Seasonal patterns in biomass smoke pollution and the mid 20th-century transition from Aboriginal to European fire management in northern Australia

Abstract

Aim Globally, most landscape burning occurs in the tropical savanna biome, where fire is a characteristic of the annual dry season. In northern Australia there is uncertainty about how the frequency and timing of dry season fires have changed in the transition from Aboriginal to European fire management. LocationIn the tropical eucalypt savannas that surround the city of Darwin in the northwest of the Northern Territory of Australia. MethodsOur study had three parts: (1) we developed a predictive statistical model of mean mass ( µ g) of particulates 10 µ m or less per cubic metre of air (PM 10 ) using visibility and other meteorological data in Darwin during the dry seasons of 2000 and 2004;(2) we tested the model and its application to the broader air shed by (a) matching the prediction of this model to PM 10 measurements made in Darwin in 2005, (b) matching the predictions to independent measurements at two locations 20 km to the north and south of Darwin and (c) matching peaks in PM 10 to known major fire events in the region (2000-01 dry seasons); and (3) we used the model to explore changes in air quality over the last 50 years, a period that spans the transition from Aboriginal to European land management. ResultsWe demonstrated that visibility data can be used reliably as a proxy for biomass burning across the largely uncleared tropical savannas inland of Darwin. Validations using independent measurements demonstrated that our predictive model was robust, and geographically and temporally representative of the regional airshed. We used the model to hindcast and found that seasonal air quality has changed since 1955, with a trend to increasing PM 10 concentrations in the early dry season. Main conclusionsThe results suggest that the transition from Aboriginal to European land management has been associated with an increase in fire activity in the early months of the dry season.

The pyrohealth transition: how combustion emissions have shaped health through human history

“…Aboriginal Australians have a long tradition of burning tropical savannas to create wildlife habitat and 'humanize' the landscape [54][55][56]60,61]. The transfer from Aboriginal to European types of savanna burning that occurred from 1950s onwards has been shown to be associated with larger and more frequent fires resulting in increased population exposure to fire smoke pollution in the Northern Territory capital city, Darwin [62]. Traditional Aboriginal burning practice has also been shown to benefit the health of Aboriginal people by providing physical activity, and facilitating access to nutritious foods, traditional lands and cultural practices [63].…”

Understanding complexity in savannas: climate, biodiversity and people

“…Savannas have been influenced by fire for millennia with the reduction of tree cover by burning resulting in the evolution of highly biodiverse ecosystems, and facilitating the rise of C 4 grass-dominated floras and associated faunas [33]. Fire, not surprisingly, limits tree growth, as clearly shown by fire exclusion experiments [34,35]. Increased anthropogenic use of fire as a tool for clearing savannas has a profound influence on the amount of burned biomass [35,36].…”

“…Fire, not surprisingly, limits tree growth, as clearly shown by fire exclusion experiments [34,35]. Increased anthropogenic use of fire as a tool for clearing savannas has a profound influence on the amount of burned biomass [35,36]. To understand the changing nature of the impact of fire there is a need for better information on the temporal and spatial character of fire activity and land cover impacts at the landscape level [37].…”

Water uptake and composition of natural Australian cloud condensation nuclei

“…A distinct diurnal trend in the ratio of activated cloud condensation nuclei at 0.5% supersaturation and particle number was observed, with ~40% ± 20% of BBA acting as CCN during the night and ~60% ± 20% during the day. This increase in CCN activity corresponded with an increase in the hygroscopicity from 0.04 ± 0.03 to 0.07 ± (Bowman et al, 2007). Concurrent aircraft measurements would be required to investigate the penetration and evolution of smoke into upper levels of the troposphere.…”

“…These late dry season fires may have been lit intentionally to trigger the onset of rainfall following the formation of pyro-cumulus clouds, among other ecological reasons (Bowman and Vigilante, 2001;Bowman et al, 2007). Under non-Indigenous management, early dry season prescribed burns are commonplace in order to reduce the severity of late dry season fires .…”