On some level (in particular, the top-of-the-atmosphere level where net-global accounting is done), the effects of greenhouse gases are simple: because they trap outgoing radiation from the surface, the planet must warm to radiate more by Planck's Law and remain in equilibrium with the incoming solar radiation. This instantaneous 'radiative forcing' mechanism is relatively well-understood, although the subsequent heat transports and the final regional climate effects in equilibrium are less so. Radiative forcing occurs more or less uniformly around the globe, hence the term 'well-mixed greenhouse gases' [WMGHGs] for CO2, CH4, N2O, and several others that, once emitted, exist in the atmosphere for decades or centuries and move all around during that time. For near-surface air pollution, this is not the case. [Air pollution can broadly be thought of as any gas or suspended particle that is harmful to human health, and so is of most interest at the surface.] The compounds comprising it, like ozone, sulfates, and nitrates, have short lifetimes (a few days is typical) and thus are present in high concentrations only in or immediately downwind of source regions. This makes prediction more difficult, and of course also means that the differences in impacts between regions are larger.
Another fundamental difference between greenhouse-gas 'pollution' and air pollution is the divergence of their trends in recent decades: the former is generally still rising, while the latter is falling, at least in developed countries, as a consequence of strict legislation. Today's pollution hotspots are in places like Harbin, China and New Delhi, while the legendary suffocating smogs of Los Angeles and London -- that in terms of some components like sulfur dioxide were multiple times worse than even present-day pollution episodes in East Asia -- are mostly a thing of the past. To give a flavor of the severity, for the week ending Dec. 13, 1952, average SO2 concentrations in London were five times higher than the daily maximum recorded during Beijing's severe episode in Jan. 2013, resulting in something like 10,000 excess deaths. With such short lifetimes, these kinds of regional-pollution events respond to economic fluctuations, with impacts on the health of our most vulnerable citizens measured in the thousands, in addition to exhibiting seasonal patterns in intensity and location both in the U.S. as well as in China. And like smoking and obesity, it seems the more one reads, the more correlations one discovers between air pollution and elevated health risks: diabetes, for example.
So, in a practical/political sense, regional air pollution control should be easier to agree on because the benefits of management are more immediately tangible. It's a case of big costs and big benefits, and politicians tend to dwell more on the first, which is why there is hesitation to impose regulations despite the consensus net benefit, and also why it is only once countries reach a certain level of wealth that they begin imposing limits on pollution of their own accord. The decision is often a conscious one: at low income levels, the benefits are judged as outweighing the costs, but this analysis changes with income (i.e. following a Kuznets curve). Is there reason for optimism then? The overall downward trends in the developed world suggest there is, although this optimism must be tempered by the recognition that reducing pollution might result in other unintended and undesirable counter-effects like increasing temperatures over the Atlantic that have been held down by reflective sulfate aerosols. Also, increasing summertime temperatures will promote the formation of more photochemical smog, especially in already-polluted urban areas. In the near-term, traditional foci of air pollution like coal-fired power plants and smelters continue to have significant negative health impacts on people living in their vicinity -- which is suggested by the figure at left below, and even more so by the figure at right. The values over 1,000 expected cancer cases per million residents in the righthand figure are in census tracts adjacent to steel factories. These are the scales on which people are affected, and so must be the scales ultimately considered in policy decisions if future progress is to be as remarkable as what has been achieved since the palette of London smogs included Madeira wine, yellow, dun, heavy brown, green-yellow, and faint yellow.
At the bottom of the page is an interactive real-time map from aqicn.org, a group in China that compiles observations from around the globe into user-friendly formats using the EPA 0-500 scale for overall air quality. It works best in Firefox or Chrome.
Two articles ("Holiday Lights from Space" and "A Hazy Road to Mecca") in the December issue of the Urban Climate News highlight an important aspect of climatology, and of urban climatology in particular: the wildcard of human influence. Some climate decisions can be strongly influenced by human relationships writ large, of course, as is always the case in geopolitics; but the phenomena described in the two articles above cause us to reflect upon the ability of ordinary people to together modify the environment around all of them when behaving in a coordinated way. Holidays are a perfect example, whether they mean a person treks to Mecca or simply sets out decorations in their yard. Analyzing the changes during holidays are fun, but, more importantly, time variations allow for a better understanding of how sensitive the climate system is (on local and regional scales especially) to human actions. The midweek peak in southeast-U.S. precipitation is one good example of this. Zooming out to the scale of years, economic crises leave their signature in emissions trends as well.
It's not just in or near cities where the human temporal fingerprint can be detected, but any place where people gather in numbers for a length of time and bring with them the trappings of an energy-intensive lifestyle. (This is to a certain extent just a case of the general rule that adding more people can turn any previously sustainable activity, like slash-and-burn agriculture, into an unsustainable one.) The authors of the study examining environmental conditions along Hajj routes ingeniously made the connection between the huge concentrations of people — however temporary — and the probable resultant impacts. After all, from a human-health perspective, the degree to which adverse conditions are problematic is proportional to the product of their severity and the number of people exposed. Indeed, the study found that heavy traffic, constrictive tunnels, and high temperatures were the main factors in producing locally extreme levels of carbon monoxide, methane, and toxic volatile organic compounds [VOCs]. These spikes were not representative of regional conditions, but the 100- or 1000-km average isn't what matters for something like air quality; the 1-m average is. I don't expect that models will be able to soon, if ever, capture the effects of traffic jams in Meccan tunnels on a handful of days surrounding Ramadan. But keeping in mind that goal lays out a useful scientific trajectory for, in the coming decades and beyond, moving from bulk averages to localized and fleeting values.
Achieving that goal calls for more inspired studies that look at temporary perturbations to the time- and space-average anthropogenic climate effect. Typically, regional studies focus on the space average, or the time average but primarily for periods defined by ambient environmental conditions like heat waves. Both require parameterization to get down to the level of human experience, which is something like one meter in space and one minute in time. Although perhaps clearest for air quality, I think there is a purpose to closer examination of other types of detailed time-varying climate information as well. The amount of lights during holidays may be a proxy for overall increases in energy usage, which would have implications for the management of electrical capacity, especially when two stressor events overlap (e.g. a hot holiday afternoon). Large events like festivals or games, whether in urban areas or far away from them, may also have discernible environmental effects for attendees or residents of the surrounding regions. Or they may not. The point is, in a world of 'smart' and 'just-in-time' allocation of resources, such questions are not just worthy of consideration from an impacts standpoint — they may be able to tell us something useful about the behavior of climate systems as well as that of ourselves.