Earth observation can reveal how different parts of the land are being used and the status of vegetation growing on the surface. EO in the optical domain allows us to look particularly at land cover related to forests, crops, water, and other areas of very high importance for humans. Monitoring land cover, along with the related property of land use, is a particularly important application of EO, including in relation to: monitoring deforestation or reforestation; vegetation quantities and health; monitoring crop production and the effects it may have on food security; and tracking other human activities such as urban development. Information spanning over decades can also indicate some of the effects of climate change.

Measuring land cover and land use change is important on local, regional, national, and international scales, and underpins many of the international efforts to monitor and respond to man-made and climate-related impacts on the environment. An example of this is the framework for valuing tropical forests as a ‘sink’ for carbon and tracking both deforestation and degradation (the so called ‘REDD+’ framework).

Land use and land cover mapping have been core applications of EO from its inception to today, and so data spanning over several decades is now available. The NASA Landsat missions (since the early 1970s) along with France’s SPOT missions (since the mid 1980s) have monitored land use for more than a generation, and Sentinel-2 has now taken up the mantle, providing high quality multispectral image data over the globe every few days.

The measurements from satellite EO which are most relevant to climate change have been defined in a list known as the ‘Essential Climate Variables’ (ECVs). These ECVs are defined and laid out by the Global Climate Observing System, a group in which a number of different international organisations co-operate. There are 50 ECVs in total which are measured to support the work of the UN Intergovernmental Panel on Climate Change .

Copernicus, the EU’s programme for Earth Observation, will provide a wide range of data products linked to vegetation and land use:

• Leaf Area Index (LAI) - this quantifies the area of leaves and tells us how much leaf area is available for photosynthesis, water and CO2 exchange.

• Fraction of Absorbed Photosynthetically Active Radiation - this is related to how much photosynthesis is going on. If we track photosynthesis, and add it up over the course of a year, we can measure canopy productivity - how much new material the plant can accumulate (effectively the mass it adds each year, much of it being Carbon of course). FAPAR is also indicative of the health of the plant; it responds to plant stress due to heat, cold, drought, insect and parasite infestation and so on, as these all serve to reduce photosynthesis.

• Fraction of Green Vegetation Cover - as the name suggests, this deals with the fraction of the ground covered by vegetation

• Normalized Difference Vegetation Index (NDVI) – this is calculated based on the contrast between the red and NIR reflectance as described above – this time, just turned into a single number which indicates the ‘greenness’, or health and amount of vegetation. NDVI correlates quite strongly with the rate of canopy photosynthesis, although only empirically, and so we need to be careful interpreting these values.

• Vegetation Productivity Index - this is a measure of the health of vegetation comparing current NDVI to long-term averages. The repeat observations in short time intervals provided by satellites provides the immediate and the long-term data.

• Vegetation Condition Index - this measurement compares the current NDVI with the same time in different years.

• Dry Matter Productivity - this is a measure of growth rate.

Forests in many parts of the world have been chopped down to make way for pasture and cattle grazing, crops, oil palm and other plantation forests, or urban development. This drastically reduces how much carbon dioxide is taken out of the atmosphere by trees, and also allows soil to be weathered more easily. Observation from space can measure the amount of land covered by trees, whether it’s decreasing over time, or in some cases increasing because of planned reforestation and afforestation.

Loss of vegetation combined with other environmental factors can lead to desertification. This can be measured through the change of colours of the landscape. Long-term measurements have revealed the creep of the Sahara Desert.

Clues about water management can also be taken from space. The effects of the decision to drain water from the Aral Sea to irrigate the desert in the former Soviet Union are very strikingly visible from orbit. This can serve to highlight the wider environmental and human costs very effectively, particularly to policy-makers and other non-specialists.

Measurement of the area of water bodies is another Essential Climate Variable. Sentinel-2 is also monitoring elements of water quality, such as algal blooms. Satellite pictures combined with radar data can monitor the health of wetlands, which are a hotspot for biodiversity, and at higher latitudes form a key part of the natural ‘methane budget’.

Featured Educators:

• Dr Mathias Disney

View featured satellites on the satellite tracking app

• Landsat-7

• Sentinel-2

• SPOT-4

• SPOT-5

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