STOCKHOLM – Particle emissions into Earth’s atmosphere affect both human health and the climate. So we should limit them, right? For health reasons, yes, we should indeed do that; but, paradoxically, limiting such emissions would cause global warming to increase. Why?The combustion of fossil fuels, wood, and other biomass increases the amount of airborne particles, which, in a somewhat simplified manner, we can describe as "white” or "black.” Both types can be found in varying amounts in all emissions. Most black particles stem from small-scale and inefficient burning of biofuels, and, in Asia and Africa, from the burning of agricultural waste. By contrast, white particles consist largely of sulfur from the burning of coal and oil.Because black particles contain soot and absorb sunlight, they are believed to increase global warming. White particles, however, reflect some of the incoming sunlight back into space, producing a cooling effect on Earth’s climate.Indeed, according to the United Nations’ Intergovernmental Panel on Climate Change, the cooling effect of white particles may counteract as much as about half of the warming effect of carbon dioxide. So, if all white particles were removed from the atmosphere, global warming would increase considerably.The dilemma is that all particles, whether white or black, constitute a serious problem for human health. Every year, an estimated two million people worldwide die prematurely, owing to the effects of breathing polluted air. Furthermore, sulfur-rich white particles contribute to the acidification of soil and water.Lowering the level of black particles in the atmosphere would benefit both human health and the climate. Measures aimed at accomplishing this could be particularly effective in countries where emission standards for diesel-fueled vehicles have not yet been introduced, and in countries, especially in Asia and Africa, where rural dwellings are heated by primitive stoves and food is prepared over open fires, causing large emissions of soot particles.In 2011, the United Nations Environment Program published a report comparing measures targeting soot particles and other so-called "short-lived climate pollutants” to measures reducing CO2, which showed that the former would achieve a more rapid decrease of global warming in the coming decades. The proposal in this report is now gaining support among governments in many countries and may become an important complement to measures to limit CO2 emissions of.Naturally, measures targeting soot and other short-lived particles must not undermine efforts to reduce CO2 emissions. In the long term, emissions of CO2 and other long-lived greenhouse gases constitute the main problem. But a reduction in emissions of soot (and other short-lived climate pollutants) could alleviate the pressures on the climate in the coming decades.So, what do we do about white particles? How do we weigh improved health and reduced mortality rates for hundreds of thousands of people against the serious consequences of global warming?It is difficult to imagine that any country’s officials would knowingly submit their population to higher health risks by not acting to reduce white particles solely because they counteract global warming. On the contrary, sulfur emissions have been reduced over the last few decades in both Europe and North America, owing to a desire to promote health and counter acidification; and China, too, seems to be taking measures to reduce sulfur emissions and improve the country’s terrible air quality. But, in other parts of the world where industrialization is accelerating, sulfur emissions continue to increase.One way out of the dilemma could be to allow higher sulfur emissions in sparsely populated areas where the soil is not vulnerable to acidification. Here, emissions from ship traffic on the open seas come to mind. However, this is probably not a viable alternative.The most reasonable conclusion is that we should limit particle emissions to reduce health risks. As a consequence, the greenhouse effect would come more fully into view, which we could then hope would further strengthen the resolve to reduce emissions of CO2 and other greenhouse gases.Nobel laureate Paul Crutzen has suggested another solution: manipulate the climate by releasing white sulfur particles high up in the stratosphere, where they would remain for several years, exerting a proven cooling effect on Earth’s climate without affecting human health. In 1991, the eruption of Mount Pinatubo in the Philippines created a haze of sulfur in the higher atmosphere that cooled the entire planet approximately half a degree Celsius for two years afterwards.Other methods of geoengineering – that is, consciously manipulating the climate – include painting the roofs of houses white in order to increase the reflection of sunlight, covering deserts with reflective plastic, and fertilizing the seas with iron in order to increase the absorption of CO2.According to the Royal Society in Britain, releasing sulfur particles into the stratosphere would probably be the most cost-effective method. But our goal should be to avoid such a hazardous enterprise, which future generations would have to continue for hundreds of years. Instead, we should aim to reduce emissions of CO2 and other pollutants that contribute to global warming.