A future short of breath? Possible effects of climate change on smog
Smog arrives in U.S. northeastern and midwestern states with the summer’s merciless heat and still air, a brown haze that hangs in the sky until cool air rolls in and provides a welcome respite. In Los Angeles and Missoula, it thickens in mountain basins like soup in a pot. Houston’s smog worsens in September, when sea breezes off the Gulf of Mexico die down. For the rest of the world, although the composition and severity of smog can vary greatly from region to region, a pattern has emerged: Warm temperatures, pollutants, and sunlight often work together to produce unhealthy conditions, the dangers of which are just now becoming known. The United States and other countries have attempted to address the issue of air pollution. On “bad air days,” when particularly smoggy conditions are predicted, newscasters or authorities urge people to take public transportation and warn asthmatics and others with heart or lung conditions to stay indoors.
As more has become known about what meteorological conditions favor smog formation, predictions for the next day’s air quality have become increasingly accurate. But what about the long-term picture? In coming decades, climate change will likely have a large impact on temperatures at the Earth’s surface and on day-to-day weather patterns. Will higher temperatures at the surface favor smog formation? Or will these higher surface temperatures contribute to lofting of the smoggy air toward higher layers of the atmosphere? How will changes in cloud cover impact surface air quality? If a warmer atmosphere can hold more water vapor, will cloud cover increase, thereby slowing down the production of smog? Finally, how will developing countries adopt new technologies without further degrading their air quality in a changing world?
Smog consists of a mixture of chemicals, some in gas-phase and some in the form of tiny particles. Smog starts with the emissions of gases like nitrogen oxides, volatile organic compounds (VOCs), and sulfur dioxide, and of particles like organic carbon and soot. Many of these constituents form during combustion processes. Every time someone starts a car or a coal-fired power plant kicks in, the high temperatures of combustion cause the release of more smog precursors into the air. But some smog precursors have natural sources. For example, for reasons that are not entirely clear, many trees and grasses emit VOCs like isoprene or a class of molecules called monoterpenes. Plant biologists are still trying to sort out why plants evolved to emit these chemicals and what protective effect the molecules could have on leaf structure. Even tiny organisms in soil emit high quantities of N[O.sub.x] as they convert other forms of nitrogen into usable energy.
But this short answer–that higher temperature means worsening air quality–neglects many competing and complicating factors. For example, the source gases that form secondary organic particles condense less readily at higher temperatures, which could mean fewer such particles in the atmosphere in the future.
One of the main factors influencing pollution levels is the frequency and duration of stagnation episodes. Stagnant air traps air pollution, allowing the chemicals to interact and the products of emissions to accumulate. Stagnation episodes typically occur during the summer, when the heat and humidity can become unbearable, but they can occur at other times of the year as well. In December 1952, a cold air mass moved off the English Channel and parked over London for five days. The cold air trapped the plumes of soot emanating from coal-fired stoves and factories, leading to a thick, dirty haze. This smog event, known as the Great Smog of 1952, may have led to as many as 12,000 premature deaths in the days and months that followed.
In the coming decades, as climate changes, will such stagnation episodes take place more frequently? Will they last longer when they occur? To understand how future climate change could affect stagnation episodes, it is helpful to think about how such episodes in the present-day atmosphere come and go. Over mid-latitudes, stagnation is one phase of an endlessly repeating weather pattern. First, a cold front comes through from the west, bringing rain and cool weather. After the passage of the cold front, winds weaken, the sky clears, and temperatures begin to climb. The air may stagnate. Soon another cold front arrives. A wedge of cool or cold air pushes in, lifting the warm (and possibly polluted) air eastward and poleward. What drives these cold fronts is the Earth’s heat imbalance. The sun deposits most of its energy in the tropics, and the ocean and atmosphere respond with several mechanisms that redistribute that energy. Cold fronts contribute to this redistribution by pushing warm air toward the colder, higher latitudes.
It should be emphasized, however, that all the studies described above made one very large assumption: that the emissions of smog precursors related to human activity would stay constant through future decades. In fact, it is quite possible that such emissions will decline, as new technology is put in place and existing regulations on pollution are tightened. The main value of these studies is that they make clear the climate change penalty that will be needed to overcome to meet air quality standards.
Particles have a more complicated role influencing climate. Most particles, such as sulfate or organic particles, reflect incoming sunlight like tiny mirrors and therefore lead to cooling. Soot particles, on the other hand, absorb incoming sunlight and outgoing infrared radiation, making the net effect of these particles difficult to calculate. Some studies have suggested that plumes of soot and sulfate particles emanating from South Asia may have led to local cooling of the Indian subcontinent, diminishing the strength of summer monsoon. This effect could counteract the warming influence of greenhouse gases, which most models predict will intensify the monsoon.