A Semi-Physical Framework for Attributing Long-Term Urban Warming in Desert Cities: Integrating Climatic Forcing and Urban Energy Dynamics over Riyadh (1985–2024) and Projections to 2050

Cities are not just collections of buildings; they are living systems that breathe, store heat, and change the air around them. For decades, scientists have known that as the planet warms, cities get hotter too. But there is a persistent puzzle in understanding exactly why a specific city feels so much warmer than the countryside surrounding it. Is the heat coming from the global climate changing, or is it coming from the city itself? The answer is rarely just one or the other. It is a complex mix of two forces working together: the slow, steady warming of the region's atmosphere and the intense, local heating caused by human activity, concrete, and the sheer energy we use to live our lives. Untangling these two threads has been difficult, especially in places like the desert, where the sun is already fierce and the ground holds very little water to cool things down.

In a new study focused on Riyadh, the capital of Saudi Arabia, researchers have built a clear way to separate these two sources of heat. They looked at forty years of temperature data, from 1985 to 2024, to see how the city has changed. The team developed a method that acts like a filter, allowing them to look at the background warming of the region and the extra warming caused by the city's growth as two distinct things. They found that while the region is indeed getting warmer, the city is doing something more. As Riyadh expanded, its population grew, and its energy use skyrocketed, the city began to trap and release heat more efficiently, amplifying the natural warming trend. The researchers also looked ahead to 2050, showing that the choices a city makes about how it grows and how much energy it uses will determine just how much hotter it gets in the future.

The story of Riyadh's heat is one of rapid transformation. Over the last four decades, the city has grown from a smaller settlement into a massive metropolis. During this same time, the average temperature in the region has been rising steadily. The researchers wanted to know how much of that rise was just the natural climate changing and how much was the city's own doing. To figure this out, they created a model that does not rely on complex, computer-heavy simulations that require thousands of data points. Instead, they used a simpler approach that connects the dots between what they could measure: the temperature, the number of people, the size of the city, and the amount of energy being used.

They started by looking at the long-term temperature record. The data showed a clear, unbroken line of warming. Over the forty-year period, the average annual temperature in Riyadh rose by about 2.4 degrees Celsius. This is a significant shift, equivalent to a rate of about 0.60 degrees Celsius every ten years. This warming was not just a few hot years in a row; it was a consistent trend that statistical tests confirmed was real and not a fluke. However, knowing the city got hotter was not enough. The team needed to understand why.

To separate the causes, the researchers broke the temperature rise into two parts. The first part was the "background" warming, which represents the heat the region would experience even if the city didn't exist. This is the large-scale climate change affecting the entire Arabian Peninsula. The second part was the "urban amplification," which is the extra heat generated specifically by the city. This extra heat comes from three main sources: the heat people and machines release directly (like air conditioners and cars), the way the city's density concentrates that energy, and the fact that buildings and roads store heat during the day and release it slowly at night.

The researchers introduced a new way to measure the city's energy load, which they called the Thermal Energy Index. Think of this index as a way to measure not just how much energy a city uses, but how efficiently that energy is packed into the space. As a city grows denser and uses more energy per person, this index goes up. The study found that as Riyadh grew, this index rose steadily. This meant the city was becoming better at trapping heat. The more people lived there, the more energy they used, and the more the city's surface changed from sand to concrete, the more the city amplified the natural warming.

The model they built was able to recreate the actual temperature history of Riyadh with high accuracy. When they compared the model's predictions to the real temperature records, the two matched up very closely. This gave the researchers confidence that their method was working. It showed that the combination of regional warming, direct human heat, and the city's ability to store and concentrate that heat could explain almost all of the warming observed over the last forty years. The model also revealed that the city's growth acted as a multiplier. It didn't just add a little bit of heat; it made the background warming feel stronger.

Looking forward, the researchers ran three different scenarios to see what might happen by 2050. In a "low-impact" scenario, where the city grows slowly and uses energy more efficiently, the warming continues but at a slower pace. In a "moderate" scenario, which follows the current path of development, the warming continues at the same rate as the past few decades. But in a "high-impact" scenario, where the city grows rapidly and energy use surges, the temperature rises much faster. The gap between these three paths widens over time. This shows that the future temperature of the city is not just a matter of what the global climate does; it is heavily influenced by how the city chooses to develop.

The study highlights a crucial point for city planners and residents alike. While no city can stop the background warming caused by global climate change, cities have a lot of control over the extra heat they generate. By managing how dense the city becomes, how much energy is used, and how buildings are designed to store or release heat, a city can change its own thermal future. The research suggests that the choices made today about urban growth will determine whether the city becomes significantly hotter in the coming decades or if it can manage to keep the warming in check.

The findings from Riyadh offer a clear lesson for other rapidly growing cities, especially those in hot, dry climates. The heat we feel is a partnership between the sky and the street. The sky provides the background warmth, but the street, with its buildings, traffic, and people, decides how much of that warmth is trapped and amplified. By understanding this relationship, cities can make smarter decisions. They can plan for growth that does not come at the cost of unbearable heat. The study proves that we can separate the global signal from the local noise, giving us a clearer picture of how our cities are changing and what we can do about it.

Ultimately, the work done in Riyadh shows that urban warming is a solvable problem, at least in terms of understanding it. It is not a mystery that requires impossible data or supercomputers to solve. It is a matter of looking at the energy we use, the people who use it, and the ground we build on. As cities continue to expand, the ability to measure and manage this extra heat will become increasingly important. The study provides a tool to do just that, offering a way to see the future of a city's climate and to shape it before the heat becomes too much to bear.