Ozone is also found in the lower atmosphere, a region called the troposphere, but normally at very low concentrations that are not harmful. A small amount of tropospheric ozone is transferred down from the ozone formed naturally in the stratosphere. The remainder is formed in the lower atmosphere by the action of sunlight on natural and human-made substances. There is evidence that tropospheric ozone concentrations in the Northern Hemisphere have increased since pre industrial times because of human activity and that this has resulted in a positive radiative forcing. However, the observations of the most recent decade show that the upward trend has slowed significantly or stopped.
Ozone depletion is observed to cause an increase in UV-B radiation at the Earth's surface.
Monitoring data show that the growth in concentrations of ozone-depleting chemicals in the atmosphere is slowing, consistent with the declining production required by international agreements. Because tropospheric and lower stratospheric ozone are greenhouse gases, their depletion results in global cooling. The magnitude of the effect, however, is critically dependent upon the altitudes at which depletion occurs, and must be thoroughly understood in order to unravel the detection of climate change from global observations from satellites, radiosondes, and surface data. The future changes in ozone and ozone-depleting gases are likely to be important for accurately predicting the temperature trends of the coming decade. As a consequence, the cooling effect of ozone depletion will no longer compensate for increasing burdens of greenhouse gases, and the climate could warm much faster in the next few decades.