Human Land Use And Global Dust Flux

It is now widely accepted that human activities have resulted in significant changes to the Earth’s atmosphere, oceans, and terrestrial environments. While climate change is perhaps the best-known of these changes and has received the most attention from scientists to try to comprehend its causes and the magnitude of its effect, it is becoming increasingly understood that humans are having large impacts on other earth systems and processes.

Many of the areas where people are causing changes are interlinked through the complex web of feedbacks that form the Earth’s biogeochemical cycles, and, as such, it is no surprise that alterations to one ecosystem can result in changes to the Earth’s climate and consequently affect other environments, and so on. It is, therefore, necessary to understand and attempt to quantify the many changes that are occurring as a result of human activity in order to better comprehend how these changes and feedbacks will interact with one another and affect the world in which we live.

Our research investigates whether the expansion and intensification of human land-use have resulted in increased wind erosion and attempts to quantify the scale of any changes in dust flux to the Earth’s atmosphere. We did this by investigating deposition trends over centennial to millennial timescales in multiple sedimentary archives around the world, such as ice cores, sub-ocean cores, and cores extracted from lakes and peat bogs. By compiling 25 of these sedimentary records from across the globe and performing statistical analysis on the level of dust deposition recorded within them, we were able to calculate an average increase in dust emissions due to the intensification of human activity.

The timing of an increase in dust deposition in many of the cores was contemporaneous with the introduction and intensification of modern industrial agriculture in the dust source regions supplying dust to the core sites. For example, a core from a peat bog in the Snowy Mountains of New South Wales, Australia, showed an abrupt increase in dust deposition in the 1870s, soon after the introduction by European colonialists of intensive livestock grazing across the Murray Darling basin, now a major agricultural region that is directly upwind of the core site. The impact of human land disturbance on dust emissions is confirmed by the comparison of numerous other study types, such as meteorological records, airborne sediment sampler datasets and remote sensing by satellites, spanning time scales from months to decades and ranging from spatial scales of hundreds of metres to hundreds of kilometres, which combined show in almost every case substantial and significant increases in dust emissions as a result of human disturbance of previously wild or semi-wild landscapes.

Taken together, the results of the global compilation of sedimentary records that we analyzed showed that dust flux around the world increased by an average of 2.1 times in the period since the beginning of the Industrial Revolution (1750 CE). Of the 25 records, sixteen showed statistically significant and sustained increases in dust flux ranging from 120-420 % pre-disturbance levels. Only one sedimentary archive, from the Ross Sea in Antarctica, recorded a decrease in dust flux during this period. The nine records that did not record any change in dust deposition were all located in remote, high altitude areas, far removed from dust transport trajectories, such as the Tibetan plateau, the South American Altiplano, and inland Antarctica.

Our analysis of the different study types that measured the scale of human impact on dust flux found that the measured effect was greatest in studies with the smallest spatial and temporal scales such as airborne sediment sampling, which measured the average minimum-maximum disturbance factor to dust emissions at 3.6-45 times that of pre-disturbance levels. Modeling studies appeared to consistently underestimate the impact of anthropogenic activity on increased dust emissions compared to all other study types, with an average minimum-maximum disturbance factor of 1.3-1.8 times.

This is important because aerosols are a key factor in climate models through their direct influence on the way the Earth’s atmosphere reflects and absorbs the Sun’s radiation, but also because of their role in providing particles for water droplets to condense around, which aids the formation of clouds. Dust is also known to impact climate through changing the albedo and therefore the heat-absorbing properties of ice and snow that it lands on, leading to increased melt. Furthermore, dust can have an indirect effect on climate by delivering nutrients such as iron and phosphorous to the Earth’s oceans and rainforests supporting and increasing primary productivity that results in the drawdown of Carbon Dioxide from the atmosphere. This is not to mention other important impacts, such as the deleterious effect on public health of breathing air heavily laden with aerosol particulates.

The net signal of these dust feedbacks is likely to have been a cooling of the climate at a time when global civilization’s emission of greenhouse gasses has only been increasing. As such, the implications of this research are that changes to human land use in the future may result in unanticipated climate consequences based on the way in which dust emissions interact with the Earth’s other biogeochemical systems.

These findings are described in the article entitled A global doubling of dust emissions during the Anthropocene? recently published in the journal Global and Planetary ChangeThis work was conducted by James Hooper and Samuel Marx from the University of Wollongong.

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