Accredited official statistics

Spring index

Updated 10 December 2024

Applies to England

© Crown copyright 2024

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This publication is available at https://www.gov.uk/government/statistics/england-biodiversity-indicators/18-spring-index

Last updated: 2024

Latest data available: 2023

Introduction

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 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).

Data for this indicator can be found in the published datafile. We also have a similar indicator at the UK level.

Type of indicator

Context indicator

Type of official statistics

Official statistic

Assessment of change

Assessment of change in the timing of spring events: No assessment - indicator provided for context only showing how changes in climate, particularly temperature, are associated with changes in the timing of biological events.

Key results

The Spring Index for England has high year-to-year variability, but since 1998 the annual mean observation dates have been 8.8 days in advance of the average dates in the first part of the 20th century (Figure 1).

The advancement of spring events is strongly linked to warmer temperatures in March and April. The mean observation dates in 2011 are the earliest for which there are records. The warmest April in the Central England Temperature series occurred in 2011 and resulted in an especially early spring that year.

Figure 1: Index of the timing of biological spring events (number of days after 31 December) in England, 1891 to 1947, and 1998 to 2023

Source: 1891 to 1947 – Royal Meteorological Society, 1998 to 2023 – UK Phenology Network (Nature’s Calendar, currently funded by the People’s Postcode Lottery, Postcode Green Trust)

Notes about Figure 1

  • Index denotes number of days after 31 December (for example, day 121 = 1 May).
  • Not directly comparable to previous versions of the indicator following a change in methodology in 2019.

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 seven degrees Celsius.

The average value of the Spring Index between 1998 and 2023 is 8.8 days earlier than the average value for 1891 to 1947. The index shows a strong relationship with mean temperature in March and April (Figure 2), and it advances more rapidly when the mean temperature equals or exceeds 7 degrees Celsius. The average advance in dates for the Spring Index is 3.8 days for every one degree increase in Celsius when the mean March to April temperature is 7 degrees Celsius, and 7.4 days for every one degree increase in Celsius when the mean March to April temperature is above 7 degrees Celsius.

Figure 2: Relationship between Spring Index and Central England Temperature (CET), 1891 to 1947, and 1998 to 2023

Source: 1891 to 1947 – Royal Meteorological Society, 1998 to 2023 – UK Phenology Network (Nature’s Calendar)

Notes about Figure 2

  • Index denotes number of days after 31 December (for example, day 121 = 1 May).
  • CET = Central England Temperature series, in degrees Celsius.

Further detail

For further information on the methods used in this indicator, please see the Technical Annex below.

Relevance

Phenology is the study of the timing of recurring natural events in relation to climate, and such observations provide year-on-year information on how nature is responding to a changing climate. This indicator illustrates the stress that one aspect of climate change (spring warming) can place on biological systems. Differential responses among species may cause problems for life cycles (e.g. pollinating insects emerging out of synchrony with flowers opening in spring), increasing vulnerability to extreme events such as late frosts, disruption of food webs, and changing the balance of competition between species.

Acknowledgements

Thank you to the many people who have contributed by providing data and to the many colleagues who have helped produce this indicator. In particular, thanks go to all those citizen scientists contributing their data to Nature’s Calendar and to the Woodland Trust.

Technical Annex

Methodology

The England 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 appearance of orange tip butterfly (Anthocharis cardamines) and first sighting of a swallow (Hirundo rustica).

These four events were chosen for the following reasons:

  • good England coverage
  • well recognised and easily recorded species
  • good historical data
  • both early and later spring events are included
  • the observations cover plants, a vertebrate, and an invertebrate
  • a strong phenological response is observed

For data collected up to and including 2018 the overall index was compiled by calculating an annual mean across all sites where all four biological events were recorded. As there was no missing data, the England Spring Index could be calculated in two ways: the average of the four events or the average of the locations (the same value will be returned). After the England Spring Index was calculated, the raw data was not used in the calculation again.

In order to include more data in the calculation the method was changed in 2019 and the overall index was compiled by calculating an annual mean for each species using all available data and averaging these four means. This new method of calculation (see below) has been applied to all previous years retrospectively and has been submitted for external review.

The number of locations will vary from year to year. The England Spring Index varies year to year mostly because of differences in the temperature.

The average date of these events is now (1998 to 2023) about 8.8 days in advance of the average for the period 1891 to 1947. The England Spring Index shows a very significant relationship with mean March-April Central England Temperature, which appears to be stronger when mean temperatures exceed 7⁰C. For the England Spring Index, the relationship in years with mean March-April temperatures below 7⁰C was:

England Spring Index = 150.20 - 3.76 x Temperature, R2 = 39.31%, p<0.001

The equivalent for years when mean March to April temperatures equalled or exceeded 7⁰C was:

England Spring Index = 175.15 - 7.40 x Temperature, R2 = 78.75%, p<0.001

Data for the 1891 to 1947 period comes from the phenological network of the Royal Meteorological Society as published in the Quarterly Journal of the Royal Meteorological Society. This was a voluntary network of recorders that submitted their phenological observations to a central coordinator. All these data have now been digitised and loaded into the Nature’s Calendar database. The UK Phenology Network/Nature’s Calendar project runs along broadly similar lines. As a citizen science project, it also involves volunteers contributing their observations. It is run and coordinated by the Woodland Trust and funded by the People’s Postcode Lottery, Postcode Green Trust. Here we use their data from 1998 onwards. The number of records varies from year to year, but the current scheme is substantially bigger than the earlier one.

Since locations reflect the distribution of volunteer recorders they are not evenly spread across England, but rather reflect the density of human population, with a greater number of recorders in the South-East and fewer in the North.

The reliability of the data is dependent on the frequent and timely observations of the natural world by the volunteer recorders. The data show such a strong relationship with temperature that we have confidence in their reliability.

Changes to the method in 2019

  • Previously, the Spring Index was calculated using only data from locations for which all four Spring Index species have been recorded. Recorders gave a postcode and could record anywhere within 10 km of that postcode.

  • However, in 2017 (when Nature Calendar’s website was launched), recording location changed to plotting records on a map and each recording location is now the area of a circle with only a 20m radius.

  • This change means that it is much less likely that a single recorder can record all four Spring Index species in a single location.

  • The new method (as applied to data utilised since the 2020 publications) takes into account all the individual sightings submitted.

  • Consequently, the new method increases the amount of data that can be used in the calculation. It also allows an additional year of data (1998) to be included in the dataset.

Development plan

Since our previous publication we have adapted the language and visualisations used in this indicator. We are keen to hear from our users about these changes, as well as our published development plan, please email us.

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