* Governments, others work to protect forests

* Measuring forests can give benchmark for carbon trading

* Forests store carbon dioxide as they grow

By Alister Doyle, Environment Correspondent

OSLO, Oct 20 (Reuters) - Space agencies and Google Inc <GOOG.O> are helping an international project to monitor forests by satellite to fight global warming, the head of an international earth observation group said on Tuesday.

Deforestation from Brazil to Indonesia is blamed for emitting about a fifth of all greenhouse gases from human activities -- plants soak up carbon as they grow and release it when they burn or rot.

"The only way to measure forests efficiently is from space," said Jose Achache, director of the Group on Earth Observations (GEO), which is linking governments, space agencies such as NASA and others in a new partnership to measure forests.

The system would aim to make annual assessments of forest carbon stocks, compared to a current five-year cycle.

Google, which offers satellite images via its Google Earth site, would contribute with a related project, Achache told Reuters in a phone interview from London. Details of the company's involvement would be given in November.

A 190-nation U.N. climate pact due to be agreed in Copenhagen in December is likely to approve a plan to slow deforestation in tropical nations. That may include putting a price on carbon stored in trees as part of a new market.

"Investors will want some kind of guarantee that when they are putting money into forests that the forests ... will remain there and remain in good condition," Achache said.

NASA, ESA

America's NASA, the European Space Agency (ESA) and national space agencies of Japan, Germany, Italy, India and Brazil were among those taking part in the forest mapping.

Costs would be low, Achache said, since satellite data were already being collected for other purposes. GEO's members include 80 governments as well as U.N. organisations.

Seven countries would act as pilot projects in 2009-10 -- Australia, Brazil, Cameroon, Guyana, Indonesia, Mexico and Tanzania -- based on satellite images taken in recent months.

Satellite images from the U.S. Landsat go back to 1972 -- enabling the world to work out deforestation rates by comparing images with snapshots of current forests. A base year of 1990 might be used, in line with the U.N.'s Kyoto Protocol for cutting industrial emissions.

Under the satellite project, a first phase was to show how much of a country was forested. A second phase would be to work out how much carbon was locked up in each type of forest.

Stephen Briggs, head of ESA's Earth Observation Science, Applications and Future Technologies unit, said radar images of forests can measure carbon above ground since the microwaves are scattered by passing through vegetation.

"We need some form of validated, assured mechanism," he said. Assessments of carbon stocks from space need to be calibrated against measurements taken on the ground.

David Singh, head of Conservation International in Guyana, said forest credits could help the South American nation.

"So far we have low deforestation rates. But there is an upgraded road joining northern Brazil to coastal Guyana. That has the possibility of opening the region," he said.

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