1000 Years Of Atmospheric Sulfate Concentrations In S. Asia Recorded By Himalaya Glacier - GRL

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Duan, Keqin; Thompson, L. G.; Yao, T.; Davis, M. E.; Mosley-Thompson, E.

<1> A sulfate record covering the period A.D. 1000–1997 from the Dasuopu glacier in the Himalayas reveals that this site is sensitive to anthropogenic activity originating in southern Asia. Prior to 1870 atmospheric sulfate concentrations were relatively low and constant, but thereafter concentrations have increased and since 1930 the rate of increase has accelerated rapidly. This accelerating trend in sulfate deposition is paralleled by growing SO2 emissions over southern Asia resulting from the increased energy demand. The concentration of sulfate deposited in the last 50 years exceeds that for any prior 50-year period in the last millennium. Unlike the Greenland ice core-derived sulfate concentrations that have declined since the 1970s, sulfate concentrations deposited on the Himalayan ice fields continue to increase, having nearly doubled since 1970. This reflects regional differences between Europe and Asia in source strength and transport pathways for atmospheric sulfate, as well as differing degrees of environmental regulation.

<2> Sulfate aerosols cause climate forcing by reflecting and absorbing radiation, as well as through indirect effects that alter cloud cover and cloud albedo . It has been suggested that their effects might be sufficient to moderate the positive forcing by anthropogenic greenhouse gases on regional scales . The increased atmospheric sulfate in southern Asia is already implicated as a cause of local surface temperature declines and reductions in monsoon rainfall . Thus, knowledge of the distribution and concentration of sulfate aerosols is required to model and evaluate any associated climate changes . Unlike the long-lived greenhouse gases which are distributed uniformly over the globe, sulfate lifetimes are generally a week or less , resulting in substantial spatial and temporal variations. Although sulfate histories have been reconstructed from polar ice cores , no comparable records exist for tropical and subtropical regions and this limits effort to study the global sulfate cycle and its role in regional climate changes.

<3> Ice core records from Greenland and the European Alps suggest that atmospheric sulfate has increased since the Industrial Revolution, primarily due to increased SO2 emissions from North America and Europe . However, since 1970 sulfate concentrations in the Greenland cores have decreased due to air pollution control measures in North America and Europe . In Asia, although the fossil energy-related SO2 release is nearly an order of magnitude smaller than that in North America and Europe , Asia is rapidly catching up. Roughly 40% of the world's population lives in southern and eastern Asia, and hence the potential for increased pollutant emissions is high and closely linked to the growing demand for energy. Although variations in sulfate emissions over Asia have been estimated for the last few decades , little is known about the longer term trends. Here we present a 1000-year record of sulfate concentrations over southern Asia, deduced from a high resolution ice core drilled on the Dasuopu glacier in the central Himalayas.

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Figure 2. (a) Annual and decadal average sulfate concentration (ppb) for the Dasuopu ice core, central Himalayas, during the period 1600–1997 and 1000–1997, respectively. (b) Annual sulfate concentration (ppb) from 1600 to present in the enlargement. Examples of volcanic events recorded by sulfate spikes are: Coseguina (1835, spike b); Krakatau (1883, spike c); Novarupta (1912, spike d). Spike a reflects a monsoon failure and the source for spike e is unknown. Enhanced EPS <1.3 MB>


Figure 3. (a) Sulfate trend concentrations (ppb) in the Dasuopu ice core and SO2 emissions (triangle) (1012 g yr−1 or megatons) over southern Asia . (b) Sulfate trend concentrations (ppb) in the Greenland B16 ice core and the anthropogenic SO2 emissions for the United States (cross) and Eurasia (dot) . Enhanced EPS <1.4 MB>

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<10> Comparisons of the Greenland sulfate records with SO2 emission estimates for the United States and Europe suggest that Greenland sulfate levels were controlled mainly by North American emissions in the first half of the twentieth century, whereas the major increase detected after 1950 originates mainly from European emissions . The difference in sulfate deposition trends between Dasuopu and Greenland since 1970 almost certainly reflects differing regional source strengths. To examine this, the Dasuopu and Greenland sulfate trends, detected using SSA, are compared with SO2 emissions estimated from energy and industry statistics for Europe, the United States and southern Asia (Figure 3). The 1970 peak in Greenland sulfate concentrations is also evident in the SO2 emissions from Europe, (and to a much lesser extent also in those from the U.S.), pointing to European anthropogenic sources as the primary contributors to sulfate levels in Greenland ice since the 1950s. The differing trends for sulfate deposition on Dasuopu and for SO2 emissions in Europe and the United States clearly point to another important SO2 source. The close correspondence of the sulfate trend on Dasuopu and SO2 emissions over southern Asia (Figure 3a) indicate that south Asian anthropogenic sources have dominated the concentrations of sulfate levels deposited on Himalayan glaciers since the 1930s. Figure 3 reveals that sulfate concentrations in the Dasuopu ice core increased by a factor of 2.5 from 1951–1997 while SO2 emissions over southern Asia increased 6 fold over the same period .

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