"Determining why the height of the tropopause is increasing gives us insights into the causes of the overall warming of the lower atmosphere," explains Tom Wigley, an NCAR senior scientist and co-author of the article. "Although not conclusive in itself, this research is an important piece in the jigsaw puzzle."
Benjamin Santer of the Lawrence Livermore National Laboratory is the lead author of the article, "Contributions of Anthropogenic and Natural Forcing to Recent Tropopause Height Changes." Wigley and four other NCAR scientists contributed to the article. NCAR's primary sponsor is the National Science Foundation.
Although numerous past studies have pointed to human activities as a leading cause of global warming, this is the first to evaluate impacts on the tropopause. It also provides evidence that temperatures are rising in the troposphere, the lowest layer in the atmosphere.
The tropopause provides a unique window into atmospheric temperatures because it is situated at the upper boundary of the troposphere, where temperatures cool with increased altitude, and at the lower boundary of the stratosphere, where temperatures warm with increased altitude. Observations and climate models both show that the tropopause, which is about 5 to 10 miles (8 to 16 kilometers) above Earth's surface depending on latitude and season, has risen by several hundred feet since 1979. Although this height increase does not directly affect Earth, it is important as an indication that the troposphere is becoming warmer and the stratosphere is becoming cooler. But until now, no study has looked into how much of the tropopause height increase could be attributed to natural causes and how much to human impacts on the atmosphere.
The research team looked at five variables--two natural and three human-related--that could contribute to the height increase: solar radiation, volcanic activity, emissions of greenhouse gases (such as carbon dioxide), emissions of sulfur dioxide, and levels of tropospheric and stratospheric ozone. The team used the NCAR/Department of Energy Parallel Climate Model to conduct a series of seven experiments. The first five analyzed each factor's impact on the atmosphere in isolation. The sixth looked at the combined impact of the two natural factors, solar radiation and volcanic activity. The seventh assessed the impact of all the factors combined. The impacts were compared with observed changes in tropopause height, which were inferred from two sets of data--one from NCAR and the National Center for Environmental Prediction, and the other from the European Centre for Medium-Range Weather Forecasts.
The results showed that the depletion of stratospheric ozone combined with human emissions of greenhouse gases accounted for more than 80 percent of the rise in the tropopause. Ozone depletion (caused largely by human emissions of chlorofluorocarbons, or CFCs) was significant because it cooled the stratosphere, while greenhouse gases warmed the troposphere. The other factors had much smaller impacts. Solar activity made a small contribution to warming in the troposphere and stratosphere, while sulfur dioxide emissions from both human-related activities and volcanic eruptions slightly cooled the troposphere.
The study also gives support to scientists, including Wigley and Santer, who believe temperatures in the upper troposphere are increasing. Researchers have been at odds over whether satellite data indicate that atmospheric temperatures are rising or stable. But a new data set produced by researchers at remote sensing systems in Santa Rosa, California, and analyzed by Santer, Wigley, and other scientists in Science earlier this year indicates that global temperatures in the lowest several miles of the atmosphere rose by one-third of a degree Fahrenheit (about 0.2 degrees Celsius) between 1979 and 1999.
"The increase in the height of the tropopause appears to support the data set that shows the troposphere is warming," Wigley says.