Scientists say they've made the first direct observations confirming an increasing greenhouse effect at the Earth's surface caused by carbon dioxide.
The finding confirms in nature what has already been predicted by computer models and seen in laboratory experiments, that continued emissions of carbon dioxide are causing warming of the surface of the Earth, researchers say.
"We're actually measuring the fact that rising carbon dioxide concentrations are leading to the greenhouse effect," says the study lead author Dan Feldman at California's Lawrence Berkeley National Laboratory. "This is clear observational evidence that when we add carbon dioxide to the atmosphere, it will push the system to a warmer place."
The finding is the first direct confirmation of the impact atmospheric CO2 has on the balance between the energy coming in from the Sun and the outgoing heat from the Earth.
When more energy flows in than flows out, it creates a greenhouse effect through what scientists term radiative forcing.
The researchers measured that forcing at the surface of the Earth at two long-established atmospheric research locations operated by the Department of Energy, one in Oklahoma and one above the Arctic Circle on the North Slope of Alaska.
In Alaska 3,300 measurements were made, while in Oklahoma researchers gathered data from 8,300 measurements made on an almost daily basis.
CO2 was found to be responsible for a significant increase in radiative forcing detected at both locations from 2000 to the end of 2010, they found.
"We see, for the first time in the field, the amplification of the greenhouse effect because there's more CO2 in the atmosphere to absorb what the Earth emits in response to incoming solar radiation," Feldman says.
The scientists used sensitive spectroscopic instruments in Alaska and Oklahoma to measure thermal infrared energy traveling down through the atmosphere to the surface, instruments able to detect the telltale signature of infrared energy from CO2.
During the 10-year study period, concentrations of measurable carbon dioxide in the atmosphere increased at both of the research sites to 22 parts per million, the researchers reported.
Other researchers said they were not surprised by the findings.
"This is another direct piece of evidence that supports that the increase in carbon dioxide is indeed contributing to global warming," says Dave Turner, an atmospheric scientist with the National Severe Storms Laboratory in Oklahoma, who was not a participant in the study.
The researchers said they would continue their studies to see what other greenhouse gases such as methane are contributing to global warming.
