Air Pollution as a Climate Forcing: A Workshop

Day 2 Presentations

Climate Effects of Aerosols in China and India

Surabi Menon*+, James Hansen*, Larissa Nazarenko*+, Makiko Sato*+, Yun-Feng Luo°
* NASA Goddard Institute for Space Studies, New York, NY, U.S.A.
+ Center for Climate System Research, Columbia University Earth Institute, New York, NY, U.S.A.
° National Nature Sciences Foundation of China, Haidian, China

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We carry out climate simulations that suggest that human-made aerosols may tend to increase rainfall in South China and decrease rainfall in North China during the summer rainy season, in the sense of the observed trend in recent decades ("flood south, drought north"). This result occurs when we use a dark aerosol single scatter albedo (SSA = 0.85), which is typical of current aerosols in China with their large portion of black carbon (BC). The aerosols also cause summer cooling in Eastern China, of a magnitude comparable to that observed in the past 50 years.

In a companion experiment we remove the BC absorption, i.e., we employ white aerosols (SSA ~ 1). This experiment does not yield the strong changes in rainfall patterns and the surface cooling is reduced. As expected, the dark aerosol experiment yields global mean warming and the bright aerosols yield global cooling.

The increased local cooling and the changes of rainfall in the dark aerosol experiment are associated with changes in vertical atmospheric motions over China that appear to be driven by the absorbing aerosols. This results in increased cloud cover over China in our climate model, which is contrary to data for observed cloud changes (Kaiser, 1998). However, observed annual clouds show a small increase in the south, and the amplitude of the diurnal cycle of surface air temperature, which is arguably a proxy measure of cloud cover change, has decreased in China. An alternative interpretation would be that simulated changes of temperature and precipitation are more robust than simulated cloud cover changes; climate models are notorious for their inability to simulate cloud cover changes realistically. We note that, over the Indian ocean, the added aerosols yield an increased cloud cover in our simulations. This is contrary to the decreased cloud cover due to soot solar heating in the modeling study of Ackerman et al. (2000), but it is consistent with the observed small increase of cloud cover over both the northern and southern Indian Ocean in January-April 1952-1996 (Norris, 2001). Finally, we note that, although the absorbing aerosols cause an increase in the local cloud cover in our experiments, they cause a decrease in global cloud cover, consistent with results of Hansen et al. (1997).

Figure 1: Aerosol optical depth (a) and radiative forcing at the tropospause (b) and surface (d) for the China and China + India experiments. Observed June-July-August surface air temperature change is shown in (c).

If it is confirmed that absorbing aerosols cause increased rainfall in the south and decreased rainfall to the north, there are a number of possible implications. The recent trend toward increased plumes of dust from North China, with adhered toxic contaminants, is often attributed to overfarming, overgrazing and destruction of forests (French, 2002). Our experiments suggest the possibility of an alternative explanation: human-made absorbing aerosols in remote populous industrial regions that alter the atmospheric circulation. Conceivably a similar phenomenon, with increasing dark aerosols in India, could contribute to an increasing drought tendency in Afghanistan. If these inferences are correct, an obvious remedy would be to decrease particulate pollution, especially BC aerosols.

Figure 1a shows the optical depths at wavelength 0.55 µm for the aerosols that provide the added climate forcing in our simulations. The optical depths over China are based on surface solar radiation observations of Luo et al. (2001). These aerosols thus represent somewhat of an exaggeration of the effect of anthropogenic aerosols, as some of the measured aerosols must be natural. However, as a compensation, we employed the optical depth measured at 0.75 µm by Luo et al. (2001) for our aerosol optical depth at 0.55 µm. Thus if the actual aerosol optical depth is about two-thirds anthropogenic, our experiment may provide an approximate representation of the anthropogenic effect. Over India and the Indian Ocean we use aerosol optical depths from Collins et al. (2001) that are based on assimilation of AVHRR satellite retrievals into a chemical transport model. In both cases we employ a proportion of black carbon (BC) such that the aerosol single scatter albedo is about 0.85, which is representative of ACE-Asia and INDOEX measurements (Ramanathan et al. 2001).

Figure 2: Calculated surface air temperature and precipitation changes for China and China + India aerosol experiments.

The calculated aerosol radiative forcings at the tropopause and at the surface are shown in Figs. 1b and 1d, respectively. The observed temperature change between 1951 and 2001, based on the linear trends, is shown in Fig. 1c.

Figure 2 shows the simulated changes in summer (June-July-August) surface air temperature and precipitation for the experiments with absorbing aerosols (SSA = 0.85). Two experiments are shown: one with aerosols added only over China and one with aerosols added over China, India, and the Indian Ocean. A companion figure, not shown, indicates that the regional aerosols can cause significant climate effects at a distance, suggesting the possibility that climate trends in regions with relatively little local air pollution could be affected by air pollution in other regions.


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