Analysis of MODIS albedo changes over steady woody covers in France during the period of 2001-2013
Planque, Carole ; Carrer, Dominique ; Roujean, Jean-Louis
Using MODIS satellite data, this study examines the presence and causes of albedo trends in vegetation in France during the last decade. Special attention is paid to steady woody areas. During the last decade, 0.31% of the land surface of France has been identified as being affected by a sudden albedo shift due to land cover changes (LCC). Excluding these sudden changes in land cover, we show a significant (decreasing) albedo trends over 3.5% of France (p-value < 0.01). In most cases (84%), trends were identified in non-vegetated and non-steady vegetated covers; however, gradual albedo trends were observed in steady woody covers in over 0.1% of the surface area of France. The majority (59.9%) of these woody covers were broadleaved tree covers; the remainder was divided between needle-leaved and mixed tree covers and shrub cover. On average, in the steady woody covers affected by gradual albedo trends, the visible albedo shift was ? 0.0088 between 2001 and 2013, i.e., a loss of 20.1-29.8% depending on the woody cover type. This decrease in albedo was supported by a significant gradual increase (p-value < 0.05) in the normalized difference vegetation index (NDVI). This increase in greenness appears to be linked to the under-use of woody areas in France, leading to denser forests. The impact of these gradual albedo trends on the radiative forcing (RF) was estimated and compared to the impact of LCCs. During the 21st century in France, the impact on the radiative forcing of steady woody covers could reach + 0.2 W.m? 2. This value is 2.5 times larger than IPCC estimates based on past vegetation changes.
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