Study of the reflective power of solar radiation in pastures

What is meadow albedo? How can it be used to mitigate climate change? The Albédo prairies project seeks answers to these questions.

Lower greenhouse gas emissions, carbon storage in grasslands… are levers to deal with climate change already known to everyone. But do you know what albedo is?

“It is the part of solar energy which is reflected from a surface (a meadow, for example), towards space. The higher it is, the more the considered surface has a “cooling” effect”, explains Idele. of albedo value is between 0 and 1, and the closer it is to 1, the more reflective the surface. Thus, an albedo of 0.25 corresponds to 25% of reflected radiation. A “clean” surface, with snow for example, will reflect more energy into space. Therefore, the earth heats up less and there is less infrared radiation in the atmosphere.

Some examples of albedo:

Albedo values ​​of different surfaces. (©Idele)

Albedo is measured using an albedometer which, thanks to sensors oriented towards the sky and the ground, will calculate the difference between the radiation that descends from the sky and that which is reflected from the surface (soil, meadow, crops, etc.). ).

it reflection of solar radiation from pastures it is probably the third lever that will allowmitigate climate change. This is why a research project Alpedo meadows is being developed with the main objectives:

  • “to characterize the spatio-temporal variability ofprairie albedo in France, for different pasture management methods and pedo-climatic situations;
  • to identify and quantify levers for increasing albedo in order to mitigate climate change, from the plot to the scale of territories;
  • to strengthen the arguments in favor of sustainable growth of herbivores based on optimal use of pastures. »

The first example of measurements carried out in experimental farm of Trévarez showed that the grassy land at this site had an albedo of about 0.24. When this pasture is grazed or mowed, it decreases, but grazed pasture appears to have less impact on albedo than mowing: grazing reduces albedo by 7% over 3 weeks, while a mowed pasture will reduce albedo by 14% over 31 days . . All this also depends on the immediate loading of UGB per ha per day.

“When we mow, we remove the grass immediately, it takes longer to grow than if we graze,” explains Pierre Mischler from Idele.

Similarly, a degraded grassland, with visible soil, reduces the albedo and therefore has a “warming effect”. The final lesson: the albedo of a meadow remains higher than the albedo of a crop in general, whether grazed or mowed.

And Pierre Mischler concludes: “The first analyzes show that the meadow has one cooling effect compared to a bare baseline. Grass-based systems have a higher average albedo, so during hot periods the cooling effect is greater.

In total, 10 experimental areas are included in this project, which will enable the study of several pedo-climatic contexts.

Three actions are on the agenda:

  • analysis of climatic and anthropogenic determinants of albedo variations at the plot scale,
  • satellite mapping of albedo and grassland management practices,
  • analysis of albedo effects at the farm and territory scale, compared to other pillars of climate change mitigation: carbon storage and GHG reduction.

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