B4. Pressure from climate change

Spring Index


Type: Context indicator


Indicator Description

This is a context indicator, and is not assessed; it is shown to highlight a biological response to climate change and a potential pressure on biological systems.  It shows the impact of temperature change on the timing of biological events such as flowering or migration in the spring. The UK Spring Index is calculated from the annual mean observation date of the following four biological events: first flowering of hawthorn (Crataegus monogyna), first flowering of horse chestnut (Aesculus hippocastanum), first recorded flight of an orange-tip butterfly (Anthocharis cardamines), and first sighting of a swallow (Hirundo rustica).




Since 1999, the annual mean observation dates have been around 6 days in advance of the average dates in the first part of the 20th century.  

The Index shows a strong relationship with mean temperature in March and April, and it advances more rapidly when the mean temperature equals or exceeds 7 degrees Celsius.  


Figure B4i.  Index of the timing of biological spring events (number of days after 31 December) in the UK, 1891 to 1947, and 1999 to 2017.


Notes: * Number of days after 31 December (e.g. day 121 = 1 May).

Source: 1891 to 1947 – Royal Meteorological Society; 1999 to 2017 – UK Phenology Network.


This is a contextual indicator showing how changes in climate, particularly temperature, are associated with changes in the timing of biological events.

The Spring Index for the UK has high year-to-year variability, but since 1999 biological events in the spring have occurred around 6 days in advance of the average dates in the period 1891 to 1947 (Figure B4i).  The figures published since 2015 are slightly different to those published previously as a result of data correction in the underpinning database. 

The advancement of spring events is strongly linked to warmer temperatures in March and April.  The mean observation dates in 2011 were the second earliest for which there are records.  The warmest April in the Central England Temperature series (1659 onwards) occurred in 2011 and was almost certainly influential. 


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Last updated: July 2018

Latest data available: Spring 2017

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