By Deborah Zabarenko, Environment Correspondent
WASHINGTON, April 23 (Reuters) - The amount of two key greenhouse gases in Earth's atmosphere rose sharply in 2007, and carbon dioxide levels this year are literally off the chart, the U.S. government reported on Wednesday.
In its annual index of greenhouse gas emissions, the U.S. National Oceanic and Atmospheric Administration found atmospheric carbon dioxide, the primary driver of global climate change, rose by 0.6 percent, or 19 billion tonnes last year.
The amount of methane increased by 0.5 percent, or 27 million tonnes, after nearly a decade of little or no change, according preliminary figures to scientists at the government's Earth System Research Laboratory in Colorado.
Methane's greenhouse effect is 25 times more potent than carbon dioxide's, but there is far less of it in the atmosphere. Overall, methane has about half the climate impact of carbon dioxide.
The primary source of carbon dioxide is the burning of fossil fuels, which is increasing, with China now the world's biggest emitter, said Pieter Tans, who studies greenhouse gases at the laboratory. The United States is second.
The greenhouse gas index, based on data from 60 sites around the world, showed that that last year's carbon dioxide increase added 2.4 molecules to every million molecules of air, a measurement known as parts per million, or ppm.
OFF THE CHART
Carbon dioxide levels were about 270 ppm in the mid-18th century, before the wide use of fossil fuels that began with the Industrial Revolution. Last year's levels were near 390 ppm, and they have been rising more steeply over the last three decades, Tans said in a telephone interview.
"The average (annual rise) over the last five or six years has been 2 ppm and that is actually steeper than it has been in previous decades," he said. "This whole decade the rate of increase has accelerated, and we have a very clear candidate (for the cause) and that's emissions from burning fossil fuels."
The rise continued in 2008, according to a chart of global carbon dioxide emissions online here, which showed world emissions of this gas heading off the chart at over 386 ppm.
"It's gloomy," Tans said. "With carbon dioxide emissions, we're on the wrong track, it's obvious. And I'm also fully convinced that we're in actually quite a dangerous situation for climate."
The increase in methane emissions after years of little change may indicate that methane locked for thousands of years in frozen Arctic soil known as permafrost is being emitted into the atmosphere as the soil melts.
"What used to be in the deep freeze is now being taken out in the warming," Tans said.
It is also possible that the 2007 rise in methane emissions is due to some other cause. Methane emissions rose sharply between 1978 and 1998 and then leveled off.
(Editing by Cynthia Osterman)