1072. Definition, Impacts, and Causes of Heat Waves

It seems that the frequency and intensity of heat waves have been increasing recently not only in Japan, but around the world as well. The Copernicus Climate Change Service (C3S) has compiled a summary of the definition, impacts, and impact mechanism of heat waves on its website.

Defining a heatwave

There is no internationally agreed definition of a heat wave, and the threshold for a heat wave varies depending on the climate, geography, and social conditions of each region and country. The definition may also vary depending on how the temperature data is used. For example, the European State of the Climate 2023 defined a heat wave as "a period of at least three consecutive days when both the daily surface air temperature minima and maxima are higher than the highest 5% of values for the day in question during the 1991–2020 reference period."

Heat wave impacts

In most parts of the world, heat waves are accompanied by high temperatures. According to the World Health Organization (WHO), heat stress is the leading cause of weather-related deaths and can exacerbate underlying diseases such as cardiovascular disease, diabetes, mental health, and asthma, increasing the risk of accidents and the transmission of some infectious diseases.

Heat waves, especially when combined with long periods of dry weather and drought, have significant environmental and socio-economic impacts. Periods of extreme heat affect water availability in agriculture, industry, and domestication by reducing soil moisture, reducing river flow, and depleting groundwater storage. Reduced water supply can lead to disputes over water resources and increased costs for water withdrawal and distribution.

In agriculture, heat waves can severely damage crops and reduce yields. Higher-than-average temperatures stress plants, impair growth, and make them more susceptible to heat-related illnesses. Livestock are also affected, increasing the risk of heat stress and reducing productivity. Moreover, marine heatwaves can affect fisheries and aquaculture industries due to rising water temperatures, resulting in reduced yields, economic losses for producers, and higher food prices for consumers.

What causes a heat wave?

Heat waves are mainly caused by anticyclonic systems that trap warm air in a specific area. These high-pressure systems (also known as anticyclones) compress the air, creating a dome of heat that raises the temperature of the Earth's surface. Since there is no cloud cover under high pressure, there will be more solar radiation, which will further heat the ground and the air above it, which can lead to long-term heat waves.

Climate change is also an important factor contributing to the increase in the frequency and intensity of heat waves. Rising temperatures due to the accumulation of greenhouse gases in the atmosphere increase the likelihood of extreme heat waves. In addition, climate change can alter atmospheric circulation patterns, resulting in persistent anticyclones that encourage heat waves.

Other factors include urbanization and changing land use. Urban areas are experiencing a heat island effect, with higher temperatures than surrounding rural areas due to dense infrastructure and limited green space. Land-use changes due to deforestation and agriculture can also affect local temperature and humidity levels by altering the energy balance on the Earth's surface.

Heat wave mitigation and adaptation

The frequency, intensity, and duration of heatwaves are likely to continue to increase, and the combined effects of climate change, urbanization, and population ageing will severely impact public health in the future. Therefore, mitigation and adaptation strategies are essential to protect vulnerable populations. Urban planning is expected to play a role, such as promoting natural cooling by increasing green spaces and improving urban road network layout and building design to reduce the urban heat island effect. In addition, promoting energy-efficient buildings and cooling systems will help reduce overall greenhouse gas emissions while managing increased energy demand during extreme heat periods.

Public health efforts are also important to mitigate the effects of heat waves. Early warning systems can alert communities to impending heat waves and allow them to take preventative measures.

The ultimate way to effectively combat the increasing frequency and intensity of heatwaves is to take decisive action on climate change. It is essential to reduce greenhouse gas emissions through the transition to renewable energy sources, the introduction of energy-efficient technologies, and the adoption of sustainable practices.

Contributor: IIYAMA Miyuki, Information Program