Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
economic and technological assumptions, estimate a rise in global mean temperature of 1-3.5°C (best estimate 2°C) between 1990 and 2100. In all cases,
the average rate of warming would probably be greater than any in the past 10,000 years, though actual annual-to-decadal changes would include
considerable natural variability. A general warming is expected to lead to an increase in the occurrence of extremely hot days and a decrease in the
occurrence of extremely cold days. Regional temperature changes could differ substantially from the global mean value, and there are many uncertainties
about the scale and impacts of climate change, particularly at the regional level. The mean sea level is expected to rise 15-95 cm (best estimate 50 cm) by
2100, with some flooding of low-lying areas. Forests, deserts, rangelands, and other unmanaged ecosystems would face new climatic stresses, partly as a
result of changes in the hydrological cycle; many could decline or fragment, with some individual species of flora or fauna becoming extinct. Because of the
delaying effect of the oceans, surface temperatures do not respond immediately to greenhouse gas emissions, so climate change would continue for many
decades even if atmospheric concentrations were stabilized.
Achieving stabilized atmospheric concentrations of greenhouse gases would demand a major effort. For CO2 alone, freezing global emissions at their current
rates would result in a doubling of its atmospheric concentrations from pre-industrial levels soon after 2100. Eventually, emissions would have to decrease
well below current levels for concentrations to stabilize at doubled CO2 levels, and they would have to continue to fall thereafter to maintain a constant CO2 concentration. The radiative forcing of greenhouse gas levels (including methane, nitrous oxide, and others, but not aerosols) could equal that caused by a
doubling of pre-industrial CO2 concentrations by 2030 and a trebling or more by 2100.
The international community is tackling this challenge through the United Nations Framework Convention on Climate Change (UNFCCC). Adopted in 1992,
the Convention seeks to stabilize atmospheric concentrations of greenhouse gases at safe levels. More than 170 countries have become Parties to the
Convention. Developed countries have agreed to take voluntary measures aimed at returning their emissions to 1990 levels by the year 2000, with further
legally binding emissions cuts after the year 2000 proposed at Kyoto in late 1997. Developed countries have also agreed to promote financial and
technological transfers to developing countries to help them address climate change.
Source: IPCC, 1996a.
Box 1-2. Stratospheric Ozone Depletion, UV-B Radiation, and the Montreal Protocol
Although ozone can be measured throughout much of the atmosphere, most of it is found in the stratosphere in a layer centered about 20 km above the
Earth's surface. Stratospheric ozone is beneficial to life on Earth because it blocks much of the dangerous ultraviolet light (UV-B) radiated by the sun. If
unnaturally high levels of UV-B radiation reach the Earth's surface, many forms of life can be harmed. For instance, UV-B can cause skin cancers in humans
and may reduce crop yields.
Natural ozone amounts in the stratosphere result from a balance of production and loss processes involving chemistry, meteorology, and solar radiation.
Since the 1960s, however, increases in atmospheric concentrations of human-generated chlorine- and bromine-containing compounds (principally
chlorofluorocarbons and halons) have caused additional ozone loss. This trend has resulted in declines in stratospheric ozone amounts at middle and high
latitudes in both hemispheres. The most dramatic manifestation is the Antarctic ozone hole, where more than half of the ozone is destroyed in a 6-week
period each spring. In recent winters, similar features-but with half the ozone loss (20-30%)-have been observed over the Arctic, and at northern midlatitudes
a long-term decline of 5-10% has occurred over the past 20-30 years. Annual amounts of biologically active UV-B radiation have increased by about
10% over mid-latitudes since 1979. No significant loss of ozone or increase in UV-B radiation has been found in the tropics.
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