Resolution of temperature problem proves global warming a reality

The resolution of a debate over differences in surface and atmospheric temperature data has led US scientists to conclude that warming of the Earth's surface is "undoubtedly real," and that surface temperatures in the past two decades have risen at a rate substantially greater than average for the past 100 years.

Climate models generally predict that temperatures should increase in the upper air as well as at the surface if increased concentrations of greenhouse gases are causing the warming. Now, a report by the National Research Council of the National Academies (NRCNA) shows that the differences between the surface and upper-air trends does not invalidate the conclusion that the Earth’s temperature is rising.

The report examines the fact that the Earth’s surface temperature has risen about 0.4 to 0.8°C in the last century, while data collected by satellites and balloon-borne instruments since 1979 indicate little if any warming of the low- to mid-troposphere – the atmospheric layer extending up to about 5 miles from the Earth’s surface.

“The differences between the surface and upper-air trends in no way invalidates the conclusion that the Earth’s temperature is rising,” said John M. Wallace, chair of the panel that wrote the report and professor of atmospheric sciences at the University of Washington in Seattle. “But the rapid increase in the Earth’s surface temperature over the past 20 years is not necessarily representative of how the atmosphere is responding to long-term, human-induced changes, such as increasing amounts of carbon dioxide and other greenhouse gases. The nations of the world should develop an improved climate monitoring system to resolve uncertainties in the data and provide policy-makers with the best available information.”

The report shows that satellite and balloon data correspond with surface temperature observations when natural events and human activities which cool the upper atmosphere are accounted for in atmospheric models. For example, natural events such as the eruption of Mt. Pinatubo in 1991 tend to decrease atmospheric temperature for several years. Burning coal and oil produces tiny aerosol particles in the atmosphere that can have a cooling effect. Upper-air temperatures can also be reduced by depletion of ozone in the stratosphere caused by chlorofluorocarbons and other chemicals being emitted into the atmosphere.

Because global warming is a long-term process that can be masked by year-to-year climate variability, warming trends are most clearly revealed by surface temperature measurements – which have been recorded daily at hundreds of locations for more than a century. This data indicates that the Earth is, in fact, warming, the panel said. Satellites have been collecting data from the upper atmosphere for only about 20 years.

The differences between surface temperature and upper-air temperature records also may be partially attributed to uncertainties in temperature measurements, the panel said. A better climate monitoring system is needed to ensure continuity and quality in data collection. Measurements should include not only temperature and wind, but also ozone, water vapour, clouds, and aerosols. Scientists need to perform a more comprehensive analysis of the uncertainties in surface, balloon, and satellite temperature data. Natural as well as human-induced changes should be accounted for in model simulations of atmospheric temperature variability.

Data also needs to be accessible in a form that enables a number of different research groups to use and improve them, the report says. To ensure access, data should be available in electronic databases to the entire scientific community.

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