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Scientists have criticised a study published last year which suggested that earlier estimates of worldwide tropical deforestation and atmospheric carbon emissions were too high.

The original study by Frederic Achard and his colleagues — which was based on satellite imagery and published in Science last August — calculated that deforestation rates were 23 per cent lower than current UN Food and Agriculture Organisation estimates (see 'Global deforestation has been overestimated').

But in an article in this week's issue of Science, Philip M Fearnside of the Brazilian National Institute for Amazonian Research and William F Laurance of the Smithsonian Tropical Research Institute in Panama claim that the study contains serious flaws.

For example, they say that estimates of forest biomass were far too low, that the researchers failed to include dead material in the study, and that incorrect assumptions were made about regeneration of forests on abandoned land.

"The cumulative effect of these omissions and other [assumptions] is a large underestimate of greenhouse gas emissions," say Fearnside and Laurance.

The authors of the original study reject this assertion, saying that they "find their comments less than convincing for the Brazilian Amazon,” and that their critics “produce no evidence at all contesting our global biomass estimates or global deforestation rates."

But they also acknowledge that lack of local data on forest biomass remains a major problem in making global estimates about deforestation.

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