The global warming culprit we hear the most about is carbon dioxide (CO2), but human activity produces a host of other, shorter-lived pollutants that act as "partners in crime" in contributing to climate change.
Until recently, most of the attention paid to these pollutants has centered around their detrimental effects on air quality and human health – the pollutants include fine particles such as black carbon and gases that form smog.
But because these pollutants disappear from the atmosphere relatively quickly, they also give us an important opportunity to put the brakes on the rapid rise in global temperature. If people around the world can reduce the amounts that they emit, everyone will see an immediate benefit and help avoid dangerous tipping points in the climate system over the next few decades.
My colleagues Nadine Unger and Drew Shindell at the NASA Goddard Institute for Space Studies (GISS) and I just published a paper in the journal Atmospheric Environment that offers additional insight into the climatic role of these pollutants. Our 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.
For this paper, we delved into emissions from two key sectors, transportation and power generation, for the U.S. and the world. We primarily used a global climate model developed at NASA GISS that simulates the transport of pollutants by wind and the chemical and physical reactions that transform the pollutants into smog and particles. The model also calculates the warming or cooling effect of the different pollutants.
One of our important findings is that transportation is a particularly good sector to target quickly for emissions controls because it produces a lot of black carbon (think: diesel exhaust) and ozone-producing gases, in addition to CO2. In contrast, emissions cuts in the power generation sector do not offer the same short-term opportunity. That sector emits little black carbon, but it does create much sulfate particle pollution. Sulfate particles are bad for air quality and acid rain, but in the short term actually counteract the warming effects of CO2 emissions. Of course, it is essential to clean up the power sector to address long-term climate damage from CO2, as well as health problems from sulfate particles, ozone smog and other pollutants. But short-term opportunities to slow global warming are more significant in the transportation sector.
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: A hefty benefit for the climate.
The switch to a zero-emissions or electric power source would decrease the warming effect if you just consider CO2 emissions. (Though increased CO2 emissions from the electricity generation sector would offset the decrease in direct emissions from vehicles to a certain extent.)
But reducing the non-CO2 pollutants provides even more benefit for the climate. Zero-emission or electric transportation would greatly reduce black carbon emissions. The short-term benefits to be gained from focus on the transportation sector are important for policymakers to note.
Last week's announcement by President Obama on national greenhouse gas emissions standards for passenger cars and light trucks is a significant step in this direction. Further action is needed to clean up the exhaust from existing heavy-duty trucks and other diesel-powered transport, both in this country and internationally.
Unger and her colleagues are working to expand the published analysis to include a full suite of economic sectors, including industry, non-road transport and agriculture, and additional greenhouse gases such as methane and nitrous oxide.
Look for another paper in the near future.