Multi-decadal climate variability and associated hydrological responses.
Climate change's impact on particulate matter pollutions
The transport and removal processes of particulate matter pollutants are affected by greenhouse gas-driven global warming. We are examining using CESM large ensemble how the global warming may change the pollution level in the 21st century given a fixed emission at present-day level.
Climate response to aerosols
The regional climate change is under the influence of aerosols, in addition to greenhouse gases. We conduct GCM simulations with the Community Earth System Model (CESM) to assess the climate response to black carbon aerosols, with a focus on the Himalaya snow pack (Xu et al., 2016, ACP). We have also studied the general circulation response to different types of aerosols and demonstrated that ocean temperature gradient plays a key role (Xu and Xie, 2015, ACP). Recently, we start to contrast the role of aerosol and GHGs in the future projection of heat extremes (Xu et al., 2015, Climatic Change), precipitation extremes (Pendergrass et al., 2015, GRL) and terrestrial aridity (Lin et al., 2016, Climatic Change).
Radiative forcing of carbonaceous aerosols
The radiative forcing of carbonaceous aerosols (black carbon and organics) in current global climate models is biased due to (1) the underestimated emission inventory and (2) the negligence of organic solar absorption. We carry out observational studies utilizing satellite and ground network observations to quantify the direct radiative forcing of black carbon and organics and develop a novel method of partitioning the solar absorption between the two species (Bahadur et al., 2012, PNAS). We apply this method to a regional study over California (Xu et al., 2013, JGR) and show solar absorption due to organics should be included in the climate model. In addition, black carbon emissions over Asia are underestimated by a factor of two to four (Xu, 2014, Ph.D. dissertation).
Short-lived pollutants and climate change mitigation
Mitigation of short-lived climate pollutants is considered a win-win solution to fight climate change and air pollution. We project the global mean temperature change in the 21st century and show that mitigation of short-lived pollutants plays a vital role in reducing future global warming (Ramanathan and Xu, 2010, PNAS) and sea level rise (Hu et al., 2013, Nature Clim. Change). Specifically, we study the role of HFCs (hydrofluorocarbons, as a substitute of ozone-depleting substances) (Xu et al., 2013, ACP) in future climate change. The ultimate goal is to provide useful information for policy makers (Xu and Zaelke, 2013, Our Planet). The quantitative approach we use here rely on simple models and statistical methods, which are also my interests of research.