Many research needs related to factors that influence the atmosphere and other components of the physical climate system are discussed in the chapters of Part II, and many of these needs have also been summarized in other recent reports. For example, many of the conclusions and research recommendations in Understanding Climate Change Feedbacks (NRC, 2003b) and Radiative Forcing of Climate Change (NRC, 2005d), such as those highlighted in the following two paragraphs, remain highly relevant today:
The physical and chemical processing of aerosols and trace gases in the atmosphere, the dependence of these processes on climate, and the influence of climate-chemical interactions on the optical properties of aerosols must be elucidated. A more complete understanding of the emissions, atmospheric burden, final sinks, and interactions of carbonaceous and other aerosols with clouds and the hydrologic cycle needs to be developed. Intensive regional measurement campaigns (ground-based, airborne, satellite) should be con ducted that are designed from the start with guidance from global aerosol models so that the improved knowledge of the processes can be directly applied in the predictive models that are used to assess future climate change scenarios.
The key processes that control the abundance of tropospheric ozone and its interactions with climate change also need to be better understood, including but not limited to stratospheric influx; natural and anthropogenic emissions of precursor species such as NOx, CO, and volatile organic carbon; the net export of ozone produced in biomass burning and urban plumes; the loss of ozone at the surface, and the dependence of all these processes on climate change. The chemical feedbacks that can lead to changes in the atmospheric lifetime of CH4 also need to be identified and quantified. (NRC, 2003b)
Two particularly important—and closely linked—research topics related to forcing and feedback processes in the physical climate system are clouds and aerosols. Aerosols and aerosol-induced changes in cloud properties play an important role in offsetting some of the warming associated with GHG emissions and may have important implications for several proposed strategies for limiting the magnitude of climate change (see Theme 4). Cloud processes modulate future changes in temperature and in the hydrologic cycle and thus represent a key feedback. As noted later in this chapter, the representation of cloud and aerosol processes in climate models has been a challenge for many years, in part because some of the most important cloud and aerosol processes play out at spatial scales that are finer than global climate models are currently able to routinely resolve, and in part because of the complexity and limited understanding of the processes themselves. Continued and improved observations, field campaigns, process studies, and experiments with smaller-domain, high-resolution models are needed to improve scientific understanding of cloud and aerosol processes, and improved parameterizations will be needed to incorporate this improved understanding into global climate models.
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