Science Briefs

Transportation Pollution and Global Warming

The main anthropogenic global warming culprit is carbon dioxide (CO2), but human activity produces a host of other, shorter-lived pollutants that contribute to climate change, among them gases that react to form ozone smog and fine particles such as black carbon. Until recently, most of the attention paid to these short-lived pollutants focused on their threat to human health. But because these pollutants disappear from the atmosphere relatively quickly, global efforts to reduce their emissions can produce an immediate benefit and help avoid dangerous tipping points in the climate system over the next few decades.

Bar graph of climate impacts from Global and US Transportation and Power sectors

Figure 1. Climate impacts measured in terms of radiative forcing from the global and U.S. on-road transportation (ORT) and power generation (PG) sectors. The CO2 radiative forcing shown is for the 20-year time horizon. The sum of total non-CO2 and CO2 forcing is indicated above each bar. + View large GIF or PDF of figure

Our new study offers additional insight into the climatic role of these pollutants. These findings come at a time when activity on domestic and international climate policy in general and on black carbon policy in particular is ramping up.

We calculated the overall warming effect of the transportation and power generation sectors, two of the main contributors to CO2 emissions, for the U.S. and the world. Effective mitigation of global climate change requires action in these sectors for which technology change options exist or are being developed. We primarily used a global climate model developed at the NASA Goddard Institute for Space Studies that simulates the transport of pollutants in the atmosphere by winds and the chemical and physical reactions that transform the pollutants into smog and particles and eliminates them from the atmosphere. The model also calculates the warming or cooling effect of the different pollutants. The results are shown in Figure 1.

We found that transportation would be a particularly good sector to target for emissions controls because it emits a lot of black carbon (most notably through diesel exhaust) and ozone-producing gases in addition to CO2. In contrast, the power generation sector emits little black carbon, but instead creates much sulfate particle pollution, which although bad for air quality and acid rain, cancels out the warming effect of the sector's CO2 emissions in the short-term.

Bar graph of climate impacts from Global and US Transportation and Power sectors

Figure 2. Climate impacts measured in terms of radiative forcing of conversion to plug-in hybrid electric vehicle fleet in the U.S. and globally for two replacement energy sources: (S1) zero emissions renewable sources, and (S2) electric power generation sector in current state. The CO2 radiative forcing shown is for the 20-year time horizon. The sum of total non-CO2 and CO2 forcing is indicated above each bar. + View large GIF or PDF of figure

We also considered a hypothetical example of switching the transportation sector to a zero-emissions or electric power source, such as in plug-in hybrid electric or pure electric technologies. The result was a hefty benefit for the climate. Such a switch would decrease the warming effect when looking just at CO2. (Increased CO2 emissions from the electricity generation sector would to some extent offset the decrease in emissions from vehicles.) But non-CO2 pollutants provide an added benefit for the climate. The technology shift greatly reduces black carbon emissions. Furthermore, switching to electric power, generated predominantly with coal at present both in the U.S. and worldwide, increases emissions of cooling sulfate particles, further reducing global warming.

Of course, the power sector also needs to be cleaned up to address long-term climate damage from CO2, as well as health problems from sulfate particles, ozone smog and other pollutants. Our results indicate that technology change options that target specific economic sectors may invoke decadal scale climate effects from the air pollutants that dominate the CO2 effects. Assessment of the full impacts of technology and policy strategies designed to mitigate global climate change must consider the climate effects of ozone and fine aerosol particles.

Reference

Unger, N., D.T. Shindell, and J.S. Wang, 2009: Climate forcing by the on-road transportation and power generation sectors. Atmos. Environ., 43, 3077-3085, doi:10.1016/j.atmosenv.2009.03.021.