The greening and wetting of the sahel have leveled off since about 1999 in relation to SST

Tiexi Chen*, Shengjie Zhou, Chuanzhuang Liang, Daniel Fiifi Tawia Hagan, Ning Zeng, Jun Wang, Tingting Shi, Xin Chen, A. J. Dolman

*Corresponding author for this work

Research output: Contribution to JournalArticleAcademicpeer-review

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 languageEnglish
Article number2723
Pages (from-to)1-17
Number of pages17
JournalRemote Sensing
Volume12
Issue number17
Early online date23 Aug 2020
DOIs
Publication statusPublished - 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).

FundersFunder number
NUIST research startup fund2241051301004, 2241021301065
National Natural Science Foundation of China41561124014, 31570464, 41375099, 41605039
Ministerie van Onderwijs, Cultuur en Wetenschap024.002.001
Natural Science Foundation of Jiangsu ProvinceBK20160625
Netherlands Earth System Science Centre
National Key Research and Development Program of China2017YFB0504000

    Keywords

    • Greening
    • Precipitation
    • Sahel
    • Sea surface temperature
    • Vegetation
    • Wetting

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