Biomass burning, land-cover change, and the hydrological cycle in Northern sub-Saharan Africa

Charles Ichoku,K Elena Willmot,Shahid Habib,Jimmy Adegoke,Churchill Okonkwo,Jejung Lee,Luke T Ellison,Toshihisa Matsui,Charles K Gatebe,Amin K Dezfuli,Eric M Wilcox,Frederick S Policelli,John Bolten,Jun Wang

doi:10.1088/1748-9326/11/9/095005

Abstract

The Northern Sub-Saharan African (NSSA) region, which accounts for 20%–25% of the global carbon emissions from biomass burning, also suffers from frequent drought episodes and other disruptions to the hydrological cycle whose adverse societal impacts have been widely reported during the last several decades. This paper presents a conceptual framework of the NSSA regional climate system components that may be linked to biomass burning, as well as detailed analyses of a variety of satellite data for 2001–2014 in conjunction with relevant model-assimilated variables. Satellite fire detections in NSSA show that the vast majority (>75%) occurs in the savanna and woody savanna land-cover types. Starting in the 2006–2007 burning season through the end of the analyzed data in 2014, peak burning activity showed a net decrease of 2–7%/yr in different parts of NSSA, especially in the savanna regions. However, fire distribution shows appreciable coincidence with land-cover change. Although there is variable mutual exchange of different land cover types, during 2003–2013, cropland increased at an estimated rate of 0.28%/yr of the total NSSA land area, with most of it (0.18%/yr) coming from savanna. During the last decade, conversion to croplands increased in some areas classified as forests and wetlands, posing a threat to these vital and vulnerable ecosystems. Seasonal peak burning is anti-correlated with annual water-cycle indicators such as precipitation, soil moisture, vegetation greenness, and evapotranspiration, except in humid West Africa (5°–10° latitude), where this anti-correlation occurs exclusively in the dry season and burning virtually stops when monthly mean precipitation reaches 4 mm d−1. These results provide observational evidence of changes in land-cover and hydrological variables that are consistent with feedbacks from biomass burning in NSSA, and encourage more synergistic modeling and observational studies that can elaborate this feedback mechanism.

Highlights

The Northern Sub-Saharan African (NSSA) region is the trans-African latitude zone bounded to the north and south by the Sahara and the Equator, respectively
To simplify the interpretation of these complex land-cover change vectors, we focus on croplands, whose expansion is one of the major drivers of burning in NSSA (e.g. Andela and van der Werf 2014)
Changes were observed in precipitation, soil moisture, normalized difference vegetation index (NDVI), and surface evapotranspiration in certain parts of the region, it is not easy to clearly establish a generalized cause-and-effect relationship between biomass burning and these hydrological cycle indicators mainly because of the difference in the seasonality between them

Summary

Introduction

The Northern Sub-Saharan African (NSSA) region is the trans-African latitude zone bounded to the north and south by the Sahara and the Equator, respectively This region is subjected to intense biomass burning during the dry season each year (e.g. figure 1), contributing 20%–25% of the global total annual carbon emissions from fires (e.g. van der Werf et al 2006, 2010, Roberts and Wooster 2008, Schultz et al 2008). Several results inferred that regional weather patterns forced by the North Atlantic SST have more influence on the Sahel regional climate than land–atmosphere interactions (Folland et al 1986, Giannini et al 2003, 2008, Lu and Delworth 2005, Hoerling et al 2006, Dai 2011, Nicholson and Dezfuli 2013). Improved modeling of the observed variability in precipitation requires knowledge of both SST and land–atmosphere interactions (Wang et al 2004)

Methods

Results

Conclusion