“When we try to pick anything by itself, we find it hitched to everything else in the Universe”, John Muir (1838-1914)
I use a computer model of the Earth system to study atmospheric chemistry and global change. Carbon dioxide is the most important single contributor to human-induced climate change. However, climate is also strongly influenced by shorter-lived gases (methane and ozone) and aerosol particulates (sulfate, soot, dust), collectively called short-lived climate forcers (SLCFs) that have complex effects involving both warming and cooling. Carbon dioxide, other long-lived greenhouse gases (GHGs) including nitrous oxide and halocarbons, and the SLCFs are often linked through common emission sources. Many of the SLCFs are associated with other environmental problems including acid rain and the degradation of air quality. For instance, ozone and particulates are known to damage human and ecosystem health and have detrimental impacts on agriculture. A major challenge faced by humanity is that 50% of the expected GHG warming due to the industrial revolution is masked by the net cooling effect of the pollution aerosol particulates.
In previous research, I have examined future climate and air quality impacts of tropospheric ozone, sulfate and methane for a broad range of possible scenarios. My research has shown that ozone precursor gases have an indirect cooling effect on climate by promoting the gas-phase oxidation of sulfur dioxide to form sulfate aerosol. Over heavily polluted subtropical regions, this cooling may outweigh the warming effects through increased ozone (Unger et al., PNAS, 2006). Recent work has focused on attribution of human-induced climate change by economic sector, a valuable approach for identifying policy options that tackle a range of different pollutants and activities (Unger et al., PNAS, 2010). Considering all forcing agents (SLCFs and GHGs), the most warming sectors on short timescales (20-30 years) are road transportation, household biofuel burning and animal husbandry. On longer timescales (100 years), power and industry as well as road transportation are the largest contributors to warming. A key related activity is investigation of the full impacts of policy-relevant climate mitigation proposals. For example, conversion of the U.S. gasoline/diesel vehicle fleet to plug-in-hybrid electric vehicles would have unambiguous benefits to climate when the replacement energy is supplied either by the electric power sector in its current state or zero-emission clean sources (Unger et al., Atmos. Env., 2009).
Current studies include: (1) U.S. FAA funded project to quantify the effects of commercial air transport on climate and air quality, (2) the role of reactive carbon compounds from vegetation in biogeochemical-climate feedbacks, (3) the impacts of heterogeneous chemistry and changing aerosol loading on climate forcing by ozone and methane, and (4) solar radiation management using stratospheric sulfate aerosol. Additional interest lies in the development of climate metrics for policy.
Key motivational questions are: Is accelerated warming due to future air pollution abatement inevitable? Can we treat air pollution and climate policies under a common framework? How will future climate change affect surface air quality in different regions? What are the best actions for us to take now to address climate and air pollution?