When it comes to pollutants that alter the Earth's climate, heat-trapping carbon-dioxide gets most of the attention. And rightly so. But there are other pollutants out there that also fiddle with the planet's thermostat in various ways, and they can have a significant impact, too. As the IPCC noted in its 2007 report, aerosol pollution—which includes everything from sulfur-dioxide to particulates—actually has a cooling effect that partly offsets the warming effect of greenhouse gases. The tiny particles in the air tend to reflect sunlight back into space, and they also seem to make clouds more reflective. Scientists can test this whenever a big volcano erupts and belches up sulphates. (See this chart for a breakdown of the different factors altering the Earth's temperature.)
As it turns out, though, the cooling effect of aerosols is actually fairly worrisome—mainly because it won't last forever. As Anil Ananthaswamy recently reported for New Scientist, countries like China and India are rapidly trying to clean up their aerosol pollution. Satellite measurements show that China's making especially big strides in this regard. After all, particulates have been linked to lung disease and sulfur-dioxide is a main cause of acid rain. No one wants to keep all that pollution around. But there's a catch: As those particles get mopped up, they can no longer counteract the warming effects of greenhouse gases.
What's more, since aerosols don't linger in the air for very long, cleaning them up can lead to fairly quick changes. According to work done by the European Commission Joint Research Centre, as aerosol pollution shrivels, the short-term rate of overall warming could leap from the current 0.2°C per decade to as much as 0.3°C or 0.4°C per decade. (Although I'd caution here that, whenever I ask climatologists what aspect of their field still has the most uncertainties, they frequently mention aerosols—it can be extremely difficult to pin down the effects and various side effects of these small particles.)
The reason that's potentially alarming is that, in many ways, the pace of climate change can matter even more than the overall level of warming. Many species need time to adapt to shifts in temperature. Ananthaswamy points to a paper by two Dutch researchers that suggests that 83 percent of forest ecosystems around the world would struggle to cope with temperature increases of more than 0.3°C per decade.
That said, it's not all bleak. There's another easy-to-tackle pollutant out there that also affects the Earth's climate—only this time in the opposite direction. It's black carbon, a sort of soot that's produced from diesel engines or wood cookstoves. The dark black carbon particles settle on ice and snow and absorb sunlight that would otherwise be reflected. Recent studies have suggested that black carbon may be responsible for a significant chunk of glacial melt in many areas, especially in the Himalayas. And the good news is that this soot isn't terribly tricky to clean up—it mainly involves filters for diesel engines or cleaner-burning stoves in developing countries.
Finally, it's worth adding that there are all sorts of lesser-known greenhouse gases that don't get nearly as much press as carbon-dioxide gets. Methane, which comes from sources like landfills and sewage plants and industrial farms, packs a much bigger punch than CO2, but it also doesn't remain in the air for very long, which means we could make a lot of immediate progress in slowing the pace of warming by reining it in. Recently, Stacy Jackson of UC Berkeley suggested that it might make sense to tackle shorter-lived gases like methane in a separate treaty, so that those efforts don't get bogged down in the (more contentious) fight over CO2. We'd still have to tackle CO2—it's still the single biggest driver of global warming out there—but making rapid headway on secondary pollutants could help buy us some time.
(Flickr photo credit: Stefan Gara)