Response of Urban Heat Stress to Heat Waves in Athens (1960–2017)

Description

In many European cities, heat stress is increasing in the summer. The cities of the eastern Mediterranean, such as Athens, are most vulnerable.

The large urban area of Athens has experienced increasing trends in air temperature in general and heat wave frequency, especially since the mid-1970s. The annual number of days with maximum air temperature above 35°C is increasing, and so is the number of days with extreme heat stress.

Heat stress is not only determined by air temperature, but also other parameters, like wind speed, humidity and insolation. They also determine how we experience a hot day. These parameters can be combined into heat stress indices. These heat stress indices,  reflect thermal comfort better than temperature alone. For the city of Athens, the trends of 4 commonly used heat stress indices have been quantified for the period 1960-2017 on an hourly basis.

The results show that the population of Athens is exposed to a significant, increasing risk of heat stress since the 1960s, further increasing in the last two decades. This increase varies from 0.3%/decade to 0.9%/decade, depending on the heat stress index and is expressed in the additional time per year that the inhabitants of Athens are exposed to conditions of extreme heat stress. Despite this statistically significant trend over the entire study period, a decline is observed during the last decade.

The population of Athens is exposed to significantly higher heat stress levels in recent decades, compared to the past ones, during the nighttime hours, when the human body should cool down. The time that people are exposed to high heat stress has increased at a faster rate for nighttime than for daytime conditions.

The occurrence of the first heat stress conditions has shifted to earlier in the year. The occurrence of the last heat stress conditions has shifted to later in the year. As a result, the period of heat exposure has expanded.

Reference information