Impacts of worldwide vehicle emissions in 2030 compared with 2000. a–d, Changes are shown under the baseline (left side by region) and tight-standard scenarios (yellow, right side by region). Negative valuations indicate savings. Ranges include the estimated forcing uncertainty on the basis of published observations and modelling results (a, .67% confidence interval (CI); uncertainty due to concentration–response relationships only (b, health; d, health valuation; 95% CI) and uncertainty due to concentration–response metrics (c, crops). Much of the uncertainty is systematic, so differences between scenarios can be significant even if uncertainty ranges for the two scenarios overlap. Click to enlarge.

Shindell and colleagues used a comprehensive computer model and climate simulator—one of the first capable of accounting for the role of aerosols—that shows vehicle exhaust exacts an enormous toll in all countries and especially in the developing world.

The scientists used modeling techniques developed at GISS to compare a baseline scenario that assumes existing emission standards remain unchanged in coming decades with a second scenario that has most countries adopting stringent emission standards. The aggressive scenario assumes, for example, that China, India, and Brazil adopt Euro 6 standards by 2015, a regime that would reduce emissions of particulate matter by about 85%, nitrogen oxides by about 65%, and carbon monoxide by about 70% for passenger vehicles.

The aggressive scenario assumes major emissions reductions in Latin America, Africa, and the Middle East, the regions with the laxest emissions standards. Emissions rules in North America are slightly more stringent than European standards already, so in North America the baseline and aggressive scenarios were identical.

The analysis also breaks down potential health benefits by region and finds benefits varied widely. Overall, the modeling found that stricter standards would prevent the most deaths in China, India, and North Africa, regions where unfiltered soot-producing diesel engines remain ubiquitous.

While reductions in particulate matter tend to produce local health benefits, the scientists found health and agricultural benefits from reduced ozone disperse more widely. That means for some countries—India, for example—changes in emissions from neighboring countries could have as much impact as local emission changes.

The new study shows that the same measures that benefit human health and agriculture would also make a significant dent in climate change in the near term.

While it is well-established that carbon dioxide released by vehicles contributes to global warming, it has been much less clear how the combination of shorter-lived aerosol particles vented by vehicles such as black carbon, sulfate, and organic carbon affect climate.

While some of these aerosols reflect sunlight and produce a cooling effect, others absorb light and warm the atmosphere. Aerosols from vehicles can also impact the development of clouds in ways that have poorly-understood consequences for climate.

Shindell et al.’s modeling shows that stringent emissions standards would reduce 0.20°C (0.36°F) of warming in the Northern Hemisphere from 2040 to 2070. That’s largely because more stringent standards would reduce emissions of black carbon, a constituent of soot, and carbon monoxide, a precursor of ozone. In comparison, the Northern Hemisphere has warmed by about 0.3°C (0.54°F) per decade in the last three decades.

Though the stringent standards would provide a clear climate benefit in the near term, the impact of accumulating carbon dioxide from vehicles is so large that there would still be an overall warming impact from vehicle emissions, albeit a lesser one than if they were not enacted.

—Drew Shindell

As with the health benefits, the model projects the climate impacts of more stringent standards would vary significantly depending on the region. Cooling effects of sulfates, which highly are reflective, are minimized over parts of the Earth such as ice sheets and deserts that are also highly reflective, while the same areas exaggerate the warming from soot.

Emissions from India, for example, produced a particularly strong regional warming response because of the close proximity of large swaths of snow and ice in the Himalayas. The same was true of the Middle East and North Africa because of deserts in the region.

To date, most studies have looked at the health, agricultural, or climate impacts of emissions in isolation. Shindell et al.’s analysis is one of the first to analyze the closely-intertwined impacts together.

Resources

• Shindell, D., G. Faluvegi, M Walsh, S.C. Anenberg, R. Van Dingenen, N.Z. Muller, J. Austin, Koch. D., and G. Milly (2011) Climate, health, agricultural and economic impacts of tighter vehicle-emission standards. Nature Climate Change, 1, 59-66, doi: doi: 10.1038/nclimate1066