Passage I

Since 1880, Earth's average surface temperature has increased by 0.5°C. This global warming has caused ocean surface temperature increases and the melting of parts of the polar ice caps. Two scientists discuss the cause of these events.

Scientist 1

Global warming is caused by human activities that have significantly increased the atmospheric concentration of CO₂ above the concentration prior to 1880. CO₂ is one of several greenhouse gases (gases that trap heat emitted from Earth's surface). Human activities, primarily the burning of fossil fuels, have added significant additional CO₂ to the air (6 billion tons per year at present). This has increased the atmospheric CO₂ concentration by 30% since 1880. The higher the atmospheric CO₂ concentration, the warmer Earth's average surface temperature. While atmospheric CO₂ concentrations have been steadily increasing over the past 18,000 years, the annual rate of increase over the past 100 years has been 10 to 100 times faster than the rate of increase prior to 1880. Eight of the warmest years on record have occurred since 1979.

Scientist 2

Changes in atmospheric CO₂ concentration have little or no effect on average surface temperature. Global warming is caused by an increase in the energy output of the Sun and by regular changes in the shape of Earth's orbit. A less elliptical orbit corresponds to a warmer climate. These 2 factors explain at least 80% of the observed surface temperature increase. The rest can be explained by decreases in the amount of atmospheric sulfate aerosols and increases in the stratospheric ozone content. The 6 billion tons/yr of CO₂ from human activities is a very small proportion of the total 186 billion tons/yr of CO₂ that enter the atmosphere. Studies of climate records over hundreds of thousands of years show that average surface temperature has increased and decreased in regular cycles that coincide with increases and decreases in solar energy output. This parallel relationship is particularly evident in the steady warming of average surface temperature since the end of the last ice age 18,000 years ago.