely dry. In the winter, the descending branch of the Hadley cell will not reach the region, even considering future expansions. Therefore, as the cell expands, the most significant drying is expected to occur during the transitional seasons. The cell will prevent rain not only during the summer months, June to September, but it will also shift northwards, thereby preventing rain both before and after this period. The Eastern Mediterranean winter is shortening because the Hadley cell reaches the region for a longer duration each year.

Two key insights emerge: first, according to the Clausius-Clapeyron equation, future rainfall events are projected to intensify due to increased atmospheric water content.  Second, the expansion of the Hadley cell suggests reduced precipitation and fewer rainy days during transitional seasons in the Eastern Mediterranean. What is happening in reality? Are these the changes that have been observed?

A group of researchers from the Hebrew University and the Tel Aviv University found that the number of rainy days in Israel has been decreasing by an average of about two percent per decade since 1975, while the intensity of rain on rainy days has increased by an average of about two percent per decade. The study suggests that rain events will intensify, but the number of rainy days will decrease by an average of seven days per year, resulting in similar amounts of precipitation each year. These findings are consistent with conclusions drawn by the Meteorological Service. 

The researchers also found that during the transitional seasons, the number of rainy days has decreased at twice the rate observed on average—about five percent per decade. This trend aligns with the forecast based on the expansion of the Hadley cell. Changes in humidity levels and wind patterns offer valuable insights into how rainfall trends in the region are expected to change due to greenhouse gas emissions. Many precipitation patterns can be understood and predicted using basic scientific principles, not solely through complex computer models.