This is the conclusion of a team of scientists led by Ben Santer, of Lawrence Livermore National Laboratory. And this is the result of human activity. Nature explains what's going on in "Top of sky is receding."
The top of the troposphere -- the atmosphere's lowest layer -- has risen by several hundred metres since 1979, mostly because of transport and industrial emissions.
The troposphere's height could even serve as a kind of barometer for the extent of global environmental change.
The researchers computer-modelled five possible causes for the shift, three human and two natural. They looked at changes in greenhouse-gas levels, sunlight reflected from airborne solid particles, atmospheric ozone concentration, the Sun's output of heat and light, and dust injected into the atmosphere by volcanoes.
In "Why is the tropopause getting higher?," PhysicsWeb adds that their computer models "show that about 80% of this increase was directly caused by human activity."
The National Center for Atmospheric Research (NCAR) published its own report, "Rising height of atmospheric boundary points to human impact on climate" which gives more details about the troposhere.
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.
And will it affect us?
The study also gives support to scientists, including Wigley, an NCAR senior scientist, 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 full report, "Contributions of anthropogenic and natural forcing to recent tropopause height changes," is published by Science (Vol. 301).
Sources: Philip Ball, Nature, July 25, 2003; Belle Dumé, PhysicsWeb, July 24, 2003; National Center for Atmospheric Research (NCAR), July 24, 2003
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