Seen from space, the biosphere stricto sensu appears greenish in colour if the chlorophyll vegetation is fairly dense, or in shades of light brown, or even pale yellow, as it becomes scarcer, giving way to bare ground and desert sands. The biosphere is in symbiosis with the climate, since temperature and water play an important role in the functioning of ecosystems. The evaporation brought about by ecosystems is a powerful energy consuming system which limits warming and helps to control surface temperatures. On the other hand, a decrease in such vegetation cover has a strong warming effect.

The biological processes underlying function and growth are dependent on chemical reactions that are closely linked to temperature: a stage of non growth (temperature below -10 or -5°C), a growth stage peaking at an optimum (around 30-35°C), and a stage of decrease in growth with a lethal threshold (40-50°C). So at high latitudes or at high altitudes, growth becomes diffi cult, adapted species become scarce and a decrease in biomass and biodiversity is observed: this is called cold desert.

Biosphere is alive
The greatest biodiversity corresponds to the highest temperatures in areas of non-limiting water (e.g. the equatorial zone: water and temperature are not limiting), where the energy needed for photosynthesis is abundant and harnessed by these very fertile ecosystems.

However, in such regions the lack of water over long periods of time can lead to temperatures that exceed the lethal threshold of 50°C, thus diminishing ecosystems and their sustainability, biomass and biodiversity. Moving up from the tropics towards temperate regions, the water budget rapidly becomes less negative, and temperatures become suitable for good growth, production of biomass and biodiversity.

Moving up to high latitudes, temperatures become cool, then cold, limiting biomass and biodiversity. The biosphere is alive and adapts to the climate: respiration releases carbon dioxide, transpiration releases water into the atmosphere, all signs that the process of photosynthesis is taking place, using solar energy to produce organic matter.

In the last ten years or so, with the arrival of a new generation of infrared sensors, we have learnt how to observe and quantify such photosynthetic activity from space by estimating the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR). This corresponds to the fraction of solar energy absorbed by vegetation and is a fundamental piece of data for monitoring ecosystems and their development. Monitoring and warning systems, mainly based on satellite observations, are in the process of being planned and set up with the aim of lending support to analyses of ongoing changes.

In addition to the detailed analyses of photosynthetic activity available since the mid 1990s, it is possible to create – on the basis of observations by NOAA meteorological satellites – indices that characterize vegetation in a simplified way. For instance, the widely used Normalized Differential Vegetation Index or NDVI. Although tricky to use quantitatively, this tool can be used qualitatively to indicate the presence of active vegetation. And, on top of that, we can use satellite archives to look back thirty years into the development of this part of the biosphere. Since the early 1970s, this region has been affected by an unprecedented drought, and its vegetation has dwindled due to the disappearance of monsoon rainfall. For around the last twenty years, even though rainfall has remained well below normal levels, the situation of the environment has improved, as is shown by the map of NDVI index change, which confi rms that vegetation is returning to the Sahel. However, changing rainfall patterns only explain part of this greening.
The rest is due to the direct action of humans on the biosphere.

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