Abstract
The Sahel, a semi-arid climatic zone with highly seasonal and erratic rainfall, experienced severe droughts in the 1970s and 1980s. Based on remote sensing vegetation indices since early 1980, a clear greening trend is found, which can be attributed to the recovery of contemporaneous precipitation. Here, we present an analysis using long-term leaf area index (LAI), precipitation, and sea surface temperature (SST) records to investigate their trends and relationships. LAI and precipitation show a significant positive trend between 1982 and 2016, at 1.72 × 10 -3 yr -1 (p < 0.01) and 4.63 mm yr-1 (p < 0.01), respectively. However, a piecewise linear regression approach indicates that the trends in both LAI and precipitation are not continuous throughout the 35 year period. In fact, both the greening and wetting of the Sahel have been leveled off (pause of rapid growth) since about 1999. The trends of LAI and precipitation between 1982 and 1999 and 1999-2016 are 4.25 × 10-3 yr -1 to - 0.27 × 10 -3 yr -1, and 9.72 mm yr -1 to 2.17 mm yr -1, respectively. These declines in trends are further investigated using an SST index, which is composed of the SSTs of the Mediterranean Sea, the subtropical North Atlantic, and the global tropical oceans. Causality analysis based on information flow theory affirms this precipitation stabilization between 2003 and 2014. Our results highlight that both the greening and the wetting of the Sahel have been leveled off, a feature that was previously hidden in the apparent long-lasting greening and wetting records since the extreme low values in the 1980s.
Original language | English |
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Article number | 2723 |
Pages (from-to) | 1-17 |
Number of pages | 17 |
Journal | Remote Sensing |
Volume | 12 |
Issue number | 17 |
Early online date | 23 Aug 2020 |
DOIs | |
Publication status | Published - Sept 2020 |
Funding
This research was supported by the National Key R&D Program of China (NO. 2017YFB0504000), the National Natural Science Foundation of China (NO. 31570464, 41561124014, 41375099, 41605039), the NUIST research startup fund (NO. 2241021301065 and 2241051301004), and the Natural Science Foundation for Young Scientists of Jiangsu Province, China (grant no. BK20160625). A.J. Dolman acknowledges support from the program of the Netherlands Earth System Science Centre (NESSC), financially supported by the Ministry of Education, Culture and Science (OCW) (grant 024.002.001).
Funders | Funder number |
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NUIST research startup fund | 2241051301004, 2241021301065 |
National Natural Science Foundation of China | 41561124014, 31570464, 41375099, 41605039 |
Ministerie van Onderwijs, Cultuur en Wetenschap | 024.002.001 |
Natural Science Foundation of Jiangsu Province | BK20160625 |
Netherlands Earth System Science Centre | |
National Key Research and Development Program of China | 2017YFB0504000 |
Keywords
- Greening
- Precipitation
- Sahel
- Sea surface temperature
- Vegetation
- Wetting