Sedge in Greenland, spring is almost a month earlier than a decade ago. (photograph: Pearl Bucknall/Alamy)
The climate is changing, and hopefully most of you will agree with me when I say that it is human induced change. The changing climate has many different implications and because temperature is rising globally and precipitation patterns are changing, there are various climate impacts. Some are well known, think of rising sea-levels, where others are less known, such as phenological changes. Phenology is the study of seasonal biological events, which includes growing seasons of plants. Studies show that with increasing temperatures the spring comes earlier, which can lead to, for example, false springs and changing agricultural patterns [Menzel 2002]. Maybe you noticed that some flowers started to grow in your garden or neighbourhood, but that they unfortunately did not survive due to low temperatures that followed. This phenomenon is known as false spring, which can affect agriculture and thus possibly negatively affect our food resources [Menzel 2002]. For this reason, it is important to understand what the impact is of climate change on, in this case, growing seasons of vegetation. Additionally, we can use phenological changes to understand how terrestrial ecosystems react to climate change. This could be a promising tool to quantify climate impact all over the globe. So what tool am I talking about here? What exactly is phenology, and how can we see if it changes? And more importantly: why is this so important to track these changes? In this blog I will dig into the relation between phenology and climate change to answer all these questions and more!
First of all, I want to elaborate on the emergence of phenology as a study. Ever since farmers existed, they have been using knowledge of growing seasons to estimate when to plant seeds and when to harvest their crops [Menzel 2002]. Later on, enthusiastic hobbyists started to track the blossoming of certain flowers they kept in their gardens, for example the Dutch brothers Bos observed 29 plants during the period of 1894 to 1932 [van Vliet et al. 2013]. In the 20th century larger frameworks came to existence to gather all observations, such as the Dutch Phenological Society in 1940 (ended in 1968) and the The International Phenological Gadens in the 1957. Ever since the first volunteers started to track their vegetation in their gardens, scientists have been investigating growing seasons and how it changes (www.naturetoday.com contains observations in the Netherlands). Around 1970, satellites started to also track vegetation growth, which allowed larger scale tracking of phenology This tracking happens by looking at changes in the Normalized Difference Vegetation Index (NDVI), which shows the plants activity by looking at the Near Infrared (NIR) and Red spectrum. The figure below shows how the NDVI is calculated and how it works. If you look at the NDVI over an entire year, you can see when the plant’s activity increases (the start of the growing season) and when it decreases (the end of the growing season).
Because of the large history of observations and with satellites that observe the entire globe, there is a lot of data on phenological changes, especially on growing seasons. Even though much research has been done, studies around the globe mention different outcomes, some see a small shift in growing seasons with ‘only’ 8 days per decade, where others state the growing season is moving a shocking 26 days per decade. With growing seasons coming earlier, chances of getting a false spring are higher, which is very worrying. So we see that the start of season is changing, but what exactly are the drivers behind these changes?
Temperature is not the only thing that influences the growing season, for example precipitation and day length also play an important role. Ivits et al.  investigated the changing growing seasons in Europe, and showed that temperature only plays a major role in certain climate zones, as can be seen in the figures below.
The right figure shows the different climate zones, the middle one shows the changes in the Start Of the Season (SOS) per year, and the right figure shows the change in the total Season Length (SL) per year. Apparently the growing season does not change the same in different areas. To make the relation between the phenological changes and climate change even more clear, I want to show you the next two graphs, from Post et al. .
The right figure shows that with increasing latitude global warming also increases (the black line is the mean and the grey lines are the 95% confident limits). When you compare this increase with the rate of seasonal change in the left figure, you can clearly see that higher latitude regions experience more warming. This is of course very alarming, as most of the glaciers can be found in the Arctic, Greenland, and Antarctic. With the growing season starting up to a month earlier, more and more ice will melt. With temperatures still increasing, and no clear sign of climate change coming to an end, the start of the growing seasons may come even earlier in the future.
It is alarming to see the growing seasons change so much, but tracking phenological changes can also give us insight on the scope of climate impacts. By using phenological changes as a proxy for climate impact, we are able to see where the impacts on terrestrial ecosystems are the highest. This will give us insight in the future, but even more important, it allows us to improve our adaptation policies. Using satellite derived phenological data is favourable for tracking phenology on a global scale, as this data is objective and able to cover more vulnerable areas as well (e.g. poor areas or war zones).
Phenology is an old field of study, and has helped farmers for many years now. Recently however, more and more phenological studies have been conducted to examine the relationship between observed changes and climate change, focussing mostly on temperature increase. With satellite data scientists are able to track phenological changes globally. These studies help us understand the climate impact on terrestrial ecosystems worldwide, and aid us in adapting to these changes, especially with regards to food production. So next time you see flowers popping up in your garden when you are still wearing gloves, you can explain to your neighbour the phenomenon of false spring and why it is so worrying…
Interested in this topic?
Ivits E, Cherlet M, Tóth G, Sommer S, Mehl W,Vogt J, Micale F. May 2012. Combining satellite derived phenology with climate data for climate change impact assessment. Global Planet Change. 88-99: 85-97. Available from: https://doi.org/10.1016/j.gloplacha.2012.03.010
Menzel A. September 2002. Phenology: its importance to the global change community. Climatic Change. 54(4): 379-385.
Van vliet AJH, Wichertje AB, Mulder S, van der Slikke W, Odé B. July 2013. Observed climate-induced changes in plant phenology in the Netherlands. Reg Environ Change. 14:997-1008. Available from: https://doi.org/10.1007/s10113-013-0493-8
Post E, Steinman BA, Mann ME. March 2018. Acceleration of phenological advance and warming with latitude over the past century. Nature. 8:3927. Available from: https://doi.org/10.1038/s41598-018-22258-0