In the latter half of the 20th century, it became clear that other pollutants were accumulating globally. Pesticides such as DDT and toxaphene, and industrial synthetic compounds such as polychlorinated biphenyls (PCBs), collectively known as persistent organic pollutants (POPs), have been recognized as dangerous since the early 1960s – they are toxic, soluble in fat, and accumulate in body tissue⁶. But in the 1990s two further concerns emerged: first that they are “endocrine disrupters”, disrupting hormone systems and threatening the health of both wildlife and humans⁷; and, secondly, that many are now accumulating in ecosystems globally – sometimes at higher concentrations than are present where they are first released. In a process known as “global distillation”, many of these substances evaporate into the air where they are released and then preferentially settle out in the colder air of the polar regions. Though global emissions of most POPs are falling, their presence in Arctic ecosystems continues to rise and concentrations in the diets of some Arctic inhabitants exceed tolerable daily intakes⁸. POPs are currently the subject of negotiations intended to bring them under a global agreement, with some being phased out and others tightly controlled.

Chlorofluorocarbons (CFCs), halons and other chlorine and bromine compounds were identified as a potential threat to stratospheric ozone in the 1970s. By the late 1980s, they had thinned the ozone layer at all latitudes by around 5 percent, and, in the freezing air over the Arctic and Antarctic, created ozone “holes” in which 50 to 80 percent of the ozone was destroyed for several weeks each spring⁹.

The current use of ozone-depleting chemicals is strongly regulated by international political agreement – notably the Montreal Protocol of 1987 – which called for production phase-out in the developed world by 1996, with a more gradual phase-out in developing countries. Though production phase-out in developed nations has been partly counterbalanced by growing production in developing nations, particularly China, production in these countries has been frozen at 1999 levels and must be phased out for most uses by 2009¹⁰. The ozone layer itself will take another half century to recover.

The most fundamental effect of atmospheric pollution has been on the global carbon cycle. Carbon is a key element for life. It makes up half the mass of plants and animals¹¹ and, as CO2, it is a major “greenhouse gas” responsible for maintaining the atmospheric temperature at levels fit for those organisms.

In the past 150 years, human activity has released more than 350 billion tons of carbon
into the air in the form of CO2. Though up to a half is currently absorbed by oceans or terrestrial ecosystems, this has been sufficient to raise CO2 concentrations in the air by 30 percent since pre-industrial times¹². Carbon is also present in the second most important anthropogenic greenhouse gas, methane, produced in agricultural activities such as rice paddies, the domestication of ruminants and the clearance of natural vegetation. The industrial age has seen a 145 percent rise in methane concentrations in the atmosphere¹³.

The cumulative effect of different air pollution is reducing the atmosphere’s ability to cleanse itself. Most pollutants are removed from the atmosphere through oxidation by the hydroxyl radical. Some research suggests that hydroxyl levels in the atmosphere, particularly temperate northern latitudes, are falling¹⁴. As a result, some compounds are lasting longer in the air than before, causing ever more pollution.