Extracting a History of Global Fire Emissions for the Past Millennium From Ice Core Records of Acetylene, Ethane, and Methane

Abstract

Biomass burning is an important component of the Earth system in terms of global biogeochemistry, atmospheric composition, climate, terrestrial ecology, and land use. This study examines published ice core trace gas measurements of acetylene, ethane, and methane, which have been used as proxies for paleofire emissions. We investigate the consistency of these records for the past 1,000 years in terms of (1) temporal trends in global fire emissions and (2) quantitative estimates for changes in global burning (dry matter burned per year). Three-dimensional transport and box models were used to construct emissions scenarios for the trace gases consistent with each ice core record. Burning histories were inferred from trace gas emissions by accounting for biome-specific emission factors for each trace gas. The temporal trends in fire inferred from the trace gases are in reasonable agreement, with a large decline in biomass burning emissions from the Medieval Period (MP: 1000-1500 CE) to the Little Ice Age (LIA: 1650-1750 CE). However, the three trace gas ice core records do not yield a consistent fire history, even assuming dramatic (and unrealistic) changes in the spatial distribution of fire and biomes. Substantial changes in other factors such as meteorological transport or atmospheric photochemical lifetimes appear to be required to reconcile the trace gas records. Plain Language Summary Biomass burning (wildfires) are an important component of the climate system. Understanding how biomass burning has changed in the past can help us better predict how biomass burning may change in the future. This study examines ice core measurements of three trace gases, acetylene, ethane, and methane, in order to determine if a single global "fire history" (dry matter burned) can be reconstructed over the last 1,000 years. All three ice core records indicate a large decline in their respective biomass burning emissions after 1500 CE. However, a single fire history of dry matter burned over the last 1,000 years is not easily reconstructed from the ice core records even when enacting extreme changes to the location of fires and biome types. This suggests that our knowledge of the preindustrial fire system is incomplete. There appears to be little doubt that large changes occurred in biomass burning over the past millennium, although the exact magnitude and location of those changes are not yet quantifiable.

Southern Hemisphere atmospheric history of carbon monoxide over the late Holocene reconstructed from multiple Antarctic ice archives

“…Overall, large uncertainties remain on pre-industrial fire history (Nicewonger et al, 2020b), with implication on the PI to modern evolution in aerosol radiative forcing (Hamilton et al, 2018). Nicewonger et al (2020b) used chemical-transport models to investigate if a single global burning history could be extracted from the three hydrocarbon ice core records. They could not find a consistent fire history even assuming unrealistic changes in spatial distribution of fire and biomes.…”

High-latitude fire activity of recent decades derived from microscopic charcoal and black carbon in Greenland ice cores

“…Grieman et al, 2018;Legrand et al, 2016;McConnell et al, 2018McConnell et al, , 2019Rasmussen et al, 2014;Seierstad et al, 2014;Sigl et al, 2015;Vinther et al, 2010) and preserve many different indicators of past biomass burning activity (e.g. Kang et al, 2020;Kehrwald et al, 2020;Legrand et al, 2016;Liu et al, 2021;McConnell et al, 2007;Nicewonger et al, 2020;Zennaro et al, 2014). However, some burning proxies such as black carbon cannot be strictly interpreted as indicators of fire activity during the industrial period as they contain an increasingly dominant component from fossil fuel and other anthropogenic emissions potentially overshadowing the biomass burning signal (McConnell et al, 2007).…”

Temporary pause in the growth of atmospheric ethane and propane in 2015–2018

“…The discrete, typically weekly, air sampling by cooperative global networks has been at the forefront of studies to identify and quantify long-term trends in the background air abundances of important trace gases (e.g., Masarie and Tans, 1995;Montzka et al, 2018;Nisbet et al, 2014Nisbet et al, , 2019. In parallel, higher temporal resolution in situ measurements allows for the investigation of gases' variability and of shorter term trends at specific sites.…”