Black Taxis and Global Dimming

When I recall the two years I spent working in central London in the late 1980s, my memories frequently involve traveling in one of the city's ubiquitous "black taxis" or wonderful old Routemaster buses. All of these conveyances were powered by diesel engines that contributed greatly to the incredible levels of soot I encountered. Now I see that one of the largest London taxi operators is switching to a 30% biodiesel blend that should provide a welcome reduction in particulates and other emissions. In a bizarre twist, however, reducing this pollution might have negative consequences for the entire planet.

If you don't live in a city in which particulate matter pollution (PM) is prevalent, you can't fully appreciate the benefits of reducing it. I'll never forget how shocked I was the first time the cleaners came around our Knightsbridge trading room, which overlooked a very busy street, to wipe off the phones, computer screens and keyboards. How quaint, I thought, until I saw how quickly the accumulated soot turned their rags black. For the next two years, I worried more about the state of my lungs than I ever had in L.A.

Improvements in diesel fuel quality and the retirement of the Routemaster buses, with their smoky old engines, have already made a noticeable difference in London's air pollution since then. But fuel desulfurization is a game of diminishing returns; today's ultra-low sulfur diesel fuel has already lost 98% of the particle-promoting sulfur that it contained in the late 1980s, and by 2009 new regulations will cut UK diesel sulfur levels by a further factor of five, getting as close to zero as practical. This is where biodiesel comes in. It provides environmental benefits beyond a simple dilution effect: biodiesel blends burn more cleanly than you would expect from just looking at the proportions of fuel involved. Among other things it reduces diesel soot. So this looks like a sensible way to make a highly-congested major city's public transportation even healthier and more attractive.

But when we zoom out from the local level, we find that these same particles that were fouling the air in London and other cities have also been affecting the global climate. This week's episode of PBS's NOVA science program focused on a key aspect of this, "Global Dimming." It describes how the small particles created when we burn fossil fuels, especially those high in sulfur, reflect sunlight both directly and through the formation of more reflective clouds. Scientists have known about this effect for years, but as NOVA explains, its full impact on attenuating sunlight globally has only recently been confirmed by the scientific community and by satellite measurements. Not only has this global dimming apparently altered rainfall patterns in some parts of the world, but it may well be offsetting a sizable fraction of the warming effect associated with carbon dioxide and other greenhouse gases.

In other words, a form of pollution that we've generally assumed to be mostly localized turns out to have been having global consequences, both adverse and beneficial, for decades. Moreover, its elimination, while certainly improving local air quality and mitigating downwind dimming, may accelerate global warming, perhaps dangerously so. If the interactions between these phenomena are validated, then we may need to rethink our approach to a wide range of energy and environmental issues. That's more than I can tackle in one blog posting.

Meanwhile, what about efforts to clean up London's taxis? Should they go on hold, while we sort out all the ramifications of global warming vs. particulate-related cooling? At least in this case probably not, because biodiesel addresses both problems; while it reduces particulate pollution, it also reduces greenhouse gas emissions. On the face of it, that sounds better than squeezing ever harder on the sulfur content of petroleum products, which reduces particulates but also increases greenhouse gas emissions from the energy and hydrogen consumed in the process. We will likely hear a lot more about these conflicting effects in the future. Stay tuned.