Analysis: images taken by satellites can inform us about plant growth and how this varies with climate events

By Hannah White and Jon Yearsley, UCD

In Spring 2013, Ireland's farmers were in crisis. Headlines were stark: "Helpline number for fodder crisis issued" and "20 per cent increase in dead animals as fodder crisis deepens". Across the country, farmers were struggling to find feed for their cattle because spring was unseasonably cold and the green grass of Ireland wasn’t growing. The signs of this crisis were clear far beyond the fields. They could be seen from space, and this perspective on Ireland could hold the answer to understanding how the plants in our fields and beyond respond to extreme weather events. 

From space, the fodder crisis of 2013 could be seen in the light that was reflected from our Earth’s surface (Figure 1). Satellites can capture this light and form images of our Earth’s surface. Some of this light is visible to the human eye (eg red and blue), but satellites can also detect light at wavelengths that we are unable to detect with our eyes (eg near-infrared).

Figure 1: Satellite estimates of Ireland's average anomaly in grass production for 2013 (fodder crisis), 2015 and 2018 (summer drought). These estimates use light reflected from Earth’s surface.  An average year (such as 2015) has an anomaly around zero. An anomaly below zero indicates below average grass growth. Both 2013 and 2018 show a prolonged period of extremely below average grass production across Ireland. An anomaly of -1.7 (the April 2013 value) is a 1 in 24 year event.

In isolation, these individual colours (or wavelengths of light) do not tell us much. Combined, however, they can provide useful information. For example, by looking at the difference in the amounts of reflected red and near-infrared light (called the vegetation index, Figure 2) we can determine from space how well plants are growing. This works because vigorously growing plants reflect relatively little red light compared to near-infrared, as it is the absorption of red light that fuels their growth (photosynthesis). In contrast, soil is not photosynthesising and, although it absorbs lots of red light, it also absorbs lots of near-infrared (unlike plants), giving a low vegetation index.

Figure 2. Satellite images of Dun Laoghaire harbour using reflected red light (left) and near-infrared light (middle). Combining these two images produces a vegetation index (right) that "lights up" regions where red light is being preferentially absorbed by growing plants (green areas).  The roof of the old ferry terminal (black box) reflects a lot of red light, but it is an equally strong reflector of near-infrared light, so it provides no signal of growing plants (low vegetation index represented by white regions). In contrast, the seaward side of the east harbour wall is covered by algal growth (red box). Despite being a weak reflector, the reflected red light is much weaker than near-infrared, giving a strong signal of plant growth (high vegetation index represented by green areas). The People's Park (blue box) also "lights up" as a region of vigorous plant growth due to low reflected red light. 

The real power of this data from space is not in any one image, but in the fact that satellites have been collecting this information every few days for decades. This allows scientists to compare any single event against the long-term average. A departure from the long-term average is called an anomaly. Even from space, we know that the spring of 2013 was a crisis because the anomaly was strongly negative for a prolonged period (Figure 1), meaning that plant growth was far below that expected for the time of year.

The regular nature of satellite images, and the often global extent of their cover, provide an incredible resource enabling us to see how the planet is changing over weeks, months and years. This means that we can see how widespread these dips in plant growth are, how quickly plant growth recovers following a dip and when large dips in plant growth have occurred. Plant growth is the food we eat, both directly and as fodder for livestock. More importantly, we need this growth to recover quickly from extreme climate events, so that it is consistent over time and keeps up with our constant demand. 

In our recent study, we show how satellite data can be used to see if landscapes maintain plant growth over time by determining how frequently we are seeing large dips in plant growth, and the ability to recover plant growth following one of these dips. We used satellite images taken across the whole island of Ireland to look at plant growth. In a country covered by more than 60% agricultural pasture, where plant growth in grasslands is of significant economic importance, determining how vegetation indices recover in time following a disturbance shows how plant growth might be threatened by global pressures such as climate change. 

It is possible to determine how landscapes react not only to single extreme climatic events, but also to smaller periods of unusual weather patterns or human pressures on the landscape

Over the next 30 years, extreme events such as droughts and floods are projected to increase both in number and severity. Using the measures that we have developed, it is possible to determine how landscapes across the globe react not only to single extreme climatic events such as these, but also to smaller, more prolonged periods of unusual weather patterns or human pressures on the landscape. These measures can then go on to be used to determine why some places are able to maintain plant growth better than others under climate change. 

Over recent years, we have all experienced the effects of a changing climate on plant growth, from scorched lawns to national scale fodder crises. These effects have put huge pressure on growers and farmers across Ireland and in turn, threatened the national economy and food security. Satellite images provide us with a way of keeping track of plant growth and put it into a long term context. This will help us to plan for resilient landscapes in the face of climate change. 

This work was conducted with the financial support of Science Foundation Ireland and the Department for the Economy, Northern Ireland under Grant number (15/IA/2881)

Dr Hannah White is a Postdoctoral Research Fellow in the School of Biology and Environmental Science at UCD. Dr Jonathan Yearsley is a Lecturer in Ecological Modelling in the School of Biology and Environmental Science at UCD.

The views expressed here are those of the author and do not represent or reflect the views of RTÉ