Heat is one of the issues addressed in the Delta Plan on Spatial Adaptation, along with droughtwaterlogging, and urban flooding. The KNMI’14 scenarios of the Royal Netherlands Meteorological Institute not only show that average temperatures are rising; heat extremes are increasing as well. This page provides a brief explanation of the heat issue and links to other relevant pages within and beyond the Knowledge Portal.

Why is heat increasing?

Global warming

Greenhouse gases in the atmosphere retain solar heat radiation and create a “natural” greenhouse effect. Without this effect, the average temperature on Earth would be 33 degrees lower. Human activity, such as burning fossil fuels and deforestation, releases more greenhouse gases (among which CO2, CH4, O3, H2) into the atmosphere. This boosts the greenhouse effect and results in global warming.

Heat island effect

Usually, cities are warmer than surrounding rural areas, because cities retain more heat. This is referred to as the urban heat island effect. Changes in our urbanisation (more built-up surface area, denser development, and increasing levels of paving in built-up areas) are adding to the heat island effect. The heat island effect is generated by:

  • Absorption of solar radiationby (stony) materials;
  • Lack of evaporationdue to a limited presence of greenery and water: a large proportion of the incoming radiation is converted into tangible heat;
  • Emission of heat by human activity, such as industry and households.

The difference in air temperature between cities and rural areas is largest at night. This is because after sunset, a city will cool down at a slower pace than rural areas. The temperature difference may run up to more than 7 degrees. The extent to which the heat island effect occurs is location-dependent: local spatial features determine how warm a location will get. Key factors in this respect are:

  • The proportion of built-up surface area;
  • The proportion of paved surface area;
  • The proportion of green surface area.

Other factors affecting the heat island effect are the building height – street width ratio (which affects the absorption of solar radiation), the degree of thermal radiation by buildings and other surfaces into the atmosphere, and the degree of ventilation in the street. Ergo, the layout of a street or neighbourhood affects local temperatures: the heat island effect differs from one neighbourhood to the next. The impact of heat on the human body is also highly dependent on the local layout. For example, wind has a positive effect on perceived temperatures during summery days. More information on the heat island effect of various types of neighbourhoods is provided in the book of examples entitled “Het klimaat past ook in uw straatje”.

What are the consequences of increasing heat?

The increasing heat extremes affect us in various ways. For example, increasing heat will result in health issues and add to mortality among vulnerable groups, such as the elderly and chronically ill. Furthermore, heat can kerb productivity and lead to sleep disorders. Another effect is that heat causes materials to expand. As a result, for example, bridges fail to open or close. The NAS conceptual diagrams present a schematised overview of the impact of heat in nine sectors, such as health, safety, nature, agriculture, and infrastructure. There is also a mind map indicating the impact of heat in cities.

Will the future be warmer?

Between 1880 and now, the average global temperature has risen by 0.9 degrees. In the Netherlands, temperatures have risen more: since 1901, our country has become some 1.8 degrees warmer. As a result of climate change, the Netherlands will heat up by 1 to 2.3 degrees by 2050, vis-à-vis our current climate. By 2085, the temperature rise may even increase to 3.7 degrees Celsius.

The Climate Impact Atlas shows how heat increases at a location on account of climate change. The Spatial Adaptation standardised stress test information leaflet indicates how the Atlas can be used to map out an area’s vulnerabilities.

How can we contain the impact of heat?

Various measures can be implemented to combat heat, such as adapting buildings and the infrastructure, greening gardens, and behaviour adjustments. Because vulnerability to heat is location-dependent, measures will need to be selected at the local level. An integrated approach, factoring in other climate issues (such as drought or waterlogging) is preferable.

Generally speaking, evaporation improves a city’s resistance to heat. Evaporation may be achieved by introducing green and blue elements (such as parks, green gardens, and bodies of water). Proper hydration of urban greenery is important to keep evaporation up to standard. The impact of water on urban temperatures is non-equivocal and depends on a wide range of factors, such as depth, circulation, surface area, and location vis-à-vis buildings. More information is provided in the interview on the cooling effect of water in the city. The REALCOOL research project comprises design guidelines for urban bodies of water. The animated films below show examples of heat-proofing a street or garden. The huisjeboompjebeter database of measures provides tools for residents to climate-proof their gardens. The Green-blue Grids database contains suggestions for designers and policy-makers.