Identifying Chemistry-Climate-Air Quality Connections To Inform Public Policy

Air pollutants and their precursors force the climate system by altering solar and terrestrial radiation budgets, and their distributions are in turn highly dependent upon regional climate.  We focus here on methane and tropospheric ozone, the second and third most important anthropogenic greenhouse gases, respectively.   Since methane is a key precursor to tropospheric ozone, reducing methane emissions decreases climate forcing and lessens the global public health burden by decreasing ozone levels in surface air.   We briefly review recent work indicating that controlling anthropogenic methane emissions can be a cost-effective approach to jointly mitigate ozone air pollution and climate forcing.   A warming climate in turn exerts complex feedbacks on tropospheric methane and ozone distributions by altering their sources and sinks.   These chemistry-climate interactions are examined using several century-long simulations conducted in support of IPCC AR5 with the newly developed GFDL chemistry-climate model (CM3), which includes stratospheric and tropospheric chemistry as well as aerosol-cloud interactions.   Both climate warming and projected emission changes influence the evolution of the methane lifetime under the Representative Concentration Pathway (RCP) scenarios (developed in support of IPCC AR5).  We further examine the relative roles of specific processes (e.g., temperature, water vapor, photolysis rates, anthropogenic emissions) in driving the simulated changes in methane lifetime.  We explore projected changes in ozone air quality over North America, highlighting climate-driven changes in summertime cyclone passages (which ventilate polluted regions) and in stratospheric ozone injection into the troposphere (which raises tropospheric ozone levels in winter and spring) in the most extreme climate warming scenario (RCP8.5).   Finally, we discuss major uncertainties and associated implications for climate forcing and regional air quality.  With the atmospheric component of the GFDL model, we demonstrate the potential for combining meteorological and chemical observations from in situ and space-based platforms to conduct process-oriented evaluation.  This type of evaluation should aid in determining whether chemistry-climate models are adequately representing the key processes controlling chemistry-climate interactions, a critical step towards narrowing gaps in current understanding and more confidently projecting future changes in climate and air quality.