The current era of commercial English winegrowing can be thought of as dating from 1951, when Guy Salisbury-Jones began the first planting at what would become Hambledon Vineyard. English wines now routinely garner top awards in international competitions, with one of the critical contributory factors often cited as being Britain’s warming climate. It is well documented that during the 20th century the mean annual temperature of central England rose by ca. 1.0°C (DECC, 2013). This figure includes changes that have taken place throughout all seasons, whereas viticulturalists are most interested in changes to the growing season, which in the UK roughly corresponds to the period from beginning of April to end September.
The best dataset for assessing temperature changes in central England is the Central England Temperature series (“HadCET“) maintained by, and available through, the Met Office. This averages temperature data from a number of different recording stations in order to best represent the English Midlands. It contains daily temperature records back to 1772 and plays an important role in climate change research.
A number of indices have been proposed to assess the temperature parameters of particular geographic regions. For viticulturalists the most important and widely used are Growing Degree Days (GDD; sometimes termed the Winkler Index) and the Huglin Index. The formulae used to calculate the indices from the HadCET is shown opposite.
The Growing Degree Day index over the period from 1950 to 2104 is shown in the figure with the blue bars. There is very considerable vintage variation, with large changes frequently occurring between adjacent growing seasons. Within the annual variation there is a significant underlying trend which shows an increase of 175 GDD units since 1950. As a result of this upward trend, GDD in the past 20 years for central England frequently exceeds 800 GDD units, whereas previously this only occurred in exceptional hot years such as 1959, 1976 and 1989.
The Huglin Index (figure with red bars) shows a very similar pattern to the GDD graphic, with considerable variation between vintages and an underlying upward trend. The trend line for the Huglin Index shows an increase of ca. 210 units since 1950.
These two figures lend support to the idea that the English growing season is becoming warmer and making English viticulture a less hazardous business. Nonetheless the very considerable variation between vintages means that English viticulturalists will need to continue to keep a very close eye on their vineyards if they are to harvest their grapes with optimal ripeness.