Other Environmental Themes

Global Threats: The Ozone Example

In recent decades, humans have had increasingly pronounced impacts on the biosphere, affecting processes on a global scale and jeopardizing the ecosystem functions we depend on for our survival. A major threat comes from the destruction of natural habitats and the attendant loss of biodiversity. As key species go extinct, the ecosystems they comprise falter and may eventually collapse. Native species are rapidly disappearing, while exotic species are being introduced accidentally or by design—a phenomenon that can upset ecosystems and disrupt the services they provide.

On an even larger scale, humans are influencing the composition of the atmosphere itself, boosting the concentrations of carbon dioxide and other greenhouse gases through the combustion of fossil fuels. These activities increase the likelihood of global climatic warming, while also conflicting with another broad service provided by nature—the maintenance of an atmosphere that has evolved, over time, to support life. Over the course of billions of years, photosynthesis in bacteria, algae, and plants has produced the oxygen that sustains animals. As this oxygen entered the atmosphere, a portion of it went on to form the stratospheric ozone layer, which screens out ultraviolet radiation from the sun and has enabled a plethora of life forms to thrive on the Earth's surface.

Without life, there would be no ozone in the atmosphere. But in the latter part of this century, life-in the form of human beings-has eroded this protective layer through the release of ozone-attacking chemicals such as chlorofluorocarbons used in spray cans, refrigerators, fire extinguishers and air conditioning systems. Ozone depletion is more acute near polar regions, especially over Antarctica. When the Southern ozone hole reaches its annual peak, atmospheric concentrations of the gas drop to half the normal level, resulting in a 10-fold increase in exposure to ultraviolet-B rays on the surface and ensuing damage to organisms.

The good news is that under the 1987 Montreal Protocol, 140 countries agreed to phase out the production of ozone-attacking compounds. If the terms of this agreement are upheld, a full recovery of the Earth's ozone shield could occur by the middle of the 21st century, although clear signs of such progress may not become evident for another 20 years. Despite the unavoidable delays in recovery, we must take steps now to save the ozone layer.