Passage II

Global warming can occur when solar radiation passes through the atmosphere, heats Earth's surface, and is then re-emitted as longer wavelength infrared radiation. This infrared radiation can be trapped by "greenhouse gases" in the atmosphere as it leaves Earth's surface, gradually increasing global temperatures. Greenhouse gases include water vapor, carbon dioxide (CO₂), methane, ozone, and chlorofluorocarbons.

Two scientists present their views on global warming.

Scientist 1

In the past 30 years, increased use of coal and oil and the burning of forests have steadily increased atmospheric CO₂. Burning coal and oil releases 5.6 billion tons of CO2 per year; burning forests adds another 0.4 to 2.5 billion tons and also decreases the amount of plants that would remove CO₂ from the air during photosynthesis. This increase of CO₂ in the air has increased global average temperatures and caused Arctic ice to melt. One result of continued global warming will be rising sea levels due to large-scale melting of ice in polar areas.

Scientist 2

CO₂ has not caused global warming. In the past 30 years, human activity has released not only CO₂, but large amounts of dust and aerosol particles into the atmosphere. These particles reflect large amounts of incoming solar radiation back into space before it reaches Earth's surface. This actually cools Earth slightly. From 1938 to 1976, studies showed that global average temperatures were falling while CO₂ levels were increasing.

Ice ages occurred every 2,500 years and were separated by warmer periods. Even though CO₂ levels were higher during the warmer periods than during the ice ages, the ice ages occurred regardless of the CO₂ levels in the atmosphere. During the most recent warmer period 1,000 years ago, the global average temperature was 1° to 3.5°C above that of today but the CO₂ levels were much lower than today's levels. Changes in the CO₂ levels in the past were a result of the temperature changes, not a cause of them.