What could have caused pre-industrial biomass burning emissions to exceed current rates?

摘要

Recent studies based on trace gas mixing ratios in ice cores and charcoaldata indicate that biomass burning emissions over the past millenniumexceeded contemporary emissions by up to a factor of 4 for certain timeperiods. This is surprising because various sources of biomass burning arelinked with population density, which has increased over the past centuries.We have analysed how emissions from several landscape biomass burning sourcescould have fluctuated to yield emissions that are in correspondence withrecent results based on ice core mixing ratios of carbon monoxide (CO) andits isotopic signature measured at South Pole station (SPO). Based onestimates of contemporary landscape fire emissions and the TM5 chemicaltransport model driven by present-day atmospheric transport and OHconcentrations, we found that CO mixing ratios at SPO are more sensitive toemissions from South America and Australia than from Africa, and arerelatively insensitive to emissions from the Northern Hemisphere. We thenexplored how various landscape biomass burning sources may have varied overthe past centuries and what the resulting emissions and corresponding COmixing ratio at SPO would be, using population density variations toreconstruct sources driven by humans (e.g., fuelwood burning) and a new modelto relate savanna emissions to changes in fire return times. We found that tomatch the observed ice core CO data, all savannas in the Southern Hemispherehad to burn annually, or bi-annually in combination with deforestation andslash and burn agriculture exceeding current levels, despite much lowerpopulation densities and lack of machinery to aid the deforestation process.While possible, these scenarios are unlikely and in conflict with currentliterature. However, we do show the large potential for increasedemissions from savannas in a pre-industrial world. This is mainlybecause in the past, fuel beds were probably less fragmented compared to thecurrent situation; satellite data indicates that the majority of savannashave not burned in the past 10 yr, even in Africa, which is considered "theburning continent". Although we have not considered increased charcoalburning or changes in OH concentrations as potential causes for the elevatedCO concentrations found at SPO, it is unlikely they can explain the largeincrease found in the CO concentrations in ice core data. Confirmation of theCO ice core data would therefore call for radical new thinking about causesof variable global fire rates over recent centuries.