Direct and indirect effects of anthropogenic aerosols on regional precipitation over east Asia

Another very good article has appeared which documents the very significant role of aerosols on regional climate (and thus through teleconnections on the global climate system) [and thanks for Dev Niyogi for alerting us of it!].The new paper is

Huang, Y., W. L. Chameides, and R. E. Dickinson (2007), Direct and indirect effects of anthropogenic aerosols on regional precipitation over east Asia, J. Geophys. Res., 112, D03212, doi:10.1029/2006JD007114.

The abstract reads

“A regional coupled climate-chemistry-aerosol model is developed. It is used to assess the direct and indirect effects of anthropogenic sulfate and carbonaceous aerosols on regional climate over east Asia with a focus on precipitation. The simulated direct and first indirect effects for the most part reduce the solar radiation and hence decrease the surface temperature, while the second indirect effect generates both negative solar forcing and a substantial positive long-wave forcing. It decreases the precipitation, but because of the cancelling effect, surface temperature does not change very much. With the interactively model-calculated current aerosol loading and the combined direct/semidirect/first indirect effect, the simulated precipitation is reduced by about 10% in the fall and winter and by about 5% in the spring and summer. The second indirect effect has the largest impact, by itself decreasing the fall and winter precipitation from about 3% to 20%, depending on the autoconversion scheme assumed. The semidirect effect on precipitation is relatively small. An empirical orthogonal function analysis of climatological precipitation over east Asia since the last century shows a decreasing trend of the leading modes over most of China in the fall and winter, which is generally geographically consistent with the distribution of the model-simulated precipitation reduction from anthropogenic aerosols.”

Excerpts from the paper state,

“This study attempts a more comprehensive assessment of anthropogenic aerosols’ impacts on precipitation over east Asia using a regional coupled climate-chemistry-aerosol model. Anthropogenic aerosol loadings over east Asia are especially large, thus the climatic effects are expected to be significant [e.g., Chameides et al., 1999; Schimel et al., 1996]. The regional climate model (RegCM2) [Giorgi et al., 1993a, 1993b] is enhanced with a sulfate module developed by Qian et al. [2001], a modified tracer convective transport/wet removal module [Tan et al., 2002], the implementation of a carbonaceous aerosol module, and the inclusion of all four aforementioned types of aerosol effects. This study advances the previous studies as follows: it includes all three major components of anthropogenic aerosols (i.e., sulfate, black carbon, and organic carbon); it simulates the distributions of these fields in a fully interactive manner with the meteorological fields thereby allowing for feedbacks between chemical and climatic processes; it simulates the direct, semidirect, and first and second indirect effects, individually and in concert, allowing us to assess the relative impacts of the various effects on precipitation and it compares the model-predicted geographic-distribution of precipitation trends over east Asia with an analysis of the long-term precipitation record for the region.”

“Our study has neglected the aerosol indirect effect on the microphysical processes of convective clouds and their consequent interactions with dynamical processes, and in turn the convective precipitation…….a more comprehensive evaluation of aerosol indirect effects on precipitation should address both large-scale and convective clouds from local, regional to global scales.”

“A number of uncertainties and limitations remain in both understanding and modeling of cloud physics and aerosol effects, especially the indirect ones. Various mixing states between BC and sulfate/OC aerosols (external, core-coated, and internal) lead to a range of mass absorption coefficients, with consequent radiative and climate effects [Jacobson, 2000]; Menon [2004] also mentioned that the effect from absorbing BC particles could be quite variable in both sign and magnitude depending on its vertical location relative to the clouds…”

“Although the aerosol indirect climatic effects need further clarification, our results suggest that the precipitation decreases due to both direct and indirect effects of anthropogenic aerosols, which is consistent with observations over east Asia. If pollution emissions continue to increase along with the economic development of the region, anthropogenic aerosols should become even more important in determining the climatic and environmental conditions of the region.”

The message that is reaffirmed by this study is that the role of aerosols within the climate system is significant even if there was no change in the global average temperature. The 2007 IPCC Statement for Policymakers entitled “Climate Change 2007: The Physical Basis” failed to adequately discuss this critical human climate forcing, which has such a profound impact of society (e.g. through water resources).

This failure of the IPCC assessment provide additional evidence that its goal is really not to assess the human role within the climate system, but as a very obvious vehicle to be used to make energy policy changes, irrespective of the actual effects of energy policy changes on the real climate system.

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