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Proceedings of the Symposium on Global and Environmental Monitoring

Estimates of the deforestation rate in Brazil’s Amazon region over the
last few years have varied by a factor of four. Reliable estimates are needed
because of deforestation’s contribution to global concerns such as greenhouse
warming and loss of biodiversity. A new estimate for forest area cleared
through 1989 (Tardin et al.. 1990) provides a much more reliable point of
reference than was previously available, and permits the re-evaluation of
previous data to derive deforestation rates. In the recent study, 222 LANDSAT
Thematic Mapper (TM) images covering all of the forested portion of Brazil’s
Legal Amazon region were interpreted manually on color composites of TM
bands 3, 4 and 5 at a scale of 1:250,000. The measured deforested area in the
region covered by the map sheets of that work was 394,722sq.km. Inclusion of
small deforested areas in the remainder of the region (totalling 1,930sq.km), and
adjustment for cloud cover, weighted by deforestation extent for each image,
yields a best estimate for the deforested area by 1989.
The value, 396,689sq.km ±_5%, corresponds to the average date of
August 1989.
This includes 97,643sq.km of "old" (approximately pre-1960)
secondary forests distributed continuously in the states of Para and Maranhao,
as determined from recent re-analysis of LANDSAT MSS 1977-78 images, and
5,445sq.km of flooding by hydroelectric dams. The 1989 estimate benefitted
from this year being unusually free of cloud cover over Amazonia. All but
one-third of one image were from 1989. The fact that deforestation takes place
in the dry season and LANDSAT images are also obtained in the dry season,
makes this and all LANDSAT-derived results correspond to the evolution of
deforestation smoothed over the seasonal variation, a point to be kept in mind
especially when interpreting the meaning of the dates associated with the data.
Available previous estimates include studies for images for 1978 and
1988. The 1978 study (Tardin et al.. 1980) used bands 5 and 7 of the LANDSAT
Multispectral Scanner (MSS) at a scale of 1:500,000.
The reported 77,172sq.km of clearing by 1978 was obtained with a
different methodology, for it included clearing in the "cerrado", it did not include