0.5^oC during the past 100 years. The consequences of such warming can be very dramatic and might include the thinning of the polar ice caps.

Although ozone (O₃) near the surface of Earth is a pollutant and contributes to global warming, O₃ in the stratosphere screens our planet from harmful ultraviolet sunlight. Ozone is produced naturally in the stratosphere and is destroyed by a complex series of chemical reactions, some of which are fueled by man-made chemicals.

FIGURE 9 Measurements of ozone concentrations over Antarctica from natural springtime minimum conditions in 1979 (left) to the extensive ozone hole recorded in 2000 (right).

Global concentrations of O₃ in the stratosphere are monitored by NASA and the National Oceanic and Atmospheric Administration (NOAA) using satellite-based spectrometers. The data show a dramatic seasonal loss of O₃ over Antarctica during the polar spring (October-December). While the formation of the Antarctic ozone hole is cyclic, over the last 21 years the hole has grown larger and is more depleted in O₃ and more persistent (Figure 9).

Using a variety of measuring techniques based on AMO science, atmospheric scientists have discovered a sequence of chemical reactions occurring in the polar stratosphere during winter, when no sunlight is present. These reactions, which produce chlorine molecules (Cl₂), occur on the surface of polar stratospheric clouds of ice crystals. In spring, when sunlight penetrates the region once again, the Cl₂ molecules break apart. The Cl atoms then react with O₃, destroying it and causing formation of the ozone hole.

It has recently been recognized that carbon dioxide (CO₂₎ also plays a role in the growth of the ozone hole. Paradoxically, while CO₂ contributes to global warming near Earth’s surface, it causes cooling in the stratosphere. Carbon dioxide molecules generated at ground level can eventually migrate to the upper layers of the atmosphere, where they collide with oxygen atoms. During the collision, the colliding atoms lose energy (i.e., they cool), while the CO₂ is transferred to an internal excited state. The excited CO₂ then radiates, causing a net cooling of the upper atmosphere. In the stratosphere this cooling contributes to the enhanced formation of polar stratospheric clouds, leading to greater ozone depletion. Models suggest that the doubling of CO₂ in the atmosphere, as is predicted to occur over the next century, will result in significant amounts of cooling in the upper atmosphere and, in turn, more O₃ depletion.