The difference between their current study and previous efforts, the authors say, can be explained mainly by the combination of the factors such as underlying emissions (pulse or sustained), carbon cycle model, BC efficacy, and atmospheric chemistry processes considered.

A climate mitigation policy that focuses on CO₂ emissions supports an increase in the fraction of diesel cars because passenger cars equipped with diesel engines emit on average 15% less CO₂ per kilometer than vehicles with gasoline engines with a similar power output. After the UK had begun taxing vehicles according to CO₂ emissions, the share of registered new diesel cars increased from 26% in 2002 to 38% in 2005.

However, it is not clear whether the shift from gasoline to diesel cars, which is encouraged by CO₂ taxes for vehicle registrations and fuels, would be effective as originally intended as a climate mitigation measure, given a number of non-CO₂ components emitted together. Many of them have indirect climate effects through chemical reactions or by changing the planetary albedo. NO_x, CO, and HC emissions lead to a production of O₃ (“short-term O₃ formation”) as well as a change in the levels of the main oxidant in the atmosphere, the OH radical. NO_x emissions and the produced O₃ enhance the production of OH, while CO and HC emissions reduce OH levels. The change in the OH concentration affects the lifetime of CH4, which has a longer-term impact on the O₃ production (“primary mode”). BC and OC absorbs and reflects sunlight, respectively (direct effect) and BC changes the albedo of snow/ice (albedo effect). BC and OC may also change cloud properties through the indirect and semi-direct effects. Furthermore, aerosol abundances are influenced by OH through gas-aerosol interactions.

—Tanaka et al.

To analyze whether a policy that encourages diesel cars is consistent with a long-term climate goal to cap global warming, the team calculated the global-mean temperature change that would be caused by the use of gasoline and diesel passenger cars under the previous and upcoming emission standards in Europe (EURO 3, 4, 5, and 6).

For their study, they used a simple climate and carbon cycle model combined with aggregated parametrizations of atmospheric chemistry processes (explicitly including the short-term O₃ response and the primary mode perturbation). To reduce the complexity of the task, they assumed gasoline and diesel cars exactly followed the emission standards and had a similar power output.

Nor did the study consider the impact of a fleet of aging models adhering to different emission standards, or any technology options other than standard gasoline and diesel—i.e., no shift to hybrids, plug-in hybrids, battery-electric vehicles or fuel cell vehicles. The study also focused on climate impacts alone, without comparing with the adverse health impact caused by the air pollution.

Their analysis used one-year pulse emissions as well as sustained emissions over an average car’s lifetime (15 years), and adopted the IPCC Impulse Response Function (IRF) designed to represent carbon cycle processes on various time scales.

Among their findings were:

• The newer emission standards generally lead to a smaller net warming.

• Long-term warming from diesel engines is smaller relative to that from gasoline engines for any emission standard because only the CO₂ emissions count in the long run. After 100 years, the warming from diesel cars is about 20.4% smaller than that from gasoline cars in the case of EURO 3; and 17.7% in EURO 4; 15.4% in EURO 5; and 10.2% lower in EURO 6.

• The initial temperature response is drastically different between the early emission standards (Euro 3 and 4) and the later standards (Euro 5 and 6). The magnitude of the warming caused by diesel cars under EURO 3 and 4 exceeds that by gasoline cars during the early phase of operation by up to a factor of 2.0 and 2.5, respectively due to the large influence from the short-lived components O₃ and BC.

The crossover points, beyond which diesel cars cause less warming than gasoline cars, occur closer to the emission year as the advanced emissions standards set in 5 years, 5 years, 3 years, and 2 years for EURO 3, 4, 5, and 6, respectively, because of the more stringent caps for short-lived components (O₃ precursors and BC) in the newer emission standards. In the cases of EURO 5 and 6, BC emissions from diesel cars cause only a small warming.

• During the first 10 years of contributions from the non-CO₂ components are significant, causing both warming and cooling. The short-term impacts (warming and cooling) are substantially larger for diesel engines due to the higher emissions of NO_x (leading to short-term O₃ formation) and carbonaceous particles.

• The climate response due to the short-term increase in the O₃ concentration is stronger for diesel engines than for gasoline engines. This short-term O₃ effect explains 45% of the total warming at the end of the first year.

• The only significant effect of carbonaceous particles is the BC emissions from diesel engines. At the end of the first year, the contribution from BC is as large as 43% of the total warming, but during the first decade after the emissions it decreases to 10%. In the long term (20 years or more), the contribution from BC is very small (less than 2%).

• For diesel cars, the primary mode effect reduces the warming by up to 22% (10 years after the emissions). For gasoline cars, the lower ratio of NO_x to CO + HC emissions leads to a reduction in the OH concentration and thus an increase in the CH₄ concentration, resulting in a small temperature increase (less than 5% of the net warming).

When interpreting our conclusion that CO₂ taxes are in line with efforts to reduce our impact on the climate, one should keep in mind that in general, considering relevant non-CO₂ components in addition to CO₂ is essential in designing climate-related policy instruments because non-CO₂ components play an important role in characterizing the short- to middle-term climate response—it is not unlikely that other instruments in different sectors or cases lead to a variety of climatic outcomes including those incompatible with efforts to reduce our impact on the climate. Furthermore, policy makers would need additional considerations beyond what we addressed here.

For example, the rebound effect may significantly influence our results. It has been argued that consumers use a part of the money saved through opting for a diesel engine for upgrading to a larger engine. The rebound effect may also work through increased driven mileage (in addition to larger engines). This implies that the difference between the CO₂ emissions from gasoline engines and those from diesel engines could be effectively smaller than what we obtained, reducing further the difference between the long-term climate effects expected for gasoline-fueled vehicles and that for diesel-fueled vehicles.

—Tanaka et al.

Resources

• Katsumasa Tanaka, Terje Berntsen, Jan S. Fuglestvedt, and Kristin Rypdal (2012) Climate Effects of Emission Standards: The Case for Gasoline and Diesel Cars. Environmental Science & Technology doi: 10.1021/es204190w