Dust cycle: An emerging core theme in Earth system science

Abstract

The dust cycle is an integral part of the Earth system. Each year, an estimated 2000 Mt dust is emitted into the atmosphere, 75% of which is deposited to the land and 25% to the ocean. The emitted and deposited dust participates in a range physical, chemical and bio-geological processes that interact with the cycles of energy, carbon and water. Dust profoundly affects the energy balance of the Earth system, carries organic material, contributes directly to the carbon cycle and carries iron which is vital to ocean productivity and the ocean-atmosphere CO₂ exchange. A deciphering of dust sources, transport and deposition, requires an understanding of the geological controls and climate states – past, present and future. While our knowledge of the dust cycle, its impacts and interactions with the other global-scale bio-geochemical cycles has greatly advanced in the last 30 years, large uncertainties and knowledge gaps still exist. In this review paper, we attempt to provide a benchmark of our present understanding, identify the needs and emphasise the importance of placing the dust issue in the Earth system framework. Our review focuses on (i) the concept of the dust cycle in the context of global biogeochemical cycles; (ii) dust as a climate indicator; (iii) dust modelling; (iv) dust monitoring; and (v) dust parameters. The adoption of a quantitative and global perspective of the dust cycle, underpinned by a deeper understanding of its physical controls, will lead to the reduction of the large uncertainties which presently exist in Earth system models.

Introduction

Dust as an aerosol significantly impacts on the energy balance of the Earth system through the absorption and scattering of radiation in the atmosphere and the modifications of the optical properties of clouds and snow/ice surfaces. Its importance in the Earth system has been emphasised by the IPCC 4th Assessment Report (IPCC, 2007) which highlighted the net global cooling effect of aerosols that in part, compensates for the global warming effect of the greenhouse gases. But the role of dust in the Earth system extends well beyond its impact on the radiation balance, and involves the interactions with other physical, chemical and bio-geochemical processes on global scales. Each year, about 2000 Mt dust is emitted into the atmosphere, of which 1500 Mt is deposited to the land and 500 Mt to the ocean (Table 2, Table 3, Table 5). In this process, dust carries organic matter and contributes directly to the carbon cycle and transports iron that is vital to ocean productivity and ocean-atmosphere CO₂ exchange. Thus, as Fig. 1 illustrates, the cycles of energy (E-cycle), carbon (C-cycle) and dust (D-cycle) in the Earth system are closely inter-related.

In recent years, the dust processes have become core research subjects in Earth system studies. However, gapes remain in our understanding of the driving processes, their interactions and the magnitudes of the fluxes involved in the dust cycle. These uncertainties considerably weaken the explanatory powers of Earth system models. Only with a more adequate representation of the dust cycle, can the necessary veracity be imparted to these models, so as to enable them to capture the overall functioning of the Earth system.

Traditionally, dust was mainly the subject of geomorphologists and geologists, but the last 30 years have seen a dramatic expansion of the dust research frontiers with the establishment of a diverse research community recognising the far-reaching implications of dust to the global environment. Dust research has stimulated the integration of disciplines, including geomorphology, soil physics, meteorology, fluid dynamics, air chemistry and ocean biology. It has also involved diverse methodologies, ranging from field campaigns, Geographical Information System (GIS) analyses, remote sensing numerical modeling, data assimilation as well as field and laboratory experiments.

The importance of dust cycle and the role of dust in the Earth system have been increasingly recognized in the scientific community, as clearly reflected in the recent review paper of Kohfeld and Tegen (2007), Mahowald et al. (2009) and Maher et al. (2010). These reviews well documented some of the advancement in dust research from different perspectives. Kohfeld and Tegen (2007) put forward the concept of dust cycle and examined the role of dust in the Earth system from the view point of past and present dust record. Mahowald et al. (2009) provided a detailed review on dust iron deposition, while Maher et al. (2010) examined the global link between dust, climate and ocean biogeochemistry at the present day and at the last glacial maximum. In this paper, we attempt to further the dust cycle concept and to provide a review on the role of dust in the Earth system as well as dust modeling and monitoring. We shall first introduce the concept of dust cycle and then focus on the relations between the D-cycle and E-cycle and D-cycle and C-cycle, and summarize the recent progresses in dust modeling and monitoring. In each of these sections, we highlight progresses and challenges. These will be further discussed in a series of companion papers that will provide in-depth discussions of specific aspects that can only be provided as an outline in this review.