Human disturbance amplifies Amazonian El Niño–Southern Oscillation signal

Abstract

The long-term interaction between human activity and climate is subject to increasing scrutiny. Humans homogenize landscapes through deforestation, agriculture, and burning and thereby might reduce the capacity of landscapes to provide archives of climate change. Alternatively, land-use change might overwhelm natural buffering and amplify latent climate signals, rendering them detectable. Here we examine a sub-annually resolved sedimentary record from Lake Sauce in the western Amazonian lowlands that spans 6900 years. Finely-laminated sediments were deposited from ca. 5000 years ago until the present, and human activity in the watershed was revealed through the presence of charcoal and maize agriculture. The laminations, analyzed for color content and bandwidth, showed distinctive changes that were coupled to more frequent occurrence of fossil maize pollen. As agricultural activity intensified ca. 2200 cal. BP, the 2- to 8-year periodicity characteristic of El Niño-Southern Oscillation became evident in the record. These agricultural activities appeared to have amplified an existing, but subtle climatic signal that was previously absorbed by natural vegetation. When agricultural activity slowed, or land use around Lake Sauce changed at ca. 800 cal. BP, the signal of El Niño-Southern Oscillation (ENSO) activity became erratic.

Synergy between land use and climate change increases future fire risk in Amazon forests

“…This is consistent with contemporary studies (Alencar et al, 2004;Morton et al, 2013) and with the clear signature of land use activities in charcoal paleorecords since the onset of Amazonian agriculture (Bush et al, 2007;Cordeiro et al, 2014). In fact, Bush et al (2017) analyzed a 6900-year sedimentary record in western Amazonia and found that the ENSO fire signal was strongly expressed at times of widespread agricultural activity, being otherwise undetectable in the record and most likely absorbed by natural vegetation. Deforestation rates have largely declined over the last decade (−70 % in Brazil; Nepstad et al, 2014), and several Amazon countries have committed to ambitious reforestation targets through their Intended Nationally Determined Contributions (INDC) to the United Nations Framework Convention on Climate Change (e.g., 12 million hectares by 2030 in Brazil; Federative Republic of Brazil, 2015).…”

A 7000‐year history of changing plant trait composition in an Amazonian landscape; the role of humans and climate

“…Measurements were made of 8730 bands covering the last 5000 years, during which sediments were strongly laminated (Bush et al . ). In most years the bands appeared to have been deposited as a couplet, and Bush et al .…”

“…) and heavily influenced by human activity for much of the last 7000 years (Bush et al . , ). Human influence in the region was evident from anthropogenic fire events, high sedimentation rates due to erosion and the presence of fossil maize pollen.…”

The deep human prehistory of global tropical forests and its relevance for modern conservation

“…Accordingly, scholarly assumptions about the timing of significant anthropogenic impacts on tropical forests generally point to the post-industrial era or, at the earliest, the colonial era of European 'discovery' [26][27] . Clearly, the accumulating database of archaeological and palaeoecological evidence for pre-industrial and pre-colonial tropical forest occupation and transformation has not been effectively communicated beyond a restricted set of sub-disciplines (though see [28][29][30][31] ). As a consequence, this evidence has only played a small role in discussions about the start date or characteristics of the Anthropocene (e.g.…”