The present-day supply of Fe to high-nitrate low-chlorophyll (HNLC) regions is not large enough for the ocean's biological pump to work at its maximum efficiency and fully utilise all supplied NOg\ In addition, other regions such as the central tropical Pacific and North Atlantic may be close to limitation or quasi-limited by Fe. Any reduction in dust supply will intensify limitation where it already exists and potentially induce limitation of productivity elsewhere. Either way, if aeolian Fe fluxes were lower, there should be a reduction in the rate of CO2 uptake by the ocean, which has implications for atmospheric CO2 concentrations and the rate and degree of future climate change. Under what circumstances might a reduction in dust supply to the ocean occur? Answering this question involves clarifying the two different types of anthropogenic contributions to atmospheric dust [Zender et al. (2004)]. Humans can influence the production of dust directly by altering the land surface. Alternatively, humans can indirectly affect the atmospheric dust burden through the cumulative impact of anthropogenic climate changes on the dust cycle.
The deliberate large-scale manipulation of terrestrial ecosystems has been proposed for the 'locking up' (sequestration) of carbon on land. These include, changes in soil management practices such as reducing tillage, enhancing the areal and seasonal extent of ground cover, and the 'settingaside' of surplus agricultural land, in addition to the restoration of previously degraded lands and forestation [Royal Society (2001)]. However, reduced disturbance, stabilisation of soils, and greater vegetation cover are also likely to reduce dust emissions. Since dust exerts an important control on the biological pump in the ocean, the effectiveness of carbon sequestration on land may be diminished by a reduction in carbon uptake by the ocean.
Early models of dust transport and deposition suggested that a substantial (30-50%) component of the present-day global dust supply originated in disturbed soils [Tegen and Fung (1995)]. If these soils were stabilised in the future for sequestering carbon, a substantial decrease in global dust emissions would occur. Computer models suggest that a 30% reduction in dust flux to the global ocean would lower ocean productivity to such an extent that the weaker ocean carbon sink could potentially offset the benefit of sequestering carbon on land [Ridgwell et al. (2002)], leaving atmospheric CO2 unchanged. However, subsequent satellite-based analyses suggest that the anthropogenic component is much smaller [Prospero et al. (2002)]. Furthermore, more recent attempts to match dust model simulations to the surface observations have suggested agricultural practices contribute less than 10% to the total global atmospheric dust burden [Tegen et al. (2004b)], although limited surface observations make this number difficult to ascertain exactly [Mahowald et al. (2004); Tegen et al. (2004a)]. These more recent results demonstrate that quantifying the possible reduction in dust production due to land-use changes remains an important challenge.
Socio-economic and political factors are likely to ultimately dictate any future large-scale alteration of the land surface, with changes in dust supply probably occurring on a regional scale rather than globally. For instance, a massive reforestation program is already under way in China with the specific intention of combating soil erosion and associated dust storms. Although several recent studies have demonstrated that climate factors play a strong role in determining the frequency of dust storms over China [Mukai et al. (2004); Zhao et al. (2004)], changes in reforestation could have (as yet unquantified) implications for marine ecosystems in the iron-sensitive equatorial and North Pacific.
The second means by which human activity could affect dust emissions is through anthropogenic climate change. A change in climate could drive an increase in vegetation cover in arid areas, reducing the supply of dust to the atmosphere [Harrison et al. (2001)]. The efficiency with which dust is transported through the atmosphere may also change, with any increase in global precipitation removing more dust before it reaches the open ocean. However, current model simulations have not reached a consensus regarding the impact of future climate on dust emissions. While some simulations suggest that dust emissions will decrease by as much as 60% by 2090 [Mahowald and Luo (2003)], other simulations suggest that dust emissions might even increase by as much as 10%. Thus, the response and even the direction of change of future dust emissions is highly dependent on the climate model used [Tegen et al. (2004b)].
Current computer carbon cycle models only give a relatively crude indication of the possible impacts. Such experiments do, however, serve to highlight the important link within the Earth system that is mediated by dust; a connection between carbon cycling and climate and human activities that was previously completely overlooked. The rather narrow and restricted land-atmosphere approach to carbon budgeting in the Kyoto Protocol that neglects important Earth system feedbacks involving the ocean is then too simplified.
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