Air Chemistry and Climate

Trees play an important role in the climate system by storing carbon. Scientists know that pollutants have profound impacts on climate. But untangling the exact picture is difficult because different pollutants sometimes have conflicting impacts.

When people talk about the climatic effects of pollution, the discussion inevitably turns to greenhouse gases, such as carbon dioxide (much of which is emitted by cars and trucks). Such gases trap solar radiation in the atmosphere and lead to an increase in temperatures. Without the greenhouse effect our planet would be covered in ice, and carbon dioxide occurs naturally—you're exhaling it in your breath right now. But atmospheric levels of the gas have risen by 31 percent since preindustrial times, from 280 parts per million by volume to over 370 ppmv today because of human activities.

The Chemical Climate System

One of two major goals of the Atmospheric Chemistry Division is to understand the interactions between the physical climate system, the chemical climate system, and the biosphere. Priorities are to simulate the recent past and future chemical climate states based on current climate simulations, and to study the crucial role of the upper troposphere/lower stratosphere in the physical and chemical climate system.


Rising atmospheric levels of carbon dioxide and other greenhouse gases, such as methane and nitrous oxide, are believed to be largely responsible for the increase in global temperatures in recent decades. In the 20th century, the world’s surface air temperature increased by about 0.6º Celsius (1º Fahrenheit). Research at NCAR and other institutions has indicated there is a 90% probability that, between 1990 and 2100, global temperatures will rise by 1.7 to 4.9ºC (3.1–8.9ºF), because of human influences on climate. The warming is likely to occur unevenly, with polar regions far more affected than the tropics (see sidebar).

Other pollutants, however, have a cooling effect. Tiny particles of dust, soot, and other matter that are spewed by vehicles and factories shield parts of Earth from the Sun. Unlike greenhouse gases, they reflect more energy than they absorb. Although these particles can mitigate the impact of greenhouse gases, they can’t offset it. Sulfates and other aerosols remain in the atmosphere for just a few days, and they affect only a limited area near the pollution source. Greenhouse gases, in contrast, remain aloft for years and are distributed around the globe. Computer models at NCAR and other research institutions indicate that, on a global average, sulfates and other particles cause about half as much cooling as greenhouse gases cause warming.

In addition to an increase in temperatures, we may be in for other climate changes. Research by climatologists at NCAR and elsewhere points to more severe droughts, interrupted by heavy storms and flooding. Such extreme events would force changes in farming methods and in the ways that cities handle storm runoff. Pollution also can suppress rainfall, and some research indicates it is reducing the monsoon-driven rainfall so vital for survival on the Indian subcontinent.