Baiqing Xu et al., PNAS (2009), Black soot and the survival of Tibetan glaciers

Proceedings of the National Academy of Sciences, published online before print December 8, 2009; doi: 10.1073/pnas.0910444106 

Black soot and the survival of Tibetan glaciers

Baiqing Xu^a,^b, Junji Cao^b, James Hansen^c,^*, Tandong Yao^a, Daniel R. Joswia^a, Ninglian Wang^d, Guangjian Wu^a, Mo Wang^a, Huabiao Zhao^a, Wei Yang^a, Xianqin Liu^e and Jianqiao He^d

Contributed by James Hansen, October 15, 2009 (sent for review July 6, 2009).

Abstract

We find evidence that black soot aerosols deposited on Tibetan glaciers have been a significant contributing factor to observed rapid glacier retreat. Reduced black soot emissions, in addition to reduced greenhouse gases, may be required to avoid demise of Himalayan glaciers and retain the benefits of glaciers for seasonal fresh water supplies.

*Correspondence e-mail: James.E.Hansen@nasa.gov

Link to abstract:  http://www.pnas.org/content/early/2009/12/07/0910444106.abstract

Link to free, open-access complete paper:  http://www.pnas.org/content/early/2009/12/07/0910444106.full.pdf+html

Introduction

Glaciers on the Tibetan Plateau, sometimes called Earth’s ‘‘third pole,’’ hold the largest ice mass outside the Polar Regions. These glaciers act as a water storage tower for South and East Asia, releasing melt water to the Indus, Ganges, Brahmaputra, and other river systems, providing fresh water to more than 1 billion people (1, 2). Glacial melt provides up to two-thirds of the summer flow in the Ganges and half or more of the flow in other major rivers (3). One-quarter of the population of China is in western regions where glacial melt provides the main dry season water source (4).

Tibetan glaciers have been melting at an accelerating, alarming rate over the past decade, raising the threat that many of the glaciers could be gone by midcentury (5, 6). As glaciers recede and release stored water, flow temporarily increases, but the future fresh water supply is threatened (5–8). Once headwater glaciers are gone, however, a dramatic decline in dry season water availability may ensue. Total precipitation may increase

with global warming (1, 9), but a likely result of glacier loss will be heavier spring floods and much reduced fresh water availability during subsequent dry seasons.

Climate on the Tibetan Plateau is changing rapidly; over the past three decades, the large area at altitude above 4,000 m has warmed 0.3 °C per decade (Fig. S1), which is twice the rate of observed global warming. Climate change on the plateau may have large regional effects, for example, on the Asian monsoon, and even global repercussions (3). Glacier retreat in the Tibetan Plateau presumably is driven by warming due to increasing greenhouse gases (1), but the rapidity of glacier retreat and the up to 0.3 °C warming of per decade during the past 30 years suggest additional mechanisms may be involved.

Black soot in aerosol pollution can warm the troposphere, perhaps contributing to surface melt (10–12). Absorption is caused primarily by the black carbon (BC), whereas organic carbon (OC) absorbs mainly in the UV and slightly in the visible region (13–15). Black soot incorporated in snowflakes darkens snow and ice surfaces, increasing surface melt (16–18). Simulations show that the added absorption by snow exceeds the ‘‘dimming’’ effect (reduced solar irradiance at the ground due to atmospheric aerosols) and becomes significant when BC reaches amounts on the order of 10 ng/g or more (15, 19). The Tibetan Plateau is located close to regions in South and East Asia that have been (20) and are predicted to continue to be (21) the largest sources of black soot in the world. The extensive black soot aerosols could be lofted to the high Tibetan Plateau and incorporated in snowflakes that when falling on the glaciers darken their surface, which has led to initial studies of the amount of BC and OC in the snow and ice of Himalayan glaciers (22, 23). Yet, black soot-induced reduction of snow albedo and its contribution to glacier retreat have only begun to receive attention, so there is a need for more extensive field data. Here, we report measurements of the BC and OC content in ice cores that sample snow deposited during the past half century on five widely spaced glaciers on the Tibetan Plateau.