Nowhere on Earth can you find forests as tall and vast as those in the Pacific Northwest. They make Canada's great boreal forests look like shrubs. Amazon rain forest trees are mere halflings by comparison. On a scale from one to 10, our forests go to 11.

That's strikingly clear in the first global-scale map charting the heights of the world's forests. Michael Lefsky, a former Oregon State University scientist now at Colorado State University, devised a way to combine millions of observations from NASA satellites to map tree heights around the planet.

Beyond bragging rights, the mapping effort gives scientists an important tool to track how the world's forests

and influence the rate of global warming. While the first map is a rough draft, it shows the potential for making accurate global measurements of forest growth.

"This is a very impressive technical accomplishment," says forest ecologist Stephen Mitchell, a postdoctoral researcher at Duke University.

Only a tiny fraction of the earth's land mass gives rise to tall forests. In northern California, small patches of coast redwoods exceed 200 feet and from British Columbia to Oregon, rain-drenched stands of Douglas fir and Sitka spruce routinely reach 130 feet or more. Forests along the southern rim of the Himalayas come close, as do forests in Laos, Malaysia and Indonesia. In the Amazon, average heights of forest stands seldom exceed 70 feet. In the huge tracts of boreal forest across North America and Eurasia, trees are lucky to reach 50 feet.

To make the map, Lefsky used

, which bounces laser flashes off the Earth. He calculated tree height from measurements of how much longer it took flashes to bounce back from the ground than from the tops of trees. Lefsky analyzed more than 250 million such observations recorded over seven years, which gave him lidar measurements of only 2.4 percent of the Earth's forested surface. To complete the map, Lefsky combined the data with imaging from two other NASA satellites. The journal

is publishing the

.

Mapping forest height at a planetary scale could make it easier to track how much carbon forests store, said Mitchell, who was not involved in the research. Accounting for the fate of global carbon dioxide emissions

. Forests take up some of the carbon dioxide from burning fossil fuels, but it's not clear how much.

We humans release about 7 billion tons of carbon to the atmosphere each year. About 3 billion tons of these emissions remain in the atmosphere and 2 billion tons make their way into the oceans. The rest, presumably, is taken up by vegetation, including forests. But scientists can only account for a fraction of the carbon forests are supposed to be storing.

"If you don't know where it's being stored, you can't predict how long that carbon storage will last," Lefsky said. "Should that storage of carbon stop working, a much larger fraction of carbon is going to be showing up in the atmosphere."

With more work, Lefsky said it should be possible to use lidar and other instruments onboard satellites to make an accurate global accounting of the biomass that forests contain.

"The beauty of this is that it is global and wall-to-wall over all forests," said David Turner, professor of forest ecosystems and society at OSU. "That monitoring ability will be an essential tool in the process of developing a sustainable relationship between humanity and the biosphere."

Lidar mapping may prove useful in auditing credits for carbon storage in forests.

reward landowners or companies for storing carbon by planting trees or preventing carbon from being released by protecting forests. To avoid cheating, agencies need ways to verify the amount of carbon that's really sequestered.

"I think that when we have the new generation of space borne lidar sensors, our maps will be quite accurate," Lefsky said, "probably enough to play a role in validating those programs."

--