How much drier will it be?

Springs and summers are becoming particularly drier, whilst the probability of prolonged drought is growing. But how much drier will it be? Why is drought difficult to predict? And what causes drought?

How much drier is it according to KNMI?

According to Climate Signal ’21, inland drought has increased since 1965. Coastal areas have not seen increasing drought. This can be attributed to a sharper increase in solar radiation in inland areas, compared to the coast. In addition, the coast has seen more rain. Furthermore, according to KNMI observations, over the past 30 years temperatures, solar radiation, evaporation, and precipitation deficits have increased in the Netherlands. Such changes are particularly manifest in spring. Since the 1990s, solar radiation in this season has grown by more than 4 per cent per ten years. The maximum precipitation deficit in the growing season (April up to and including September) has grown by more than 8 per cent per ten years over the past 30 years.

How much drier will the future be?

Every summer, the Netherlands is faced with drought and with a precipitation deficit. In the event of an excessive precipitation deficit, such as occurred in the summers of 2018 and 2019, drought can cause problems. The “It is becoming drier” map narrative of the Climate Impact Atlas shows that in the future, the summer precipitation deficit may increase. These figures are based on the statistics underlying the KNMI’14 scenarios. In the current climate, an extremely dry year occurring once every 10 years will entail a precipitation deficit of 225 mm at De Bilt. In the most extreme climate scenario, this precipitation deficit will have risen to 301 mm by 2050, which compares to the dry summer of 2018. By 2085, the value will rise even further to 331 mm. In addition, the KNMI statistics show that under the highest climate scenario, the average annual number of consecutive dry days will increase in the future. In 2014, De Bilt measured an average of 17 consecutive dry days, involving less than 0.3 mm of precipitation. By 2085, this number may rise to an average of 20 consecutive days annually. Climate Signal ’21 confirms, based on new climate models, that the probability of drought and precipitation deficits will increase. The probability of summer drought will be higher and dry periods may extend longer.

Why is drought difficult to predict?

According to KNMI, drought remains difficult to predict. This is due to the fact that many different factors affect the volume of water evaporating, e.g., cloud cover, humidity, wind speed, and wetness of the soil, but also the density of tree foliage and tree root depth. In addition, many processes are at play that reinforce and influence one another. Calculating the effects of such processes in models is difficult. For example, dry air will produce less rain and contain fewer clouds. Ergo, solar radiation will be higher, which will lead to more evaporation and thus increased drought.

What is causing the increases in drought?

Springs and summers are likely to become drier, whilst the probability of prolonged drought is also increasing. KNMI Climate Signal ’21 identifies several contributing factors:

• In the future, summer precipitation will likely remain constant or decrease, whereas evaporation will increase. The increase in evaporation is primarily caused by higher temperatures and boosted by a decrease in cloud cover, which will result in more solar radiation.
• The future may see more frequent easterly winds, bringing drier air to our country. This change can, in part, be attributed to a weakening of the Gulf Stream, which has a cooling effect on the North Atlantic Ocean. Concurrently, the Mediterranean area is warming. Together, these changes add to the probability of easterly winds.
• Changes in spring air currents may entail a higher probability of summer drought, as a result of various air-land processes.
• Arctic warming is adding to the probability of prolonged drought: as the Arctic and the Antarctic heat up, the force of the jet stream weakens. The jet stream is an extremely powerful high-altitude wind. A weakening jet stream will increase the probability of continuous weather situations. This may lead to prolonged periods of drought.

What else is conducive to drought?

Precipitation volumes affect the precipitation deficit. In terms of drought, however, the type of precipitation also matters. A longer period of drizzle is less conducive to drought than short torrential rains. In short torrential rain, water is drained quickly; only a limited proportion is absorbed by the water system and in the soil. Furthermore, rainwater does not easily seep into dry soil. Another factor that affects drought is the freshwater supply via the rivers Rhine and Meuse. Climate Signal ’21 expects the probability of low river water levels in summer to increase. This will exacerbate any drought.