Heat and cold — mean air temperature

Definition

The growing degree days index provides a measure of the accumulated heat available for vegetation growth. It is calculated from the accumulated sum over the year of daily mean temperature exceedances of a base temperature threshold. A base temperature of 5 °C is considered representative of most European crops. Other definitions of the index are possible, including variations in the base temperature, the use of an upper temperature threshold and restrictions for a given growing season.

Index factsheet (ETC/CCA Technical Paper): Growing degree days

Relevance

The growing degree days index has wide applicability in agriculture. It is used primarily to characterise plant phenology for a large variety of crops in Europe, but also to predict conditions that favour pest outbreaks and influence the insect generation cycle during the growing season.

Past and projected changes

Growing degree days have increased since the 1980s throughout Europe, particularly in southern Europe. Further increases are projected throughout the rest of the century under the medium- and high-emissions scenarios (representative concentration pathway (RCP)4.5 and RCP8.5), with the largest absolute change in southern Europe, whereas the increasing trend is projected to stabilise by the middle of the century under the low-emissions scenario (RCP2.6).

Further information (European Climate Data Explorer): Biologically effective degree days, 2011-2099

Annual growing degree days for the European land area and sub-regions 

Definition

The heating degree days index is a proxy for the energy required for heating buildings. It is computed from the cumulated daily deviation of the outdoor air temperature from a base temperature threshold from October to March. The base temperature threshold, period considered and calculation methodology can vary according to the local climate and applications. A base temperature of 15.5 °C is here considered representative of the pan-European scale, and daily minimum, mean and maximum temperature values are used as input variables.

Index factsheet (ETC/CCA Technical Paper): Heating degree days

Relevance

The heating degree days index is relevant mainly for energy-related applications and complements the information provided by the cooling degree days index. A decrease in heating degree days could result in a significant reduction in energy consumption for heating in Europe.

Past and projected changes

Heating degree days have decreased throughout Europe since the 1980s. The largest decrease has been in northern Europe where the absolute value is highest. The decline in heating degree days is expected to continue under all emissions scenarios. The decline could reach about 40 % by the end of the 21st century under the high-emissions scenario (representative concentration pathway (RCP)8.5), whereas the initial declining trend is projected to stabilise from the middle of the century under the low-emissions scenario (RCP2.6).

Notes:   The black lines show the annual values for 1950-2020 from reanalysis data, and the dashed horizontal lines show the means for 1986-2005. Solid grey, blue, yellow and red lines represent the ensemble medians of model simulations for the historical period and under low-, medium- and high-emissions scenarios (RCP2.6, RCP4.5 and RCP8.5, respectively) (smoothed by a 20-year moving average). Shaded areas show the 15th and 85th percentile ranges of the model ensembles. The size of the model ensemble used for each scenario is shown by the coloured numbers in the top-right corner.

Source:  ERA5 and bias-adjusted CMIP5 data.

Definition

The cooling degree days index is a proxy for the energy demand for cooling buildings. It is computed from the cumulated daily deviation of the outdoor air temperature from a given base temperature threshold from April to September. The base temperature threshold, period considered and calculation methodology of this index can vary according to the local climate and applications. A base temperature of 22 °C is here considered representative for assessing the energy demand at the pan-European scale, and daily minimum, mean and maximum temperature values are used as input essential climate variables.

Index factsheet (ETC/CCA Technical Paper): Cooling degree days

Relevance

The cooling degree days index is widely applicable in the energy sector. A significant increase in the number of cooling degree days can increase Europe’s energy use and lead to higher costs and higher impacts on the peak capacity of supply networks (EEA, 2019: Adaptation challenges and opportunities for the European energy system: Building a climate-resilient low-carbon energy system).

Past and projected changes

Cooling degree days have increased in Europe since the 1980s, with the largest increase observed in southern Europe. This increase is projected to continue, with a small increase, a doubling and approximately a quadrupling of cooling degree days in Europe projected under the low-, medium- and high-emissions scenarios (representative concentration pathway (RCP)2.6, RCP4.5 and RCP8.5), respectively, during the 21st century. The largest increase is projected for southern Europe, whereas only small increases are projected for northern Europe.

Annual cooling degree days for European land area and sub-regions