Exploring feedbacks between air pollution and climate policy

The climate of the Earth is changing in response to natural and anthropogenic forcing agents. Emissions of greenhouse gases and air pollutants have led to significant changes in the Earth’s climate systems and projections indicate that further extensive changes are likely. Increased scientific understanding into the processes responsible for climate change and the possible consequences of assumptions regarding future climate and air pollution policy is important to formulate effective response strategies based on mitigation and adaptation. Earth System Models (ESMs) can be used to make climate projections based on emissions or concentrations projections for greenhouse gasses and aerosols derived from socio-economic scenarios. Such scenarios are produced by Integrated Assessment Models (IAMs), based on detailed descriptions of population growth, energy demand and land use. There has been increasing interest in coupling different disciplines involved in climate research. The current cooperation efforts among scientists from different disciplines have led to an improved representation of climate forcings in ESMs, and of climate responses impacts in IAMs. In this thesis, we contribute to this cooperation by exploring the consequences of emission scenarios under different assumptions regarding air pollution and climate policy. To do so, we utilize a set of scenarios similar to the Representative Concentration Pathways (RCPs), developed using the IAM IMAGE. These scenarios combine scenarios with radiative forcing targets in 2100 of 2.6 W/m2 and 6.0 W/m2 with different assumptions for air pollution policies (low/high). These scenarios are subsequently used in the global atmospheric chemistry and transport model TM5. Results reveal that both climate and air pollution control policies have large-scale impacts on pollutant concentrations, often of comparable magnitude. We also find that air pollution control measures could, on a global scale, significantly reduce the warming induced by tropospheric ozone and black carbon and the cooling resulting from sulphate in the coming decades. These effects tend to cancel each other on a global scale. Next, we evaluate the equilibrium climate response to aerosol reductions in different parts of the world in 2050, using the global climate model EC-Earth. Reductions in aerosol concentrations increase downward surface solar radiation and surface temperature concomitantly in various parts of the world. The increase in surface temperature is dominated by the reduced cooling effect of sulphate which in some areas is partially compensated by the decreased warming effect of black carbon. Also, we find that aerosol reductions can significantly affect climate at high latitudes especially in the winter, mostly as a result of teleconnections between the low and high latitudes. Due to the inhomogeneous spatial distributions of air pollutants, changes in their emissions can have strong regional climate impacts. Using EC-Earth, we assess in Chapter 4 the effectiveness of different aerosol forcing agents in causing climate change in 2050. Our results show that different anthropogenic aerosol components may have a broad range ofefficacies. The results also reveal that there are large interhemispheric differences in aerosol forcings, which result in changes in circulation patterns. By using surface ozone concentrations simulated by TM5 as input to IMAGE, we estimate ozone impacts on crop production, and subsequent impacts on land use and carbon fluxes in 2005 and 2050. In the absence of new climate and air pollution policies, higher ozone concentrations could lead to an increase in crop damage in 2050 compared to present day. This may lead to a global increase in crop area notably in Asia. Implementation of air pollution policies and climate policies (co-benefits of reducing ozone precursor emissions) could limit future crop yield losses due to ozone in the most affected regions. At the local scale, the changes can be substantial.