Aerosol climate feedbacks

The main difficulty in understanding the causes of climate change is connected with the impossibility of considering climatic feedbacks sufficiently reliably. Primarily, this refers to cloud-radiation feedback, direct and indirect (by the effect on radiation properties of clouds) impacts of atmospheric aerosols on climate, and the impact of the atmosphere-ocean interaction on climate formation. 

Snowpack chemical emissions include oxidants and aerosol precursors, that also interact with climate. Oxidants determine the lifetime of greenhouse gases and aerosols impact the atmospheric radiation budget. If snowpack chemical emissions are determined by climate, feedback loops involving snow chemistry also need to be studied to assess the extent of climate change in snow-covered regions. 

The biosphere-atmosphere-climate feedbacks are quite complicated. A simplified scheme in Figure 5 shows what can happen in the atmosphere and biosphere when temperature and die CO2 concentration increase. Such conditions are expected to promote the growth of plants and, consequently, the emission of terpenoids (including isoprene), which are converted to a larger amount of aerosols that shade the Earth, also by cloud formation, to reduce the temperature (Kulmala et al, 2003, 2004). This relatively simple feedback action is complicated by several factors. 

DMS has been observed in the marine atmosphere since the early 1970s, but it was not until the mid-1980s that there was interest in this gas as being a natural source for sulfate CCN. Sulfate aerosols are, in number terms, the dominant source of CCN. The major role clouds play in the climate system leads to possible climatic implications if changes to DMS production occurred. Furthermore, the dependence of this production on environment conditions means that scope for a feedback process arises this feedback is called the Charlson hypothesis. ... 

One such feedback is the influence of clouds and water vapor. As the climate warms, more water vapor enters the atmosphere. But how much And which parts of the atmosphere, high or low And how does the increased humidity affect cloud formation While the relationships among clouds, water vapor, and global climate are complicated in and of themselves, the situation is further complicated by the fact that aerosols exert a poorly understood influence on clouds. 

Schematic representation of global iron connections linked through the state of the land surface, aerosol loading, marine productivity, and climate. Positive feedbacks are represented by solid arrowheads and negative feedbacks by open circles. Feedbacks of uncertain sign are represented by open arrowheads. Linking mechanisms are in italics. Taps represent mechanisms that modulate primary processes. Source-. From Jickells, T. D., et al. (2005). Science, 308, 67-71. (See companion website for color version.)...

As Mitra (2004) pointed out, the Indian oceanic experiment (INDOEX) was the first complex problem-oriented observational international program aimed mainly at studies of the aerosol-induced radiative and climatic forcing of regional and global climate that take respective feedbacks into account. The preliminary stage of the accomplishment of INDOEX began in 1996-1997, and the basic part of complex observations was accomplished in 1998-1999 with the participation of specialists from different countries (India, U.S.A., Western Europe, Mauritius, and the Maldives). The obtained results were based on the use of surface, ship, aircraft, and satellite observational means. 

In this concern one of the important tasks is to develop a modelling instrument of coupled Atmospheric chemistry/Aerosol and Atmospheric Dynamics/Climate models for integrated studies, which is able to consider the feedback mechanisms, e.g. aerosol forcing (direct and indirect) on the meteorological processes and climate change (see Fig. 1.2). 

The on-line integration of meso-scale meteorological models and atmospheric aerosol and chemical transport models enables the utilization of all meteorological 3D fields in ACTMs at each time step and the consideration of the feedbacks of air pollution (e.g. urban aerosols) on meteorological processes and climate forcing. 

Eckman et al. 1996 Barth et al. 2000) and aerosol indirect effects (e.g., Liao et al. 2003), with a few exceptions after mid. 1990s when truly-coupled systems were developed to enable a full range of feedbacks between meteorology/climate variables and a myriad of gases and size-resolved aerosols (e.g., Jacobson 1995, 2000 Ghan et al. 2001a, b, c). 

Jacobson MZ, Kaufmann YJ, Rudich Y (2007) Examining feedbacks of aerosols to urban climate with a model that treats 3-D clouds with aerosol inclusions. J Geophys Res 112 D24205. doi 10.1029/2007JD008922... 

Zhang Y, Hu X-M, Wen X-Y, Schere KL, Jang CJ (2007) Simulating climate-chemistry-aerosol-cloud-radiation feedbacks in WRF/Chem model Development and initial application. Oral presentation at the 6th Annual CMAS Models-3 User s Conference, Chapel Hill, NC, 1-3 Oct 2007... 

Zhang Y, Wen X.-Y, Jang CJ (2010b) Simulating Climate-Chemistry-Aerosol-Cloud-Radiation Feedbacks in Continental U.S. using Online-Coupled WRF/Chem. Atmos Environ 44(29) 3568-3582... 

It is necessary to highlight the effects of aerosols and other chemical species on meteorological parameters, which have many different pathways (direct, semi-direct, and indirect effects, etc.) and they have to be prioritised and considered in on-line coupled modelling systems. Sensitivity studies are needed to understand the relative importance of different feedback mechanisms for different species and conditions relevant to air quality and climate interactions. A concerted action to mobilise and coordinate research in this area is needed. 

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