So in the last year of their 17-year life cycle, when the soil temperature reaches about 64 degrees Fahrenheit, they crawl out of the ground by the billions.

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The satellite images picked out the right type of afforested land, and topography, which, when combined with other data on soil types, rainfall, and temperature, produced a clear "Loa Loa map" of several African countries, including Burkina Faso, Mali and Niger.

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They will look, too, at the amount of ice in the ground, and the temperature and the moisture of the soil.

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In an area of a few tens of square kilometres around this tower, the researchers will place further sensors in the soil and in local streams, to measure temperature, carbon-dioxide and nutrient levels, along with rates of root growth and the activities of microbes.

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For instance, the Hadley Centre for climate research in the UK, published research only this week suggesting that forests will speed up climate change in the future, as the soil will start to emit more greenhouse gases than the trees absorb as the temperature builds up.

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