By Robin Pomeroy

ATHENS, Sept 2 (Reuters) - Greece's huge forest fires have been blamed by some on global warming, but satellite images of smoke plumes drifting as far as Africa prompt the question: are forests a major source of greenhouse gas?

Usually it is cars, factories and power stations that are most often mentioned as sources of carbon dioxide (CO2), a gas which traps heat in the atmosphere. Trees, considered the "lungs of the planet", soak the gas up. But what if they burn?

"Global emissions from deforestation and the degradation of forests are the second single source after coal," said Stefan Singer of WWF (the World Wildlife Fund).

Every year 13 million hectares of the world's forests disappear -- an area the size of Greece -- according to the United Nations Food and Agriculture Organisation which says deforestation accounts for 18 percent of CO2 emissions.

Although paling in significance next to deforestation in the Amazon, Congo and Indonesia, forest fires in the Mediterranean might also be a net source of emissions, experts said.

Trees absorb carbon dioxide as they grow and climatologists see forests as carbon "sinks" -- places where large amounts of that element are stored. When they burn, whether in forest fires or as logs in a stove, it is released.

In the atmosphere, CO2 is the main gas which contributes to the greenhouse effect -- trapping the earth's heat which would otherwise be radiated into space.

The latest U.N. report on global warming says temperatures will rise by a best estimate of 1.8 to 4.0 Celsius (3 to 7 Fahrenheit) this century and sea levels will rise by between 18 and 59 centimetres.

CAUSE AND EFFECT

The resulting hotter, drier summers in countries like Greece could mean forests are more frequently brought to the tinder-box conditions which allowed fires to spread so devastatingly.

Greek Foreign Minister Dora Bakoyanni said the summer's devastating floods in Britain and the worst fires in Greek memory demonstrated climate change was already happening.

"From that moment everyone understood that the phenomena caused by climatic change need to be confronted with much more coordination and speed from the EU," she told a news conference.

Scientists said it was too early to judge how much C02 was released by the Greek fires which are the most intense in Europe in at least a decade and have killed 63 people.

If the trees grow back, they will eventually reabsorb the CO2. "If not, the fires will have contributed to greenhouse gas emissions," said Earl Saxon of the Geneva-based World Conservation Union (IUCN).

Bakoyanni tried to allay fears that the scorched land would be used for building. "We are determined that not the smallest piece of land will not be reforested. Nobody will build on burnt land," she said.

Any net loss of CO2 would not count against Greece's legal obligation to control greenhouse gas emissions.

Under the Kyoto Protocol, Greece, was allowed to increase its emissions by 25 percent over 1990 levels. Non man-made sources, such as wildfires do not count.

The IUCN's Saxon said forests have a natural cycle of fires and regrowth but that global warming could upset the balance. If hotter and drier summers mean more frequent forest fires, that could well mean a net emission of CO2.

"If they become more frequent, then vegetation doesn't have time to grow back and the net effect is that you lose more carbon from the eco-system than the eco-system can recapture before the next fire."

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