Abstract: Aviation emissions have been found to cause 5% of global anthropogenic radiative forcing and ∼16 000 premature deaths annually due to impaired air quality. When aiming to reduce these impacts, decision makers often face trade-offs between different emission species or impacts in different times and locations. To inform rational decision-making, this study computes aviation’s marginal climate and air quality impacts per tonne of species emitted and accounts for the altitude, location, and chemical composition of emissions. Climate impacts are calculated using a reduced-order climate model, and air quality-related health impacts are quantified using marginal atmospheric sensitivities to emissions from the adjoint of the global chemistry-transport model GEOS-Chem in combination with concentration response functions and the value of statistical life. The results indicate that 90% of the global impacts per unit of fuel burn are attributable to cruise emissions, and that 64% of all damages are the result of air quality impacts. Furthermore, nitrogen oxides(NOx), carbon dioxide (CO2), and contrails are collectively responsible for 97% of the total impact. Applying our result metrics to an example, we find that a 20% NOx stringency scenario for new aircraft would reduce the net atmospheric impacts by 700 m USD during the first year of operation, even if the NOx emission reductions cause a small increase in CO2 emissions of 2%. In such a way, the damage metrics can be used to rapidly evaluate the atmospheric impacts of market growth as well as emissions trade-offs of aviation-related policies or technology improvements.

Summary: As the Paris Agreement nears the four-year mark and signatories prepare to account for progress to date on national pledges to reduce climate-warming greenhouse gas (GHG) emissions, it’s important to note that some countries have a much heavier lift than others. A case in point is Taiwan, where more than 95 percent of the total energy supply comes from fossil fuels, nearly all of which are imported. Fulfilling its Nationally Defined Contribution (NDC) to the Paris Agreement—to reduce its greenhouse gas emissions by 50% from the business-as-usual level (428 metric tons of carbon dioxide-equivalent emissions) by 2030—may impact Taiwan’s economy due to its heavy dependence not only on imported carbon-intensive energy sources but also on international trade. The combined effect of nearly 200 other countries’ efforts to fulfill their NDCs could alter Taiwan’s trading channels and costs, and hence its domestic economic performance. To assess the overall potential impact of the Paris Agreement on the island nation’s economy therefore requires a modeling framework that takes into account the economic effects of both domestic and international GHG reduction policies.

Overcoming the limitations of previous studies of Taiwan’s economy based on a single-country modeling framework, researchers at the MIT Joint Program on the Science and Policy of Global Change have developed and applied a global, economy-wide, computable general equilibrium (CGE) model with energy use and emissions details where Taiwan is explicitly represented. The MIT model, which provides global coverage and explicit modeling for international trade, can simulate the economic effects of foreign policies including the Paris Agreement. In a study appearing in Climate Change Economics, the researchers compared the impact on Taiwan’s economy of the Paris Agreement under two scenarios—one in which Taiwan fulfills its NDC unilaterally, the other in which Taiwan does so in the context of a global effort. The study also considered the impact on Taiwan’s economy of a third scenario in which the U.S., which recently set in motion its withdrawal from the international climate accord, partially achieves its NDC through state-level efforts.