Sources of variability in short-lived climate pollutants and their effect on climate

Targeting short-lived climate pollutants (SLCPs) such as methane (CH4), tropospheric ozone (O3) and atmospheric aerosols has been proposed as an effective method of reducing the rate of near-term warming, due to their considerable contribution to climate change and relatively short atmospheric lifetimes. Many SLCPs are also air pollutants, presenting the opportunity to simultaneously tackle both climate change and air quality. The aim of this thesis is to improve understanding of factors controlling SLCPs and their role in global climate. Model simulations using the TOMCAT chemical transport model and the SOCRATES radiative transfer model are used to explore sources of uncertainty and variability of SLCPs, as well as potential as mitigation strategies. Recent evidence from proxy-records indicates that fire and biogenic emissions were likely larger in the pre-industrial era than in the present-day. Greater emissions of O3 precursors CO and NOx in revised pre-industrial emissions inventories result in increases in simulated pre-industrial tropospheric O3 concentrations, decreasing the pre-industrial to present-day radiative forcing (RF) of tropospheric O3 by up to 35%. The variability of CH4 and tropospheric O3 during El Niño is largely driven by changes in fire emissions. During the 1997 El Niño, enhanced fire emissions of CO suppressed oxidant availability and extended CH4 lifetime, causing a 7.5 ppb yr-1 increase in simulated global CH4 growth rate in 1998. Increased fire emissions also -2 lead to an increase in O3 RF of 0.03 Wm , while meteorological effects decrease -2 O3,reducingtroposphericO3RFby0.03Wm ,thusresultinginasmallnetchange to tropospheric O3. The potential of targeting SLCPs as a mitigation strategy is also assessed by investigating the climate and air pollution impacts of various emission reduction measures. It is found that maximum technically feasible reductions of anthropogenic emissions reduce global air pollution, however the removal of cooling aerosol results in an accelerated rate of warming, surpassing 2°C by 2050. Targeted measures which decrease emissions of only warming components enables mitigation of global mean temperature change of up to 0.3°C by 2050, but does not combat the growing problem of air pollution.