Azara Blog: Data on tropical forest cover is allegedly pretty poor

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Date published: 2008/01/09

The BBC says:

Data on tropical forest cover is so poor that we do not know if the forests are declining, a study has found.

Alan Grainger from the UK's University of Leeds examined UN analyses going back almost 30 years, and found that "evidence for a decline is unclear".
The UN admits there are problems with the data, but says tropical forests are certainly in retreat.

Dr Grainger is not so sure. "People have been assuming that forest cover is shrinking," he told BBC News, "and certainly deforestation has been taking place on a large scale."

But, he says, there is also evidence that in some countries, forests are expanding spontaneously.

"Our analysis does not prove that tropical forest decline is not happening, merely that it is is difficult to demonstrate it convincingly using available tropical forest area data," he writes in PNAS.

The UN reports are produced by the Rome-based Food and Agriculture Organization (FAO) in its regular global Forest Resources Assessments (FRAs).

Assembled principally using data from national forest authorities, the FRAs are widely regarded as the most accurate estimates available, which is why they are used by many researchers in the areas of forestry, land-use change and sustainability.

For his PNAS paper, Dr Grainger looked at the four most recent FRAs, published in 1980, 1990, 2000 and 2005.

Each of the individual reports showed a decline in tropical forest cover; but across the four reports, he found no trend was discernible.

This is largely because each assessment revised earlier estimates of cover. For example, in 1980 the FAO estimated natural tropical forests spanned 1,970 million hectares. But the 1990 assessment used a revised figure for 1980 of 1,910 million hectares.

The FAO says it made these revisions because better data became available, and because each assessment used different criteria.

"What you've got is a desire by the FAO for consistency inside each of its studies," commented Dr Grainger, "but that's come at the expense of consistency between studies."

It is amazing how poor this kind of data is, given how important it is to have decent data in this regard. One thing that the international community should force on researchers in this area is to make all their data permanently freely and publicly available so that data that is collected is not lost and can be validated by others.

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