Autumn is ending later and Spring is starting earlier in the northern hemisphere, according to new research.

 

A study by the University of Southampton suggests that on average the end of Autumn is taking place later in the year and Spring is starting slightly earlier.

A team of researchers examined satellite imagery covering the northern hemisphere over a 25 year period (1982 – 2006), and looked for any seasonal changes in vegetation by making a measure of its ‘greenness’. They examined in detail, at daily intervals, the growth cycle of the vegetation – identifying physical changes such as leaf cover, colour and growth.

The project was led by University of Southampton Professor of Geography Peter Atkinson, who worked with his colleague Dr Jadunandan Dash and in collaboration with Professor Jeganathan Chockalingam from the Department of Remote Sensing at the Birla Institute of Technology in India.

Professor Atkinson says: “There is much speculation about whether our seasons are changing and if so, whether this is linked to climate change. Our study is another significant piece in the puzzle, which may ultimately answer this question.”

The team was able to examine the data for specific vegetation types: ‘mosaic’ vegetation (grassland, shrubland, forest and cropland); broad-leaved deciduous forest; needle-leaved evergreen forest; needle-leaved deciduous and evergreen forest; mixed broad-leaved and needle-leaved forest; and mixed-forest, shrubland and grassland. They analysed data across all the groups, recognising that forests which have not changed size due to human intervention, for example through forestry or farming, provide the most reliable information on vegetation response to changes in our climate.

The most pronounced change found by the researchers was in the broad-leaved deciduous and needleleaved deciduous forest groups, showing that Autumn is becoming significantly later. This delay in the signs of Autumn was generally more pronounced than any evidence for an earlier onset of Spring, although there is evidence across the groups that Spring is arriving slightly earlier.

Professor Peter Atkinson comments: “Previous studies have reported trends in the start of Spring and end of Autumn, but we have studied a longer time period and controlled for forest loss and vegetation type, making our study more rigorous and with a greater degree of accuracy.

“Our research shows that even when we control for land cover changes across the globe a changing climate is significantly altering the vegetation growth cycles for certain types of vegetation. Such changes may have consequences for the sustainability of the plants themselves, as well as species which depend on them, and ultimately the climate through changes to the carbon cycle.”

The study used the Global Inventory Modelling and Mapping Studies (GIMMS) dataset and combined satellite imagery with an innovative data processing method to study vegetation cycles.

Highlights

Remote Sensing Of The Environment lists the highlights of this research as follows:

Terrestrial vegetation phenology above 45°N was analysed using remote sensing data.

Trends in vegetation phenology were analysed, controlling for land cover changes.

Latitudes 55°N to 65°N experienced the greatest changes in vegetation phenology.

Needle leaf deciduous vegetation had the maximum decrease in SOS (− 1.07 days yr^− 1).

Broad leaf deciduous vegetation had the maximum delay in EOS (+ 1.06 days yr^− 1).

Abstract

Trends in the start or end of growing season (SOS, EOS) of terrestrial vegetation reported previously as latitudinal averages limit the ability to investigate the effects of land cover change and species-wise conditioning on the presented vegetation phenology information. The current research provided more reliable estimates of the trends in the annual growth pattern of terrestrial vegetation occurring at latitudes greater than 45°N. 25 years of satellite-derived Normalised Difference Vegetation Index (GIMMS NDVI) was used and reliable vegetated pixels were analysed to derive the SOS and EOS. The rate of change in SOS and EOS over 25 years was estimated, aggregated and scrutinised at different measurement levels: a) vegetation type, b) percentage vegetative cover, c) core area, d) percentage forest cover loss, and e) latitude zones. The research presents renewed and detailed estimates of the trends in these phenology parameters in these strata. In the > 45°N zone, when only reliable pixels were considered, there was an advancement of − 0.58 days yr^− 1 in SOS and a delay of + 0.64 days yr^− 1 in EOS. For homogeneous vegetated areas (91–100% cover at 8 km spatial resolution) the 55–65°N zone showed the maximum change with − 1.07 days yr^− 1 advancement in SOS for needle leaved deciduous vegetation, and − 1.06 days yr^− 1 delay in EOS for broad leaved deciduous vegetation. Overall, the increasing trend in EOS during senescence (September to November) was greater in magnitude than the decreasing trend in SOS during spring (March to May) and the change in EOS was more consistent and greater than that in SOS.

Citation

Remotely sensed trends in the phenology of northern high latitude terrestrial vegetation, controlling for land cover change and vegetation type by C. Jeganathan, J. Dash, P.M. Atkinson published in Remote Sensing of Environment Volume 143, 5 March 2014, Pages 154–170. DOI: 10.1016/j.rse.2013.11.020

Read the abstract and get the paper here.

Source

University of Southampton news release here.

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