Earth is going through widespread and rapid change, in our atmosphere, on land and in the oceans. An important theme in Earth from Space is the use of satellite data for the detection of change – our changing landscape, cities, glaciers and even the amount of plankton growing in our oceans. As the satellite record gets longer, and the information they collect gets more detailed, Earth observation data is providing increasingly valuable information on our changing planet.
The Arctic is the fastest warming region on Earth, warming at twice the average rate of the rest of the planet.
One area of Earth that is changing dramatically, and where satellites have been critical for detecting these changes, is the Arctic. The Arctic is the fastest warming region on Earth, warming at twice the average rate of the rest of the planet. As it gets warmer, the extent of sea ice that covers the ocean in the polar north decreases. Satellites accurately map the seasonal fluctuations and long-term changes in sea ice, and these have shown that since 1980 the extent of sea ice at the height of northern summer has decreased by 2.5 million km2. This sea ice plays a crucial role in the climate of the region because sea ice has a high albedo, meaning it reflects a high proportion of the solar radiation that reaches the surface, keeping it cool. Open water is darker and absorbs more radiation, and therefore warms up more. This loss of sea ice creates a positive feedback process, whereby the warmer ocean leads to less sea ice, which in turn leads to more open ocean absorbing more radiation and further warming of ocean surface. The decrease in ocean albedo contributes to a process known as Arctic amplification, and the rapid warming that is occurring in the region.
But it is not just the ocean that is changing in the Arctic. Over recent decades satellites have detected significant changes on land as well. Plants contain a pigment called chlorophyll, which gives them their green colour. Chlorophyll is responsible for absorbing the sunlight that drives the process of photosynthesis, ultimately providing the energy for life on Earth. We see plants as green because chlorophyll absorbs less green light than the red or blue parts of the spectrum, but overall plants absorb a lot and reflect much less in the visible part of the spectrum. In contrast, in the near infrared region they absorb less and reflect much more. Satellites exploit this difference in reflectance between the visible and near infrared regions to map the extent and amount of vegetation on Earth’s surface. This differential reflectance is captured in an index known as the Normalised Difference Vegetation Index (NDVI), and since the early 1970s satellites with the ability to separate these bands have been orbit, providing immensely valuable information on global vegetation dynamics. Other surfaces, such as bare soil, do not show such a large difference in visible and near infrared reflectance, and areas with more vegetation have higher NDVI values.
Now that we have a few decades of such data, we can look for trends in the data – is it changing over time, and if so, how? Analyses over the Arctic region have revealed something dramatic. Reliable NDVI data is available from 1982 and this record shows a steady increase in the vegetation index across much of the Arctic tundra – those areas north of the tree-line where only shrubs and hardy low growing vegetation survives. Termed ‘Arctic greening’, this process is one of the most striking large-scale ecological changes seen in response to our changing climate. This trend indicates that there is more vegetation in these areas, that plant productivity is increasing. Surveys and studies on the ground confirm these ecosystems are undergoing rapid change. Vegetation is expanding into areas of bare ground and plants are getting taller and larger. But not all species respond in the same way. In particular, there is an increase in the proportion of shrub species, so the composition of the ecosystems is changing as well.
Some areas are starting to show a slowing or even reversal of the greening trend, termed ‘browning’, indicating a decrease in plant growth or even vegetation dieback.
However, not all areas are greening, and in recent years another pattern has emerged in the NDVI data. Some areas are starting to show a slowing or even reversal of the greening trend, termed ‘browning’, indicating a decrease in plant growth or even vegetation dieback. The drivers for the browning are still being investigated, but reasons given include extreme climatic events (such as warming that induces melting followed by refreezing in winter), insect outbreaks, land surface degradation and fire. Overall, the picture is very complex, but it highlights the sensitive and dynamic nature of a region undergoing dramatic change.
Changes in ecosystem species composition are harder to detect by satellite than the amount of vegetation. Hence there is a clear need for science on the ground to interpret and validate the details of the satellite signals, and to investigate the underlying processes at work. However, as the resolution of satellite imagery increases, we will be able to conduct more and more detailed science from space. While NDVI is the vegetation index with the longest record and therefore continues to be one of our most valuable, many more have subsequently been developed, taking advantage of reflectance properties across different parts of the spectrum to provide other information.
An important development in recent years has been the ability to detect a signal that is produced by plants during photosynthesis. When chlorophyll absorbs sunlight, a small amount of energy is released in the red region of the spectrum as a by-product of the energy capture process. Known as Solar Induced Fluorescence (SIF), this signal is closely linked to the rate of photosynthesis in the plants. SIF provides a new level of information, on actual photosynthetic activity rather than capacity, about our ecosystems. In 2023, the European Space Agency will launch it’s Earth Explorer Mission 8 – Fluorescence Explorer (FLEX). FLEX will be the first SIF-dedicated satellite, using a high-resolution imaging spectrometer to measure SIF across the globe and provide better insight into plant health and stress. This is just one example of the enormous advances being made in Earth observation that are helping us understand our rapidly changing planet.