Ozone is a major greenhouse gas. The greatest uncertainty in quantifying this forcing lies in reconstructing ozone concentrations in the past and projecting them into the future. Global modeling of tropospheric ozone remains a major challenge because of the complex coupling between photochemical and transport processes. The inability of models to reproduce ozone trends over the twentieth century suggests that there could be large errors in current estimates of natural ozone levels and the sensitivity of ozone to human influence. These errors could relate to emissions of precursors, chemical processes, and stratospheric influence. Lightning emissions of nitrogen oxides are particularly uncertain and play a major role in ozone production in the middle and upper troposphere where the radiative effect is maximum. Transport of ozone between the stratosphere and troposphere greatly affects upper tropospheric concentrations in a manner that is still poorly understood.
Improve understanding of the transport of ozone in the upper troposphere and lower stratosphere region and the ability of models to describe this transport.
Policies designed to manage air pollution and land use may be associated with unintended impacts on climate. Increasing evidence of health effects makes it likely that aerosols and ozone will be the targets of stricter regulations in the future. To date, control strategies have not considered the potential climatic implications of emissions reductions. Regulations targeting black carbon emissions or ozone precursors would have combined benefits for public health and climate. However, because some aerosols have a negative radiative forcing, reducing their concentrations could actually increase radiative warming. Policies associated with land management practices could also have inadvertent effects on climate. The continued conversion of landscapes by human activity, particularly in the humid tropics, has complex and possibly important consequences for regional and global climate change as a result of changes in the surface energy budget.
Apply climate models to the investigation of scenarios in which aerosols are significantly reduced over the next 10 to 20 years and for a range of cloud microphysics parameterizations.
Integrate climate forcing criteria in the development of future policies for air pollution control and land management.