Orbital-scale climate forcing of grassland burning in southern Africa

Abstract

Although grassland and savanna occupy only a quarter of the world's vegetation, burning in these ecosystems accounts for roughly half the global carbon emissions from fire. However, the processes that govern changes in grassland burning are poorly understood, particularly on time scales beyond satellite records. We analyzed microcharcoal, sediments, and geochemistry in a high-resolution marine sediment core off Namibia to identify the processes that have controlled biomass burning in southern African grassland ecosystems under large, multimillennial-scale climate changes. Six fire cycles occurred during the past 170,000 y in southern Africa that correspond both in timing and magnitude to the precessional forcing of north-south shifts in the Intertropical Convergence Zone. Contrary to the conventional expectation that fire increases with higher temperatures and increased drought, we found that wetter and cooler climates cause increased burning in the study region, owing to a shift in rainfall amount and seasonality (and thus vegetation flammability). We also show that charcoal morphology (i.e., the particle's length-to-width ratio) can be used to reconstruct changes in fire activity as well as biome shifts over time. Our results provide essential context for understanding current and future grassland-fire dynamics and their associated carbon emissions.

Tropical grassy biomes: linking ecology, human use and conservation

“…Many of these species are endangered and threated with imminent extinction [51]. Fire is a frequent disturbance in the TGBs and has been part of these systems for millions of years [17,21]; consequently, the plants and animals they contain are generally adapted to its occurrence [25].…”

“…At the last glacial maximum, TGBs extended more widely throughout Asia, Africa and the Americas than today [18 -20]. The extent of these vast biomes has shifted with glacial -inter-glacial cycles in response to changing atmospheric CO 2 concentrations, rainfall, rainfall seasonality, temperature and fire [19,21]. Given that all of these aspects of our environment are now changing, and generally at rates not previously observed, extensive alterations in the distribution and dynamics of TGBs over the coming century are inevitable and are likely already being observed ( [1,4,22], see the analysis provided by Stevens et al [23]).…”

Southern Hemisphere anticyclonic circulation drives oceanic and climatic conditions in late Holocene southernmost Africa

“…A major question in this area is whether the climate variability is synchronous with northern hemispheric variations or driven by direct insolation changes and thus antiphased to northern hemispheric signals. In the past, it was relatively uncontroversial that long term (glacial-interglacial timescale) climate variations in southernmost Africa are directly forced by local insolation (Daniau et al, 2013;Simon et al, 2015). However, recently several paleorecords have emerged showing either no evidence of direct insolation as a climatic driver (Chase et al, 2009(Chase et al, , 2010Dupont et al, 2011) or evidence for opposing trends (i.e., synchrony with the Northern Hemisphere) (Tierney et al, 2008;Stager et al, 2012;Truc et al, 2013).…”

Were Western European Neandertals Able to Make Fire?

“…This sampling problem is not limited to this particular dataset. Verardo and Ruddiman (1996), for example, found that microcharcoal records from North Atlantic cores spanning the last 200,000 years were generally decoupled from climatic conditions and did not accurately reflect terrestrial burning, which for them was unquestionably climate driven (see also Daniau et al 2013). Taken together, these observations make it questionable whether marine microcharcoal data can be used to estimate fire frequencies in a particular area, such as southwest France, and especially areas further inland.…”